DESIGN AND DEVELOPMENT OF WIRELESS BABY MONITORS
By
Eric Yi-Kuo Jen B.A.Sc., Simon Fraser University, 2002
A PROJECT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ENGINEERING
in the School of Engineering Science
© Eric Yi-Kuo Jen, 2008
SIMON FRASER UNIVERSITY
SUMMER 2008
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Revised: Fall 2007 ABSTRACT
The Philips DAP SCD510, SCD520, SCD530, and SCD540 1.8GHz/1.9GHz
DECT Digital Wireless Baby Monitors are part of the next generation digital baby monitor products that emphasize on small form factor, innovative feature set, and cost effective bill of materials. New product features include wideband audio, ambient relative humidity measurement, and carbon monoxide detection.
The objectives of this project are to design and develop these four baby monitor products and begin mass production for the 1st model within the ten-month time frame. The scope of this project includes baseband hardware design, battery charging algorithm design, and carbon monoxide detection algorithm design.
This project report presents the baseband hardware design and implementation, outlines battery charging considerations, describes the battery charging algorithm implementation, examines carbon monoxide detection requirements, and presents the carbon monoxide detection implementation in detail.
iii ACKNOWLEDGEMENTS
I would like to thank my academic supervisor, Dr. Ash Parameswaran, for his guidance throughout the course of this project. I would also like to thank my technical supervisors, Mr. Eric Ho and Mr. James Tung, for their guidance in the product development and their day-to-day opinions and feedback regarding my work. Their expert opinion and advice was very much appreciated.
Moreover, I would like to express my appreciation to all members of the engineering and product development teams at Ascalade Technologies Inc. for their contribution and support during the course of product design and development.
Last, and certainly not least, I would like to thank Ascalade Technologies Inc. for
the support and founding of this M.Eng project.
iv TABLE OF CONTENTS
APPROVAL 11
ABSTRACT III
ACKNOWLEDGEMENTS IV
TABLE OF CONTENTS V
LIST OF FIGURES VIII
LIST OF TABLES X
LIST OF ABBREVIATIONS AND ACRONYMS Xl 1 INTRODUCTION 1
1.1 COMPANY OVERViEW 1 1.2 PROJECT BACKGROUND 2 1.3 PROJECT SCOPE 3
2 DECT WIRELESS BABY MONITOR SYSTEM OVERViEW 5
2.1 PRODUCT FEATURES AND MODEL DESiGNATION 6 2.2 INDUSTRIAL DESIGN 7 2.3 DECTTECHNOLOGY 8
3 BASEBAND HARDWARE DESIGN 11
3.1 BASEBAND HARDWARE DESIGN OF THE BABY UNIT 11 3.1.1 System Controller ASIC 12 3.1.2 Power Management Block 13 3.1.2.1 Adaptor Detection and Adaptor/Battery Switch 16 3.1.2.2 Battery Charging Control Circuit.. 16 3.1.2.3 3.3V Low Drop Out Regulator Circuit 17 3.1.2.4 System Voltage Supplies 18 3.1.3 Audio Processing Block 19 3.1.3.1 Microphone Audio Input.. 21 3.1.3.2 Speaker Audio Output 22 3.1.4 Digital Interface and Control Block 24 3.1.4.1 Crystal Oscillator Circuit and EEPROM 24 v 3.1.4.2 LEDs and LED Driver Circuits 24 3.1.4.3 Keypad Input 25 3.1.4.4 LCD and LCD Driver Circuit 26 3.1.4.5 ATE Debug & Programming Interface 27 3.1.5 Sensor Processing Block 27 3.1.5.1 Temperature Sensing Hardware Implementation 27 3.1.5.2 Relative Humidity Sensing Hardware Implementation 30 3.1.5.3 Carbon Monoxide Sensing Hardware Implementation 31 3.1.6 Radio Block 36 3.2 BASEBAND HARDWARE DESIGN OF THE PARENT UNIT 39 3.2.1 System Controller ASIC 39 3.2.2 Charging Cradle Block 40 3.2.2.1 Charging Indication LED Circuit 41 3.2.2.2 Short Circuit Protection 41 3.2.3 Power Management Block 42 3.2.3.1 Cradle Detection Circuit. 44 3.2.3.2 Li-Ion Battery Charging Circuit 44 3.2.3.3 Battery Current Sense Circuit.. 45 3.2.3.4 System Voltage Supplies 46 3.2.4 Audio Processing Block 47 3.2.5 Digital Interface and Control Block 47 3.2.5.1 Vibration Motor Circuit 48 3.2.5.2 LEDs and LED Driver Circuits 49 3.2.5.3 Keypad Input 50 3.2.6 Radio Block 51 3.3 PHYSICAL IMPLEMENTATION CONSiDERATIONS 51 3.3.1 Integration of RF Circuitry with Baseband Main PCB 53 3.3.2 Utilization of Single 4-Layer Printed Circuit Board 54 3.3.3 Size Reduction of Electrical Components 55
4 BABY UNIT BATTERY CHARGING ALGORITHM 56
4.1 ALGORITHM LIMITATIONS 56 4.2 ALGORITHM DESIGN CONCEPTS 57 4.3 BATTERY TERMINAL VOLTAGE SAMPLING 58 4.4 BATTERY PRESENCE DETECTION 59
vi 4.5 BATTERY CAPACITY COUNTER 60 4.5.1 Initial Battery Capacity Counter Estimation 60 4.5.2 Battery Capacity Counter Update 63 4.6 BATTERY CHARGING CONTROL 64 4.7 AUTOMATIC SYSTEM POWER OFF UNDER Low BATTERy 67
5 CARBON MONOXIDE DETECTION ALGORITHM 68
5.1 CO DETECTION REQUiREMENTS 69 5.2 CO CONCENTRATION MEASUREMENT CALCULATION 69 5.3 CO CONCENTRATION MEASUREMENT CALIBRATION 70 5.3.1 CO Sensor Variation and Resistor Tolerance 71 5.3.2 Amplifier's Non-ideal Characteristics 72 5.3.3 ADC Quantization Error 73 5.3.4 Temperature Dependency 74 5.3.5 Humidity Dependency 75 5.4 CO ALARM CONTROL ALGORITHM 76
6 CONCLUSION 81
7 REFERENCES 83
vii LIST OF FIGURES
Figure 1: System Level Block Diagram of DECT Wireless Baby Monitor 5
Figure 2(a): Industrial Design Rendering for SCD510 7
Figure 2(b): Industrial Design Rendering for SCD520 7
Figure 2(c): Industrial Design Rendering for SCD530 8
Figure 2(d): Industrial Design Rendering for SCD540 8
Figure 3: Baby Unit High Level Block Diagram 11
Figure 4: Baby Unit Power Management Schematic Drawing 14
Figure 5: Baby Unit Power Management Scheme 15
Figure 6: Baby Unit Audio Processing Schematic Drawing 20
Figure 7: Baby Unit System Controller Audio CODEC Architecture 22
Figure 8: Baby Unit KBS Switch Matrix 26
Figure 9: Baby Unit Temperature Sensing Circuit. 28
Figure 10: Thermistor Response vs. Ambient Temperature 30
Figure 11: CO Sensor Construction and Reaction Process 32
Figure 12: Baby Unit CO Sensing Circuit 35
Figure 13: Baby Unit Radio Hardware 37
Figure 14: Parent Unit High Level Block Diagram 39
Figure 15: Parent Unit Charging Cradle Schematic 40
Figure 16: Parent Unit Power Management Schematic Drawing 42
Figure 17: Parent Unit Power Management Scheme 43
Figure 18: Parent Unit Vibration Motor Driver Circuit.. 48
viii Figure 19: Parent Unit KBS Switch Matrix 50
Figure 20: Printed Circuit Board Assembly (PCBA) Dimensional Comparison 52
Figure 21: Printed Circuit Board Layout Implementation Comparison 54
Figure 22: Battery Voltage - Battery Capacity Counter Mapping 62
Figure 23: Battery Charging Control Flow Chart 65
Figure 24: CO Detection Implementation Block Diagram 68
Figure 25: Sensor Sensitivity Compensation for Temperature Dependency 75
Figure 26: Calculated CO Alarm Threshold (13) 78
ix LIST OF TABLES
Table 1: Main Features of Philips DAP SCD51 0-540 Models 6
Table 2: Parent Unit Dimensional Comparison Between 2007 and 2008 Models51
Table 3: Battery Voltage - Battery Capacity Counter Mapping 62
Table 4: Average Current Charged/Consumed In Different System States 63
Table 5: CO Alarm Conditions Outlined in EN50291 69
Table 6: CO Alarm Conditions Derived from EN50291 76
Table 7: CO Alarm Conditions In Terms OfAccumulative CO Concentration 77
Table 8: Piecewise Linear CO Alarm Threshold (13) 78
x LIST OF ABBREVIATIONS AND ACRONYMS
AC Alternating Current ACS Adjustable Current Source ADC Analog to Digital Converter ADPCM Adaptive Differential Pulse Code Modulation ATE Automated Testing Equipment ASIC Application Specific Integrated Circuit BMP Burst Mode Processor BOM Bill Of Material CO Carbon Monoxide CODEC Coder and Decoder DAC Digital to Analog Converter DC Direct Current DCXO Digital Controlled Crystal Oscillator DECT Digital Enhanced Cordless Telecommunications DSP Digital Signal Processor EEPROM Electronically Erasable Programmable Read Only Memory ESD Electro-Static Discharge ETSI European Telecommunications Standard Institute FAD Fast Antenna Diversity FCC Federal Communications Commission FET Field Effect Transistor GFSK Gaussian Frequency Shift Keying GND Ground GPIO General Purpose Input and Output IC Integrated Circuit IIC Inter-Integrated Circuit I-Q In-phase and Quadrature -phase JTAG Join Test Action Group KBS Keyboard Scanner LCD Liquid Crystal Display LDO Low Drop-Out
xi LED Light Emitting Diode Li-Ion Lithium Ion LO Local Oscillator LOA Low Offset Amplifier MC Multi-Carrier NiMH Nickel-Metal Hydride NTC Negative Temperature Coefficient PA Power Amplifier PCB Printed Circuit Board PCBA Printed Circuit Board Assembly PCM Pulse Code Modulation PLL Phase Lock Loop PPM Parts Per Million PWM Pulse Width Modulation RF Radio Frequency RH Relative Humidity RSSI Receiver Signal Strength Indication SMO Surface Mount Device SNR Signal to Noise Ratio SPI Synchronous Peripheral Interface SRAM Static Read Only Memory SPI Serial Peripheral Interface P-Flash Program Flash Memory TOO Time Division Duplex TOMA Time Division Multiple Access UL Underwriters Laboratories UPCS Unlicensed Personal Communications Services VCO Voltage Controlled Oscillator VolP Voice Over Internet Protocol
xii 1 INTRODUCTION
This M.Eng project involves the baseband hardware design and implementation, charging algorithm design, and carbon monoxide detection algorithm design for the Philips Domestic Appliances and Personal Care (Philips DAP) SCD51 0,
SCD520, SCD530, and SCD540 1.8GHz/1.9GHz DECT Wireless Baby Monitor products. This project report is organized into five sections:
• Section one provides an overview of the company, Ascalade Technologies Inc.,
and outlines the background and scope of this project.
• Section two provides an overview of the baby monitor product, introduces the
main product features, presents the industrial designs, and provides an
introduction to the wireless technology used.
• Section three presents the baseband hardware design.
• Section four presents the Baby Unit's charging algorithm design.
• Section five presents the Carbon Monoxide detection algorithm design.
1.1 Company Overview
Founded in 1993 in Richmond, BC, Canada, Ascalade Technologies Inc. is an innovative product company that designs, develops, and manufactures digital wireless and communication products. The company offers its customers complete vertical integration in consumer product development from product
1 design to final production, and provides services in industrial design, mechanical design, electrical design, radio frequency (RF) design, embedded software design, and volume manufacturing. The company's products include digital cordless phones, Voice over Internet Protocol (VoIP) phones, digital wireless baby monitors, and digital wireless conference phones. The company has facilities in
Richmond, BC, Canada (design center and head office), Qingyuan, China
(manufacturing facility), Hong Kong, and Hertfordshire, United Kingdom.
1.2 Project Background
This M.Eng project is to develop four models of DECT wireless baby monitor products for Philips DAP under the AVEN"t9 brand name. These four baby monitor models emphasize on the small form factor, innovative feature set, and cost effective bill of materials. The main new features for these baby monitor products include the following:
• Wideband audio with 7kHz audio bandwidth provides superior audio quality
and clarity over the 4kHz audio bandwidth offered in the previous products.
• In addition to the ambient temperature measurement offered in previous
products, ambient relative humidity (RH) measurement provides parent
with enriched information about the surrounding of the baby.
• Carbon Monoxide (CO) detection provides parent with proactive warning
on one of the common risk to infant in typical households.
2 These four baby monitor products, developed by Ascalade, become a part of the
2008 product offerings for Philips DAP's line of baby monitor products. Ascalade is responsible for the design, development, and volume manufacturing for these four Philips DAP baby monitor products.
1.3 Project Scope
The development of the DAP SC51 0-540 baby monitor products requires multi-disciplinary efforts involving industrial design, mechanical design, baseband hardware design, RF hardware design, embedded software design, acoustic design, and manufacturing automatic testing equipment (ATE) development. My role as a Senior Baseband Engineer and Project Leader requires both close coordination among the development teams and direct involvement in the baseband hardware design and algorithm design for battery charging and sensor detection.
The scope of this M.Eng project work is therefore focused on the baseband hardware design, battery charging algorithm design, and CO detection algorithm design; and will have limited exposure to other aspects of the product development. The objectives of this M.Eng project are to develop the core design components in parallel with other design, implementation, and manufacturing deployment activities while meeting all design and performance requirements. The design has to fit in the industrial design under all mechanical
3 space constrains, to meet cost target on Bill Of Material (BOM), and to fulfill the target schedule for mass production. Full design verification is performed to ensure compliance with relevant mandatory ETSI, FCC, and UL safety standard.
Finally, the design has to be simple and robust enough to ensure cost effective volume manufacturing.
For simplicity, all technical discussion presented in this project report is based on the design and implementation of model SCD540 which covers all the features and is the most complex design among the four Philips DAP Wireless Baby
Monitor products developed. Nevertheless, all four models are presented in the overview sections for the completeness of this project report.
4 2 DECT WIRELESS BABY MONITOR SYSTEM OVERVIEW
Each DECT wireless baby monitor product consists of one Baby Unit, up to two
Parent Units, and one charging cradle for each Parent Unit. The Baby Unit, intended to be placed in close proximity to the baby, monitors the surrounding sound, measures the ambient temperature and relative humidity levels, and detects the presence of CO in the environment. The audio, temperature, relative humidity, and CO detection result are transmitted wirelessly to the Parent Unit which is intended to be carried by the parent(s). Figure 1 below shows the simplified system block diagram indicating the main system level components.
Parent Unit (Optional) Baby Unit #2 Identical to Parent Unit #1 I Radio Circuits I Antenna I System Controller Circuits Parent Unit #1 Memory I Peripheral Circuits Radio Circuits I Antenna Audio I Sensor Circuits I I Charging Circuits System Controller Circuits Memory I Peripheral Circuits I Display I LED II Keys I Audio I Charging Circuits I Humidity Sensor I I Display I LED I Key I ~ Temperature Sensor ~ Vibration Motor SPK I I SPK I I Batteries 0 I CO Sensor I I I MIC 0 I Batteries I MIC i AC-DC Charging Cradle 4- Adaptor Charging Circuit Mains T I I Power AC-DC Mains Adaptor Power
Figure 1: System Level Block Diagram ofa DECT Wireless Baby Monitor
5 2.1 Product Features and Model Designation
The Philips DAP SCD51 0-540 baby monitors are segmented into two Mid-Range models (SCD51 0-520) and two High-end Range models (SCD530-540). Table 1 below shows the main feature differentiation between the four models.
Table 1: Main Features ofPhilips DAP SCD510-540 Models
- - Product Feature SCD510 SCD520 SCD530 SCD540 DeCT 1.8GHz/1.9GHz RF Link • • • • Wideband Audio (7kHz audio bandwidth) • • • • Digital Volume Control • • • • Digital Sensitivity Control • • • • Voice Level Visual Indication • • • • Parent Talk-Back to Baby Unit •• • • 14-Segment LCD on Parent Unit • Dot-Matrix LCD on Parent Unit •• Dot-Matrix LCD on Base Unit • • Rechargeable NiMH Batteries (Parent Unit) • • Rechargeable Li-Ion Batteries (Parent Unit) • • Alkaline Batteries (Baby Unit) • •• Rechargeable NiMH Batteries (Baby Unit) • Pre-recorded Lullabies • • • Night-Light on Baby Unit • •• • Vibration Alarm • • Room Temperature Display • • • • Room Humidity Display • • Carbon Monoxide Detection •
6 In order to simplify manufacturing process and minimize development cost, only two different hardware designs are implemented to realize four different products.
The difference in supported features are controlled either by settings stored in
Electronic Erasable Programmable Read Only Memory (EEPROM) or by selectively populate hardware components during manufacturing.
2.2 Industrial Design
The industrial design of the Philips DAP SCD51 0-540 models emphasizes on the overall small form factor, slim Parent Unit shape, and portability. Figure 2 below shows the industrial design rendering for the SCD51 0, SCD520, SCD530, and
SCD540 models.
- ......
.. 0° ° 0 ...... '0.· • ...... • 0. 0.' -.. 'I"" Ell"" •
Figure 2(a): Industrial Design Rendering Figure 2(b): Industrial Design Rendering
forSCD510 forSCD520
7 . .. .. -:::;. .~ ,
Figure 2(c): Industrial Design Rendering Figure 2(d): Industrial Design Rendering
forSCD530 forSCD540
2.3 DECT Technology
Digital Enhanced Cordless Telecommunications (DECT) is used for the wireless communication between the Baby Unit and the Parent Unit. DECT was developed by European Telecommunications Standard Institute (ETSI) for domestic and corporate telephony applications in Europe but has since been adopted by many countries. The design of the DAP baby monitor products allows the flexibility for the products to operate in:
• 1880-1900MHz European DECT covering all European countries and
most of Asia, Australia, and South America countries
• 1920-1930MHz Unlicensed Personal Communications Services (UPCS)
covering United States of America and Canada.
8 The DECT standard provides a generic radio access technology for wireless telecommunications system comprises of a Fixed Part (the Baby Unit in our application) and one or more Portable Part (the Parent Unit in our application).
The wireless air interface defined in DECT is a Multi-Carrier, Time Division
Multiple Access, Time Division Duplex (MCfTDMAlTDD) radio access. DECT's dynamic channel selection and allocation over the 10 carrier channels assigned in
European DECT (5 channels assigned in UPCS) makes efficient utilization of the assigned frequency spectrum.
Each 10ms DECT frame is divided into 24 time slots (TDMA) organized as 12 transmit slots and 12 receiving slots (TDD). The DECT communication between a Fixed Part and a Portable Part utilizes one transmit-receive time slot pair in one of the frequency channels. The radio signal strength, Receiver Signal Strength
Indication (RSSI), is continuously monitored across all channels and all time slots.
The soft hand-over defined in DECT protocol allows the wireless communication to smoothly and dynamically change from channel to channel and slot to slot depending on the signal strength and level of interference.
Originally developed to provide the means for a portable unit to access a fixed
telecommunication network via radio, DECT permits full-duplex transmission of
300-3400Hz audio bandwidth for telephony applications. However, recent
development in DECT technology by major wireless solution providers allows
higher bandwidth full-duplex audio and data transmission over proprietary
9 implementation on top of the DECT protocol. Instead of using just one time slot pair for the duplex transmission of the digitized audio data, the proprietary implementation utilizes two pairs of time slots to increase the throughput and is capable to deliver higher bandwidth audio plus additional data payload. In this project, the design of the four DAP baby monitor products utilizes the proprietary wideband implementation developed by DSP Group Inc.
10 3 BASEBAND HARDWARE DESIGN
3.1 Baseband Hardware Design of the Baby Unit
The Baby Unit's hardware can be functionally categorized to five sections: power management, digital interface and control, audio processing, sensor processing, and radio. Figure 3 below shows the high level block diagram for the Baby Unit hardware.
VDDIO (3.0Y) System(PNX8009DBHNjController:i[~5~~~~~~~(::~ vooe 11.8V} Voltage RegUlators Radio ARM968E-S VODA/2.5V) f------VBAr RAM Temperature S4tn5or Driving CIrcuit
CO Slgn.1 Amplification Circuit
RH Sensor , l .__ ~~?!_O!_~~~~~~'!.~_ Dlg/tJ)llnterfllce end Control
KBO-KB4 _~_O_~!~ ~~~~~~'!~ j Audio Processing VMICA
LCD_COL LCD_COO, LCD_eKe, LCD_eSB. LCD_RESET
Figure 3: Baby Unit High Level Block Diagram
11 3.1.1 System Controller ASIC
The system controller Application Specific Intergraded Circuit (ASIC) used in the
Baby Unit is the PNX8009DBHN RF and Baseband Controller developed by DSP
1 Group Inc. This system controller is a highly integrated ASIC that comprises :
• An ARM968E-S microprocessor running at 80MHz
• Embedded Static Read Only Memory (SRAM)
• Embedded Program Flash (P-Flash) memory
•A Digital Controlled Crystal Oscillator (DCXO)
• Peripheral interfaces including General Purpose Input and Output
(GPIO), Inter-Integrated Circuit (IIC), Pulse Width Modulation LED
driver (PWML), Keyboard Scanner (KBS), and Synchronous Peripheral
Interface (SPI).
• Joint Test Action Group (JTAG) programming and test interface
• An embedded Burst Mode Processor (BMP)
• Four-channel hardware Adaptive Differential Pulse Code Modulation
(ADPCM) Coder/Decoder (CODEC)
• Integrated radio transceiver and radio Power Amplifier (PA)
• An Analog Processing Unit providing power management, Analog to
Digital converter (ADC), audio CODEC, and analog input/outputs.
1 Only main features that are utilized in this project are shown. Refer to [21] for detailed description of the PNX8009 family of RF and Baseband Controller. 12 3.1.2 Power Management Block
The Baby Unit is designed to be powered by either the AC/DC power adaptor or 4 size AA Nickel Metal Hydride (NiMH) rechargeable batteries. Figure 4 and
Figure in the next page shows the schematic drawings of the power management circuits and the overall power management scheme, respectively.
When the system is powered by the AC/DC adaptor, the voltage supply from the mains power (230V 150Hz AC for European countries or 11 OV 160Hz AC or North
America countries) is converted to 7.5V DC (at full load) to 11V DC (at no load) through the linear AC/DC power adaptor. This 7.5V -11V DC voltage is then down converted and regulated to 3.3V DC with a voltage regulator. If the adaptor is not plugged in, then the voltage from the NiMH batteries is regulated to 3.3V DC using this same voltage regulator.
The two inductors L1 and L2 placed in series with the power adaptor input
provides the needed immunity to Electro-Static Discharge (ESD) coming from the
adaptor power jack. Diode D1 protects the unit from possible reverse-polarity of
the voltage supply (e.g. when power adaptor with incorrect polarity is used).
13 ~------__H. ------1:------, I ChaJging Onut: a Swlt:a. CUWlt : ! Ii I l :r l I II' ~ • : n I, ' " " ' I! : a 181 i ." .n ...... "'"'''' i! I' il ~n : !l ! ': i 1.1 ~ : II i 1i ! !L__ _ I Rn ISM I ~! ~ ! N'QG~ : ------~------~ ...... '" ------.., ! ... ! i ! j .. , , .. i , , .. I '" ! ! ! i i r-- Adaptor DetecDCfI OlUJlt ,~_. ,1
,------, 3 .3V Ret;/ulatOf Q rtUI'l: ! ~ ! I ~ ! , ! !
! l. ._. .J
voc
Om
CS14 '00"1
JOK 10K
---"""""C> VRLCT"R1. [Jl '--'\rv\r---I"'"~~_C1lL llJ 470R
Figure 4: Baby Unit Power Management Schematic Drawing
14 System Controller (PNX8009DBHNj RF PA Supply
VDDO RF Transceiver I H Oscillator Supply I H Supply VDDA2.5V DC VDDA VDDI LDOi I I Analog Supply l...+ Analog-Digital ~ I I I • + Interface Supply ------~ -- VR 1 Control Logic I
VDDC 1.8V DC VDDC LD02 I I Digital Core Supply I I I H • + ------.--- -.-. VR2 Control Logic I VDDIO Peripheral Supply I Control & Detection I t • VDC 3.3V DC ( EEPROM ,------.--. l 3.3V DC iA LDO Regulator Battery ( Voltage LCD Driver I Detection I l J
Switch I RH Sensor IC ) y r ~l t ------. ( CO Sensor Amplifier I Adaptor I i J I Detection ------() ------_. 0 ( Audio Amplifier
>0 c.O IICharge• ::> 0 7.5VDC Control ..= ( I I All LEOs + l Power 4XAA NiMH J Adaptor Batteries 6 6 2100mAh 230V 150Hz AC or 11 OV 160Hz AC 4.0-6.0V
Figure 5: Baby Unit Power Management Scheme
15 3.1.2.1 Adaptor Detection andAdaptor/Battery Switch
The presence of adaptor voltage is detected by a simple transistor switch consists of transistor 06 and its associated passive components. Resistors R2, R6, and
R22 scale down the adaptor voltage so that 06 will turn on when the adaptor voltage exists. Capacitors C4 and C5 stabilize adaptor voltage and thus prevent false detection.
Since the NiMH batteries are intended to provide voltage supply to the system only when the voltage supply from the power adaptor is unavailable (i.e. when the adaptor is unplugged from the Baby Unit or during mains power failure), the presence of adaptor voltage also controls a transistor switch to disconnect the battery voltage from system supply. As shown in Figure 4, when the adaptor voltage exists, the reverse VEB biased transistor 01 disconnects the battery from the system. With the absence of the adaptor DC voltage at anode of 02, switch
01 is forward biased and thus allows the battery to power the system.
3.1.2.2 Battery Charging Control Circuit
The battery charging algorithm on the Baby Unit is a simple implementation where the system controller switches between a higher current quick charge and a lower current trickle charge. The hardware topology utilizes an open-collector PNP switch to control a second NPN switch placed in the path of the charging current.
16 This two-stage topology ensures that the higher "quick charge" current is allowed to flow into the battery even without any software control.
During trickle charge, the charging current is determined by the parallel resistors
R11, R12, R17, and R19 to be approximately 105mA. During quick charge, the charging current is increased by forward biasing transistor switch Q2 and thus adding R14 and R16 in parallel to the resistors. The reduced resistance in the charging path sets the quick charge current to approximately 21 OmA. These charging current values are determined following the battery manufacturer's recommendation and correspond to about 5% and 10% of the 21 OOmAh capacity.
The terminal voltage of the 4 NiMH batteries in series, being one of the main inputs to the battery charging algorithm, is measured by the system controller's build-in ADC.
3.1.2.3 3.3V Low Drop Out Regulator Circuit
The linear AC/DC power adaptor used does not provide voltage regulation and thus the adaptor voltage varies between 7.5V DC under full load condition and
11V DC under no load condition. In addition, the terminal voltage of the 4 NiMH
batteries in series varies between 4.4V to 6V depending on both the load current
and the charges remaining in the batteries. Therefore, a 3.3V Low Drop-Out
(LDO) voltage regulator is used to provide a clean and regUlated main voltage
17 source for the Baby Unit. This 3.3V voltage also supplies all the peripheral devices including the EEPROM memory, the Relative Humidity (RH) sensor IC, the Carbon Monoxide (CO) sensor amplifier circuit, the audio amplifier, the Liquid
Crystal Display (LCD) driver, and all Light Emitting Diodes (LED).
3.1.2.4 System Voltage Supplies
To distribute the 3.3V regulated voltage supply to other system supply domains, two simple LDO voltage regulators are implemented using the regulator control logics build-in to the system controller. These two LDO voltage regulators not only provide the required voltage regulation but also improve isolation between different system voltage domains.
As shown in Figure 4, each regulator requires one external bipolar transistor plus the necessary bias resistors. These regulators rely on the feedback control logic
hardware internal to the system controller to maintain fixed output voltage.
When the load current Ie increases, the higher VEe drop causes the output voltage
of the regulator to decrease. The control logic senses this change and reduces
the voltage at the control pins (VR1_CTRL and VR2_CTRL) , creating a higher VES
to allow higher load current. On the other hand, when the load current Ie
decreases, the voltage at the control pins increases to compensate for the lower
load current and maintains the fixed output voltage.
18 The regulated 3.3V DC voltage supply from the main regulator is distributed to the following system voltage supply domains:
• VDDA is the main analog voltage domain supplying all analog front end
circuits including the audio CODECs and is regulated to 2.5V by the VR1
LDO voltage regulator.
• VDDO supplies the Digitally Controlled Crystal Oscillator (DCXO) and is
connected to the 2.5V VDDA voltage supply.
• VDDINT supplies the analog-digital interface hardware of the system
controller and is connected to the 2.5V VDDA voltage supply.
• The voltage supplies for the RF Power Amplifier (PA) and the RF
transceiver are connected to the 2.5V VDDA voltage supply.
• VDDC supplies the digital core of the system controller and is regulated to
1.8V by the VR2 LDO voltage regulator.
• VDDIO supplies the digital 10 ports and all peripheral interfaces of the
system controller and is connected directly to the 3.3V voltage output from
the main LDO regulator.
3.1.3 Audio Processing Block
The Audio processing block includes the microphone circuit and the speaker
amplifier circuit. The audio CODECs build-in to the system controller contains
the Analog-to-Digital Convertor (ADC) and Digital-to-Analog Convertor (DAC) that
converts the analog audio signal to and from the digital linear Pulse Code 19 Modulation (PCM) samples for the system controller to perform digital audio processing and ADPCM encoding / decoding. Figure 6 below shows the schematic drawing of the audio processing circuits.
------.--.------, , MICROPHONE ORCUIT , "40 I on I 1'1 VMICA =~_-_--'V\I\r----...__._-VV'I/'_----<~_._____,"'" 470R on. C46 ~ ONO
[lJAlNl =J--_._--.-A./\f\r--'MI.CN:::''-----1 f-----. RJO C1 MC2 :1R c::.7 lOOn 018 l~ OPEN
...... GND """
C30
ONO
.,------.------.------i, SPEAKER ORCUI • 1 09
I~' m \tt 1 >0.. I~
~"""l-I 51'1
C18 [, TW T lOOn I ~4 I 1="'" _-----_.--''V~~m, ----H-.... 1-::=--' ~ 1
1
Figure 6: Baby Unit Audio Processing Schematic Drawing
20 3.1.3.1 Microphone Audio Input
The microphone circuit supplies the DC bias voltage and provides signal conditioning for the electric omni-directional microphone. The 2.0V VMICA reference voltage provided by the system controller supplies, via resistors R77,
R40, and R31, the DC bias voltage needed for the microphone's internal Field
Effect Transistor (FET). The differential audio signal from the microphone is connected to the system controller's audio CODEC inputs (AIN1 and AIP1) via the coupling capacitors C5 and C7. Additional resistors and capacitors provide signal filtering and frequency response enhancements for the audio signal. In addition, 8.2pF capacitors placed at strategic locations decouple the "TDD" noise
(commonly caused by the 10ms DECT TDD frame bursts that couples into audio circuits) from the audio signal.
Figure 7 in the next page shows the system controller's audio codec architecture.
The audio signal from the system controller's audio input pins (AIP1 and AIN1) is first amplified by a low noise 16dB pre-amplifier to magnify the small microphone signal for adequate ADC resolution. After the amplifier, the Sigma Delta ADC stage samples and digitizes the analog differential audio signal into bit streams.
The decimation filter then converts (decimates) the bit stream data to 16 bit linear
PCM samples at 32 kHz sampling rate, which is then fed into the audio sample
buffer for digital audio processing.
21 I I I I I I
AOP1 Digital Noise D/A Shaper ) AON12 I I I I I I Audio I I I Sample Buffer I I I / / AIP1 Decimation AID Filter ( <16dB AIN1 \ ~ I I I I I I 16 bit linear PCM I Bit stream I Analog signal I I I I
Figure 7: Baby Unit System Controller Audio CODEC Architecture
3.1.3.2 SpeakerAudio Output
The 16 bit linear PCM samples from the audio data buffer is first converted
(interpolated) into bit streams by the Digital Noise Shapero The DAC then converts the bit stream into analog differential audio signals at the audio output pins (AOP1 and AON12).
This AOP1/AON12 differential audio signal output from the system controller is connected, via the coupling capacitors C20 and C40, to the LM4902 audio amplifier which provides the necessary signal amplification and current drive for the 8 Ohm Mylar speaker. Additional resistors and capacitors provide signal filtering and frequency response enhancements for the audio signal. Similar to
22 the Microphone circuit, 8.2pF capacitors placed at strategic locations reduces the coupling of RF TDD noise into the audio signal.
The LM4902 audio amplifier used is a differential input - differential bridged output amplifier that is able to achieve close to 6V output voltage swing with the
3.3V supply voltage provided. The differential audio signal gain is calculated as:
A =2x R23 =2x 22k =4.4 = 12.87dB Eq 1 VD R72 10k
With the 6V maximum signal voltage swing across the 8 n speaker, the maximum electrical power of which the amplifier is able to deliver to the speaker is:
V. PsPEAKER = R;'2= (6)22J2 -;- 8 = O.56W Eq 2
The system software disables the audio amplifier via the amplifier's shutdown pin when there is no audio to be played to the speaker. Doing so not only reduces
overall system power consumption but also conceals the white noise emitted from
the speaker during silence.
23 3.1.4 Digital Interface and Control Block
The following sub-sections describe the hardware implementation for each functional block within the digital interface and control block shown in Figure 4.
3.1.4.1 Crystal Oscillator Circuit and EEPROM
The external crystal provides a 13.824MHz base clock for the system controller and the Phase Lock Loop within the system controller. As DECT does require an accurate reference frequency for its RF carrier, a Digital Controlled Crystal
Oscillator (DCXO) within the system controller compensates for the tolerance of the external crystal. In mass production, the crystal oscillator of each unit is calibrated by configuring a bank of tuning capacitors within the DCXO.
Connected to the system controller via IIC interface, a 4k byte EEPROM provides non-volatile memory storage for user preferences (e.g. volume setting), system configuration settings (e.g. DECT registration data), production calibration data
(e.g. DCXO tuning result), and other design parameters.
3.1.4.2 LEDs and LED Driver Circuits
The red and green LEDs on the Baby Unit show the status of CO alarm, system power, and the battery charging progress. In addition, 4 white LEDs provide backlight illumination for LCD display. The system controller turns on and off the
24 LEDs via simple transistor switches driven by GPIO pins. The transistor switch allows the LED's forward bias current, which is directly proportional to the optical brightness of the LED output, to be higher than the current driving capability of the system controller's GPIO.
The Baby Unit's nightlight, consists of 5 amber color LEDs, provides soothing illumination in the baby's room during night time. These LEDs are controlled by the system controller's PWML port via simple transistor switch. The system software controls the brightness of the nightlight by adjusting the pulse width of the Pulse Width Modulation (PWM) signal driving the transistor switch. With incremental increase or decrease in the PWM's pulse width, a diming effect is achieved when switching on or off the nightlight.
3.1.4.3 Keypad Input
There are 7 keys on the Baby Unit for the functions of: switch on/off power, paging
Parent Unit, switch on/off nightlight, increase speaker volume, decrease speaker volume, change lullaby music, and play/stop lullaby. These 7 keys are tact switches connected to the system controller's Keyboard Scanner (KBS) port.
Figure 8 in the next page shows the KBS key matrix of the Baby Unit.
The system controller periodically scans through each one of the 5 KBS pins by
configuring 1 pin as logic high output while keeping the remaining pins as logic
25 input pins. With each scan pass, the system controller detects for short circuit between any two KBS pins. Because only one switch is placed between any two
KBS pins, the system software can then determine which key has been pressed.
Note that while only 7 switches are used in the Baby Unit, up to (5-1)! =10 switches can be supported with the switch matrix driven by from 5 KBS pins.
r [ r J r
r-- ~ - ~ - - - ~ - 1D + ~ f-r- ~ 1----'"- ~ 1--'- ~ 1----'"-
N N N N " " " " ~ .--- .---- ~ - '" - >- ~ f-/- ~ 1-/-
N N " ~ " <------<
NmUTE - ,
Figure 8: Baby Unit KBS Switch Matrix
3.1.4.4 LCD and LCD Driver Circuit
The Liquid Crystal Display (LCD) is the main user interface output device on the
Baby Unit. The LCD block consists of a LCD glass, a flexible heat-seal cable connecting the LCD glass to the Printed Circuit Board (PCB), and a LCD driver IC.
The custom tooled LCD glass contains four 7-segment digits for the temperature
26 and relative humidity level display, a 1-line 12-character 5x7 dot matrix text display, and seven graphic icons. The LCD driver IC, communicating with the system controller via the Synchronous Peripheral Interface (SPI), provides the required step-up voltage regulation and generates the driving waveforms to the LCD glass.
3.1.4.5 ATE Debug & Programming Interface
The Automatic Test Equipment (ATE) debug and programming interface provides the necessary communication ports for testing, calibration, and firmware programming during manufacturing. The JTAG interface is used primarily to
program the content of P-Flash within the system controller; while the IIC interface
is used to program the content of EEPROM and also as the control interface
between the ATE equipment and embedded firmware during production testing.
3.1.5 Sensor Processing Block
3.1.5.1 Temperature Sensing Hardware Implementation
The temperature sensing hardware consists of an external Negative Temperature
Coefficient (NTC) thermistor, an Adjustable Current Source (ACS), and an ADC
build-in to the system controller. Figure 9 in the next page shows the Baby Unit's
temperature sensing hardware implementation.
27 ACS PT24 PT21 R1S
SR1 R13 R6 C2S DZ3 1uF S1.1K SV6 +/-1% AID
I------
Figure 9: Baby Unit Temperature Sensing Circuit
The Adjustable Current Source (ACS) inside the system controller provides a constant DC current which flows through the resistor network consists of R15 in series with the parallel configuration of R13 and the thermistor R6. The thermistor, R6, is the sensing element that exhibits a decrease in electrical
resistance with increasing temperature. As the ambient temperature changes, the thermistor's resistance changes, causing the voltage at the ACS pin to change.
The system software measures this voltage via ADC and determines the ambient temperature by using a look up table.
The Lattron LNTG49.12HW NTC thermistor used in the design has zero-power dissipation resistance of 49.12 kO at 25°C. Since the ACS provides a nominal
50 IJA DC current, feeding the ACS current source directly to the thermistor at
25°C would generate about 2.5V nominal voltage which is too high for the 0 V to
28 2.0 V ADC input voltage range. Therefore, a 51.1 kO resistor, R13, is place in parallel with the thermistor to bring down the nominal ADC input voltage to 1.253V at 25°C ambient temperature.
The thermistor sensor is physically located outside the Baby Unit's enclosure and is connected to the Baby Unit via a two-wire cable. This physical distance between the sensing element and the Baby Unit's enclosure is necessary to minimize the error in ambient temperature measurement caused by heat-generating electronic components inside the Baby Unit. Decoupling capacitor C25 is placed close to the ADC input pin to reduce noise picked up by the long wires and other passive components. Zenor diode DZ3 is placed on the
PCB but close to the thermistor wires to improve the system's immunity to
Electro-Static Discharge (ESD).
Because of NTC thermistor's non-linear relationship between resistance and temperature, a look-up table is implemented instead of approximation calculation
(for example, the Steinhart-Hart equation). This significantly reduces the processing load as the system controller is not specifically designed for complex arithmetic calculations such as the natural logarithm calculation required in the
Steinhart-Hart equation. Figure 10 in the next page shows the thermistor
resistance and the resulting ADC input voltage at O°C to 40°C ambient
temperature.
29 Thermistor Resistance and ADC Input Voltage vs. Ambient Temperature
3.5 2.00 C ..ll: ... 3.0 en 1.75 ~ ....0 "t:l CIl 2.5 1.50 .!::! ~ iii CIl E Cl ... 2.0 1.25 ....C'll z0 (5 - > CIl .... to) ::J c: 1.5 1.00 Co ....C'll E Ul u ·iii 0 CIl « 0::... 1.0 0.75 ....0 .!!! E... 0.5 0.50 CIl ~ f- 0.0 0.25 0 5 10 15 20 25 30 35 40
Ambient Temperature (Oc)
-Normalized Thermistor Resistance -ADC Input Voltage
Figure 10: Thermistor Response vs. Ambient Temperature
3.1.5.2 Relative Humidity Sensing Hardware Implementation
A RH sensor IC module, Sensirion SHT10, is used in the Baby Unit for ambient relative humidity measurements. The SHT1 0 IC contains a micro-machined capacitive polymer RH sensing element, a bandgap temperature sensor, a build-in 14-bit ADC, and a build-in IIC serial interface. Each SHT10 IC is pre-calibrated to provide ±4.5% tolerance in RH measurement and ±0.5°C tolerance in temperature measurement. The system controller periodically pulls the RH and temperature measurement data from SHT10 via the IIC interface. 30 In addition, the SHT1 D's temperature measurement data is used as the estimation of Carbon Monoxide sensor's body temperature, which is needed to calculate temperature compensation for the CO concentration measurements (see Section
5). Therefore, the SHT10 is physically located close to the Carbon Monoxide
(CO) sensor to improve the correlation between the temperature measurement and the body temperature of the CO sensor. The SHT10 is also placed close to the slot openings at the side of the Baby Unit ensure that the RH measurements are good representations of the ambient RH level.
3.1.5.3 Carbon Monoxide Sensing Hardware Implementation
The Carbon Monoxide (CO) sensor in the Baby Unit, Figaro TGS5042, is an electrochemical sensor that generates electrical current as a result of the electrochemical reaction between the ambient CO and the electrolyte's anions inside the CO sensor. The resulting electrical current, approximately linearly proportional to the ambient CO concentration, is conditioned by an amplifier and measured by the ADC build-in to the system controller.
CO Sensor's Principle of Operation
The CO sensor construction comprises of an active charcoal filter, two catalyst
layers separated by liquid alkaline electrolyte, a water reservoir, and an outer can that encapsulates all internal structures. Figure 11 in the next page shows the
overall construction and the reaction process of the CO sensor.
31 --e"---..
R
.-e"--
Figure 11: CO Sensor Construction and Reaction Process
Ambient air enters into the sensor through the diffusion pin holes located on the cap of the sensor. An active charcoal filter at diffusion intake reduces the sensor's cross-sensitivity to common ambient gas other then CO gas. The CO then reacts with the alkaline electrolyte containing very low concentration of KOH,
KHC03, and K2C03. The anodic reactions, shown in Eq3 to Eq5 in the next page, consume OH", HC03-, and col- anions and generate CO2, H20, and electron.
32 Eq 3
Eq 4
CO+ cq2- ---+ 2Cq +2e- Eq 5
The C02 and H20 generated by the anodic reactions contribute directly to the cathodic reaction at the counter electro, whereas the required electron for the cathodic reaction is supplied from an external low ohmic path connecting the working electro to the counter electro. The cathodic reactions, shown in Eq 6 to
Eq 8 below, consume 02, CO2, H20, and electrons and generate OH-, HC03-, and
2 C03 -.
l02 +H 0+2e- ~20H Eq 6 2 2
l02 +2C0 +H 0+2e- ~2HC03 Eq 7 2 2 2
Eq 8
The net reaction, shown in Eq 9 below, consumes CO and O2, generates C02, and at the same time forces electrical current to flow through the external circuit, which in term generates a voltage across the two electro of the CO sensor.
Eq 9
33 Therefore, the electrochemical sensor is self-regenerative, consumes oxygen which is readily available in ambient air, produces harmless carbon dioxide also common in ambient air, and generates electrical current that is proportional to the
CO concentration within the ambient air.
Sensor Sensitivity
The relationship between the ambient CO concentration and the resulting electrical current from the sensor can be described by the following equation:
isensor(nA) =Concentrationco(ppm)' Sensitivity( nA ) Eq 10 ppm
The sensor sensitivity (nAlppm) is a intrinsic constant and varies from sensor to sensor. The sensitivity for each TGS5042 sensor is measured by the sensor manufacturer, Figaro, and marked on the barcode label outside the sensor body.
Typical sensitivity of the TGS5042 sensor is in the range of 1.00-3.75nAlppm.
Signal Amplification
In our application, the maximum CO concentration for detection is 300 ppm (see
Section 5). With the typical 1.00-3.75nAlppm sensor sensitivity, the maximum
output current from the sensor is 0.72 IJA. According to the TGS5042 application
note, the resistance of the external electrical path across the sensor terminals
cannot be higher than 1kO in order to ensure proper sensor operation.
Therefore, a 300 ppm ambient CO concentration would be translated to just 0.72
34 mV even with the maximum 1 kO load. To improve Signal-To-Noise ratio (SNR), this small electrical signal needs to be amplified before being measured by the
ADC. Figure 12 below shows the CO sensing hardware implementation.
,..-,
SfNSOlU TGS5042-"DO
FT~ ::>-__- __- __- __--C..L...t
C4
l<#'
R25 U FU4 J\/V'------~"/\/'v OR. lAJ-t III
Figure 12: Baby Unit CO Sensing Circuit
As the sensor's output current is typically in sub-IJA range, the low noise TI
TLC272CD Operational Amplifier is chosen for its low input offset current (typically
0.1 pA) and low input bias current (typically 0.6 pA). As shown in Figure 12, the unity gain stage, U1-A, provides a 0.3 V common reference voltage for the following amplifier stage, U1-B, which scales the signal voltage to suitable levels at the ADC input. Eq 11 below shows the overall input-output relationship between the sensor output current and the ADC input voltage.
Eq 11
35 Because of the high amplification gain used, even the TLC272CD amplifier's input offset voltage, typically 1.1 mY, generates a significant 617 mV DC voltage at ADC input. In addition, any tolerance in sensor sensitivity, hardware component value, and amplifier characteristic need to be catered in the design to ensure that the signal voltage level at the ADC input does not exceed its OV - 2V range. In worst case scenario, the maximum input signal of 300 ppm ambient CO will generate
0.72mV voltage drop across resistor R1. This voltage is then shifted, amplified, and combined with the amplifier's 1.1 mV input bias voltage to (0.72 mV + 1.1 mV) x 561 + 0.3 V =1.32V at the amplifier output. This leaves 2.0 V- 1.32 V =0.6aV voltage margin at the ADC input.
3.1.6 Radio Block
The Radio Frequency (RF) block comprises of five main components: the Burst
Mode Processor (BMP), the RF Transceiver, the RF Power Amplifier (PA), the external RF circuits, and the antennas. Figure 13 in the next page shows the block diagram of the radio section.
The Burst Mode Processor (BMP) is the main RF control block that generates the
DECT slot and frame timing, controls RF transmitter and PA timing, recovers
receiver timing, handles error control coding/decoding, and processes data
scrambling and encryption. The BMP controls the PA-enable timing and the
switching between transmit and receive modes via an external PIN diode.
36 TXlRX S""';tch Control
------~ I I I I I I I BMP I I I I Digilal Demodulator
Antenna Switch Control
External RF Circuit PNX8009DBHN System Controller (RF Sub--system and ADPCM COOEC)
Figure 13: Baby Unit Radio Hardware
The Voltage Controlled Oscillator (VCO) inside the transceiver provides the Local
Oscillator (LO) frequency to the mixers in transmit and receive directions. The same LO frequency is used in the Phase Lock Loop (PLL) frequency synthesizer which supplies frequencies and clocks within the system controller. The transceiver also periodically scans and maintains a Received Signal Strength
Information (RSSI) table which records the signal strength information for each channel and each time slot. The RSSI table is then used for dynamic hand-over as defined in the DECT standard.
In the transmit direction, the baseband ADPCM data is first modulated with
Gaussian Frequency Shift Keying (GFSK) I-a modulation. The modulated signal is then up-converted to the channel frequency by mixing the signal with LO frequency. The PA, capable to drive up to 27 dBm signal into 50n load, then
37 amplify the signal for transmission. The external RF circuit provides necessary signal filtering and switching between the PA output and the antenna.
The RF signal received from the antenna is first filtered and pre-amplified to improve Signal-to-Noise Ratio (SNR). The signal is then down-converted to baseband frequency by mixing with the LO frequency. The channel filter provides the necessary adjacent channel rejection, and the hard limiter removes amplitude information from the signal for the digital demodulation. The GFSK
I-Q digital demodulator recovers the ADPCM data which is then fed into the
ADPCM decoder for further digital processing.
The Fast Antenna Diversity (FAD) hardware implementation in the transceiver dynamically switches between two antennas (via external PIN diodes) at the start of each DECT frame so that the radio communication is always carried out over the antenna that has better signal quality. Because the two antennas are physically oriented to cover different radiation direction, the two-antenna FAD implementation offers better overall RF signal quality as compared to the single antenna configuration.
38 3.2 Baseband Hardware Design of the Parent Unit
The Parent Unit's hardware can be functionally categorized into five sections: charging cradle, power management, digital interface and control, audio processing, and radio. Figure 14 below shows the high level block diagram of the Parent Unit.
Charging Cradle
ZJ(JIJJ50HzAC 0, 11CNI60HzAC System Controller: ~~~~+~~~[::J VDOIO (3.W) (P::::::~:N) l-- ______~a_~I~_. Regulator vooe (1.6V) Voltage RAM : Dlgltallnterlace and Control Circuits VR1_CTRL Regulators VDOA(2.5V) VBAl P-Flash
AOC
t---
, -:~~~~~~~~~~~~~~ -- --_. ------_. -j Audio Processing VMICA
L:N~IBe::R",ATc:::O""R_",ON::- -< ~~~~:Ult )------1
LCD_COl, LCD.COO. LCO.CKB. LCD lCO RESET LCD DrIver csa LCD Display Circuit
Figure 14: Parent Unit High Level Block Diagram
3.2.1 System Controller ASIC
The system controller ASIC used in the Parent Unit is the PNX8009DHHN RF and
Baseband Controller developed by DSP Group Inc. The PNX8009DHHN system 39 controller is very similar to the PNX8009DBHN used in the Baby Unit. In fact, for
our application, the only difference between the two ASICs is that PNX8009DHHN supports the Low Offset Amplifier (LOA) which is used for monitoring the current flowing into and out of the rechargeable batteries in the Parent Unit.
3.2.2 Charging Cradle Block
The main functions of the charging cradle are both to provide charging current to the Parent Unit and to indicate if the Parent Unit has been properly placed onto the charging cradle. When the Parent Unit is placed on charging cradle, the charging contacts at the top of charging cradle and bottom of the Parent Unit provides the electrical connections for the charging current. Figure 15 below shows the schematic drawing of the charging cradle hardware implementation.
-1-°T t <""J
.".. en .. ..--
Figure 15: Parent Unit Charging Cradle Schematic
40 The voltage supply from the mains power (230V 150Hz AC for European countries or 11 OV 160Hz AC or North America countries) is converted by the linear power adaptor to 7.5V DC (at full load) to 11V DC (at no load). The series diode 01 protects the rest of the circuit from reverse-polarity input (e.g. when the use plugs in an adaptor with incorrect polarity). This 7.5V - 11V DC voltage is then fed to the Parent Unit via the charging contacts. The voltage regulator U1 is designed-in as an option to provide additional voltage regulation if needed.
3.2.2.1 Charging Indication LED Circuit
The charging indication LED is automatically switched on with the presence of charging current. The charging current provides the necessary bias current through resistor R18 to turn on transistor switch 01, which in term lit up the charging indication LED. The brightness of the LED is set the resistor R5.
3.2.2.2 Short Circuit Protection
Because the charging contacts are electrically conductive parts exposed to the exterior of the unit, it is necessary to consider the possibility that charging contacts are short-circuited (e.g. a conductive object is placed across the
charging contacts). Transistor switches 02 to 05 are designed to shut off the
charging current when the charging contacts are short-circuited. A high enough
voltage difference between the positive and negative charging contacts is
41 required to turn on 05, which in term turns on transistor switches 02 to 04.
Therefore, when the charging contacts are short-circuited, 05 will remain off and thus switches off 02 to 04. Three transistors are placed in parallel to allow
sufficient collector current rating for the possible high transient current after the
charging contacts are short-circuited and before transistor 05 is fully switched off.
3.2.3 Power Management Block
Figure 16 below shows the schematic drawings of power management circuits, and Figure 17 in the next page shows the overall power management scheme
VIlAT VDDIO Q'Ball,"" t C -.- - ....
IIllAT C4S ~ C!9 + Cll .." 1'000 l'" 1:* "'" ------.." 101< ~CTlU.(1]
Figure 16: Parent Unit Power Management Schematic Drawing
42 System Controller (PNX8009DHHN) RF PA Supply
VDDO RF Transceiver H Oscillator Supply I H Supply VDDA2.5VDC VDDA VDDI LD01 I -I Analog Supply ~ Analog-Digital I Interface Supply r-1 t ______--- VR1 Control Logic I
VDDC 1.8V DC VDDC LD02 I H I Digital Core Supply L ______--- VR2 Control Logic I
VDDIO 3.1V DC VDDIO r---. LD03 Peripheral Supply t ------VR3 Control Logic I
Detection I 1 ~ ~ ~
~------H EEPROM ) Battery Current Sense I LCD Driver ) ( H t Vibrator ~ ______----______1 l 1 H Audio AmPlifier)
Li·lon 1 Charging Cradle ( Battery Pack I Control Detection All LEDs ) 1000mAh 3.3-4.2V
------Ch~~gi~g-C;,;d/~-i Ir------I I I I I I I I I I \--0 230V / 50Hz AC I I Charging 7.5VDC Power I I Or I I Circuit Adaptor I I J-o 110V / 60Hz AC I I I I I I I I ------.I I Figure 17: Parent Unit Power Management Scheme
43 Because the charging contacts are exposed to the exterior of the Parent Unit's plastic cases, inductors L1, inductor L2, zener diode DZ1, and zener diode DZ5 are placed close to the charging contracts to provide the needed ESD protection.
3.2.3.1 Cradle Detection Circuit
When the Parent Unit is placed on the charging cradle, the system software shows scrolling battery icon on the display as visual feedback. The detection circuit consists of a simply voltage divider and a decoupling capacitor. The voltage from the charging contacts is scaled down and fed into the system controller's build-in ADC. The system software then periodically measures the voltage level to determine if the Parent Unit is placed on the charging cradle.
3.2.3.2 Li-Ion Battery Charging Circuit
A charging management IC, Aimtron AT1547, is used for implementing Lithium Ion
(Li-Ion) battery charging. The AT1547 IC, along with an external PNP high gain bipolar transistor, provides all the necessary detection and control for three different modes of Li-Ion battery charging:
•A low current pre-charge mode that is designed to bring up the charges
within a fully discharged Li-Ion cell.
44 •A constant current charge mode is the main charging phase that fills up the
charge in the Li-Ion cell.
•A constant voltage charge mode that is designed to top-off a close to fully
charged Li-Ion cell.
The AT1547 IC provides charging status feedback, via the charging status output pin, to inform the system software if the charging has progressed to the constant voltage phase. In addition, the AT1547 IC continuously monitors the Li-Ion cell's temperature via the NTC thermistor inside the battery pack and inhibits charging current when the battery pack temperature is too high. This prevents the Li-Ion cell from overcharging which could lead to battery pack explosion.
3.2.3.3 Battery Current Sense Circuit
The current flowing in to and out of the battery pack is measured by the system software to estimate the remaining battery capacity and to control the battery capacity icon shown on the Parent Unit's display. The voltage drop across a
0.1 n resistor placed in series with the battery is amplified by the Low Offset
Amplifier (LOA) and measured by the ADC, both build-in to the system controller.
An external R-C low pass filter removes higher frequency noise before the signal is amplified by the LOA.
45 3.2.3.4 System Voltage Supplies
All system voltage supplies are derived from the battery voltage through three
LDO voltage regulators implemented with the voltage regulator control logic build-in to the system controller. As shown in Figure 17, each regulator requires an external bipolar transistor plus the necessary bias resistors. As discussed in
Section 3.1.2.4, these regulators rely on the feedback control logic implemented within the system controller to maintain fixed output voltage.
A two-stage configuration is used for the VDDIO regulator. An additional transistor switch, Q8, is connected between the Base of transistor Q7 and the regulator control signal from the system controller. This additional transistor switch ensures that the peripheral voltage supply, VDDIO, will not be switched on before the system controller's digital core voltage, VDDC, reaches proper voltage level. This prevents the peripheral devices from draining battery current before the system controller is properly powered on and running.
The battery voltage is then distributed, through these three voltage regulators, to
different system voltage supply domains:
• VDDA is the main analog voltage domain supplying all analog front end
circuits including the audio CODECs and is regulated to 2.5V by the VR1
LDO voltage regulator.
46 • VDDO supplies the Digitally Controlled Crystal Oscillator (DCXO) and is
connected to the 2.5V VDDA voltage supply.
• VDDINT supplies the analog-digital interface hardware of the system
processor and is connected to the 2.5V VDDA voltage supply.
• The voltage supplies for the RF Power Amplifier (PA) and the RF
transceiver are connected to the 2.5V VDDA voltage supply.
• VDDC supplies the digital core of the system controller and is regulated to
1.8V by the VR2 LDO voltage regulator.
• VDDIO supplies the digital 10 ports, all peripheral interfaces of the system
controller, the EEPROM memory, the audio amplifier, the LCD driver, and
all LEDs. VDDIO is regulated to 3.1V by the VR3 LDO voltage regulator.
• Because of its high peak current consumption, the vibrating motor is
supplied directly by the battery voltage.
3.2.4 Audio Processing Block
The audio processing circuits are identical in both the Parent Unit and the Baby
Unit. Therefore, the discussion on the microphone circuit and speaker amplifier circuit covered in Section 3.1.3 is not repeated again in this section.
3.2.5 Digital Interface and Control Block
Due to the similarity between the system controllers used in the Baby Unit and the
Parent Unit, the hardware implementations of the crystal oscillator circuit, the
47 EEPROM memory, the LCD and LCD driver circuit, and the ATE debug and programming interface are identical and have been covered in Section 3.1. The following sub-sections describe the hardware implementation for the vibration motor circuit, the LEOs and LED driver circuits, and the keypad input block.
3.2.5.1 Vibration Motor Circuit
An unbalanced-shaft cylindrical DC motor is used in the Parent Unit to generate vibration alarm in addition to the usual visual and audio alarms. When a DC current is applied to the motor, the rotation of the unbalanced shaft generates the desired vibration. Figure 18 below shows the vibration motor driver circuit.
MTl 1'r'5Z4TH380020021
D3
VOOIO t_------'R55/OK Q17 V'..;'v--.----'--j 0c017-40
Figure 18: Parent Unit Vibration Motor Driver Circuit
48 The system software controls the motor through a two-stage transistor switch.
Battery voltage is applied to the vibration motor only when both the system software drives the NIBR_ON signal to logic low and that the peripheral voltage supply, VOOIO, has reached proper voltage level. This prevents the vibration motor from draining the battery when the system controller is powered off either by user manual operation or by the system software itself due to insufficient battery voltage.
3.2.5.2 LEDs and LED Driver Circuits
Two pairs of red and green LEOs on the Parent Unit indicate the status of CO alarm and radio link, and 3 white LEOs provide backlight illumination for the LCO display. In addition, an array of 5 white LEOs at the top of the Parent Unit indicates the "volume" of the monitored audio that is played on the speaker.
The system controller turns on and off the LEOs via simple transistor switches driven by GPIO pins. The transistor switch allows the LEO's forward bias current, which is directly related to the optical brightness of the LEO output, to be higher than the current driving capability of the system controller's GPIO.
49 3.2.5.3 Keypad Input
There are 6 keys on the Baby Unit for the function of: switch on/off power, enter and exit UI menu, confirm selection in UI menu, activate talk-back mode, increase speaker volume, and decrease speaker volume. These 6 keys are tact switches connected to the system controller's Keyboard Scanner (KBS) port. Figure 19 below shows the KBS key matrix of the Parent Unit.
KBO l [1'2 SW_TAU KB1[1,2] SW_OK SW_ON/OFF 1 2 ..._----"-1-0--- 0--0=-2_--, TS-ll07GS-2 TS-35M-LF-3-T KB2 [1,2] SW_MENU 1 0---=2_--, TS-ll07GS-2 K63 [1,2] Figure 19: Parent Unit KBS Switch Matrix The system controller periodically scans through each one of the 4 KBS pins by configuring 1 pin as logic high output while keeping the remaining pins as logic input pins. With each scan pass, the system controller detects for short circuit 50 between any two KBS pins. Because only one switch is placed between any two KBS pins, the system software can then determine which key has been pressed. 3.2.6 Radio Block While the Baby Unit has two antennas and utilizes FAD, the Parent Unit contains only one antenna. Except for the absence of FAD, the Parent Unit's RF hardware is identical to the Baby Unit's implementation which is covered in Section 3.1.6. 3.3 Physical Implementation Considerations The Parent Unit's small form factor industrial design imposes many challenging constrains in the physical implementation. Table 2 below lists the dimensional comparison between the SCD499 Parent Unit (previous project launched in 2007) and the SCD540 Parent Unit (current project for 2008 launch). Table 2: Parent Unit Dimensional Comparison Between 2007 and 2008 Models Parent Unit 5(0499 Parent Unit 5(0540 Parent Unit % Ratio Dimension (2007 Model) (2008 Model) Overall Length lOSmm 86mm 82% Overall Width S7mm 49mm 86% Overall Thickness 30mm 23mm 77% 51 The SCD540's small form factor design requires 30% reduction in "length x width" area and 23% reduction in overall thickness as compared with last year's SCD499 model. Figure 19 below illustrates the Printed Circuit Board Assembly (PCBA) size comparison between the SCD499 and SCD540 models. SCD499 (2007) Parent Unit SCD540 (2008) Parent Unit E E Men "" 49.0 mm 43.6 mm Figure 20: Printed Circuit Board Assembly (PCBA) Dimensional Comparison As shown in Figure 20 above, the usable PCBA area in SCD540 Parent Unit has been reduced by 26% as compared to the Parent Unit from last year's model. In spite of the smaller size, the SCD540 actually packs in more hardware features such as the vibration motor and the dot-matrix display. In order to fit more electrical hardware into smaller and slimmer mechanical size, the electrical design needs to be physically implemented with higher component density, greater level 52 of integration, and smaller tolerance for design error. The following sections present the physical implementation considerations. 3.3.1 Integration of RF Circuitry with Baseband Main PCB Nearly all of the PNX8009 based products in the company utilize a common "RF Module" that contains the system controller, the crystal, the EEPROM IC, the LDO regulators, and the Radio Frequency (RF) circuits. Developing a standardized RF module offers the benefits of reduced development time, improved stability in RF performance, and streamlined manufacturing process when a large number of products utilize the same RF Module. However, in order to minimize the overall thickness of the Parent Unit, the RF circuitry is directly implemented on the same Printed Circuit Board (PCB) as all other baseband circuits. This integration requires careful PCB layout design to minimize interference and cross-talk. The voltage supply traces and return ground paths are branched out from the supply source with decoupling capacitors added at diverging nodes to minimize interference between different functional blocks through power supply. Sensitive signal, data, and audio traces are usually isolated to other circuitry with ground paths. In addition, all cross-over of signal traces are perpendicular to minimize cross-talk. 53 3.3.2 Utilization of Single 4-Layer Printed Circuit Board Instead of two 2-layer PCBs used in the previous SCD499 Parent Unit, a single 4-layer PCB is used in SCD540 Parent Unit to improve component and routing density, which reduces both the required PCB size and the overall thickness. Figure 21 shows the PCB layout comparison between SCD499 and SCD540. SCD499 Top Layer of 1't PCB SCD499 Bottom Layer of 1" PCB SCD540 Top Layer SCD540 2nd Layer SCD540 3'· Layer SCD540 Bottom Layer Figure 21: Printed Circuit Board Layout Implementation Comparison 54 3.3.3 Size Reduction of Electrical Components Instead of two AAA size NiMH batteries; a single Li-Ion battery pack is used in SCD540 Parent Unit to minimize overall thickness. The Li-Ion battery pack, although thinner, imposes stricter limitation on component heights underneath the battery pack. Therefore, miniaturized components are used whenever possible. For example, lower profile tantalum capacitors are used instead of aluminum electrolytic capacitors. Although the miniaturized components, in general, increases Bill of Material (BOM) cost, the cost saving in reduced PCB size compensates for part of the BOM cost increase. Furthermore, the higher BOM cost was anticipated in achieving the desired industrial design and thus has been considered in product cost budget. 55 4 BABY UNIT BATTERY CHARGING ALGORITHM The Baby Unit battery charging algorithm is designed to fit the charging and discharging characteristics of the NiMH rechargeable batteries while fully utilize the build-in hardware functionalities available on the system controller IC. As discussed in Section 3, the hardware implementation provides the adaptor presence status and battery voltage measurement input to the system software. Also, the hardware allows the system software to switch between two difference charging current settings. The larger current "quick-charge" is used for charging up the batteries and is usually set to 10% of the rated NiMH cell capacity. On the other hand, the smaller current "trickle-charge", usually set to 5% of the rated capacity, is intended to replenish and maintain the battery charge at full level. 4.1 Algorithm Limitations The voltage across an AA size NiMH battery cell, typically in the range of 1.0V to 1.5V, is one of the indications for the amount of charge remaining inside the battery cell. However, this voltage-capacity characteristic varies from cell to cell. In addition, the voltage across a NiMH battery cell also depends on the age of battery, the battery body temperature, and the amount of current flowing in to or out of the battery cell when the measurement is made. Nevertheless, in our application, the terminal voltage across a NiMH cell is the only physical input that 56 contains information about the remaining capacity inside the battery. Consequently, the accuracy is limited when estimating the remaining battery capacity. This is the reason behind implementing an estimation algorithm that is significantly more complicated than simple look-up tables. 4.2 Algorithm Design Concepts The objective of the software algorithm is to estimate the remaining battery capacity and to appropriately control the battery charging. The algorithm operates on both the battery terminal voltage measurement and an internal counter representing the remaining battery capacity; and processes the following items at fixed time intervals: o Measure and process battery terminal voltage. o Maintain a battery capacity counter representing the estimated numerical equivalence, in current-time unit, of the remaining battery capacity. o Reflect any battery current consumption to the battery capacity counter. o Reflect any charging current to the battery capacity counter. o Display battery icon according to the remaining battery capacity estimation. o Perform decision making on the charging control o Perform decision making on the system power on I off control 57 4.3 Battery Terminal Voltage Sampling The system software measures the battery terminal voltage via a a-bit ADC build-in to the system controller. The relationship between the numerical ADC output and the physical battery terminal voltage can be expressed by Eq12 and Eq13 below; where "VSAT' is the battery terminal voltage, and the "RafioResistOf_Divide;' is the external resistors' voltage divider ratio. The conversion reference voltage used in the ADC is 2.0V. v = ADC _Output+l. . 1 2V Eq 12 BAT 256 RatioReslstor Divider ADC _Output =(VBAT • RatioResislor_Divider . ~~J-l Eq 13 The software algorithm reads in the digital value at the ADC output every 10 sec for two purposes: (1) To monitor if the battery terminal voltage is too high as part of the battery presence detection discussed in Section 4.4. (2) To update an internal VBAT variable storing the processed battery terminal voltage. In order to prevent undesirable fluctuation in battery terminal voltage measurements (e.g. caused by noise), the VBATvariable is updated only in increments and decrements of 1. 58 Furthermore, during mass production, each unit is calibrated to compensate for hardware tolerance. The calibration result, stored in the EEPROM memory, is an offset in battery voltage measurement (VBATos) as shown in Eq 14 below. VBATvalue used in software algorithm =VBAT - VBATos Eq 14 4.4 Battery Presence Detection The battery presence detection is required for the system software to determine the charging status (i.e. the display should not indicate "charging in progress" if no battery has been installed). The detection is based on the fact that, with no battery installed, the voltage coming from the charging circuit is higher than the maximum achievable terminal voltage across the batteries. The system software periodically measures the battery voltage level, VBAT, and compares the reading with a threshold setting stored in the EEPROM memory. When 3 or more consecutive VBATvalues are higher than the threshold setting, the system software will reset the charging control algorithm. On the other hand, when 3 or more consecutive VBATvalues are lower than the threshold, the battery is considered to be present, and the system software restarts the charging control and battery counter algorithms. 59 4.5 Battery Capacity Counter The battery capacity counter is a 32-bit variable maintained by the algorithm to represent the remaining charge capacity inside the batteries. Since the algorithm updates the battery capacity counter at fixed one minute time interval, the unit assigned for the counter is the product of current (in mA), time (in minute), and a scale of 100 (representing 100%). Therefore, the maximum battery capacity that can be represented by the counter is OxFFFFFFFF = 4,294,967,295 =715,827 mAh as calculated in Eq 15 below. The maximum battery capacity counter value representing fully charged 2100 mAh NiHM batteries in the Baby Unit is 12,600,000 =OxC042CO as calculated in Eq 16 below. 1 4,294,967,295.(mA).(min).(%)x--x 1hr. =715,827mAh Eq 15 100% 60mm 60min . 2,100mAh x 100% x = 12,600,000mA . mm· % = OxC042CO Eq 16 1hr 4.5.1 Initial Battery Capacity Counter Estimation At system power up or when the batteries are installed, 3 consecutive measurements of VBAT are taken at ADC output, and the average of these 3 VBAT values is used to derive the initial approximation value for the battery capacity counter. 60 Each time the Baby Unit is powered off, the system software keeps a backup copy of the last battery capacity counter value and the last measured VBATvalue in the EEPROM memory. During system power up, the system software measures VBAT and compares it with the VBAT record in the EEPROM memory. If the difference is less than a predetermined threshold value, then the value in the EEPROM should be the best estimation of the remaining battery capacity. If the difference is significant, then it is likely that either the leakage current has significantly drained the batteries because the Baby Unit has been in power off state for a prolonged period of time, or user has replaced the batteries. In this case, the initial counter value will be estimated by mapping the measured VBAT value to predefined battery capacitor counter values via a lookup-table. Since NiMH batteries have significantly different terminal voltage characteristic during charging as compared to the characteristic during discharging, two separate lookup tables are used depending on weather the adapter is present when the Baby Unit powers up. As shown in Table 3 in the next page, each mapping table contains 4 entries, and each entry contains a VBATvalue and a corresponding counter value. The initial counter value is calculated by comparing the measured VBATvalue with the values listed in the mapping table. If the measured VBAT value falls between two entries in the mapping table, a linear interpolation is performed. Figure 22 in the next page shows the mapping with linear interpolation between entries. 61 Battery Voltage - Battery Capacity Counter Mapping 100% 90% ,? ~ ~ 80% ~... / , $ 70% s:::: ::J 0 60% / / () .a- '0 50% V I III - Q. III 40% / / () ~ / Q) 30% / ::: III ~ tIl 20% / ~ 10% f ~ ~ 0% - 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Terminal Voltage of 4 NiMH Batteries In Series ...... Oischarging ~Charging Figure 22: Battery Voltage - Battery Capacity Counter Mapping Table 3: Battery Voltage - Battery Capacity Counter Mapping VBATADC Voltage at Battery Capacity Capacity Output (Hex) ADC Input (V) Voltage (V) Counter (%) Counter Value 09 1.703 5.628 95% 11970000 a.o s:::: 00 1.633 5.396 80% 10080000 'Co... III .s::; C1 1.516 5.009 50% 6300000 enV o B3 1.406 4.647 15% 1890000 BO 1.383 4.570 0% 0 FO 1.883 6.222 75% 9450000 a.o E8 1.820 6.015 50% 6300000 .§s:::: 1.703 5.628 III 09 25% 3150000 .s::; U 02 1.648 5.447 15% 1890000 BO 1.383 4.570 0% 0 62 4.5.2 Battery Capacity Counter Update After the initial estimation upon system power on, the battery capacity counter is updated every minute to reflect the amount of current charged to or consumed from the batteries. Because the actual current flow in to or out of the battery is unknown, the algorithm relies on the knowledge of the current state of operation and the corresponding predefined average current. Table 4 below shows the average current for each of the operation states defined. Table 4: Average Current Charged/Consumed In Different System States Description Current (rnA) Current consumption - system in standby mode 112 Current consumption - system in monitoring mode 145 Additional current consumption - talk-back active 50 Additional current consumption - lullaby playback 85 Additional current consumption - nightlight turned on 40 Additional current consumption - LED turned on (each) 15 Additional current consumption - LCD backlight turned on 32 Charging current - quick charge mode 189 Charging current - trickle charge mode 95 The average current values in Table 4 are based on actual measurement data. The charging current values listed in Table 4 is about 90% of the actual charging current measured. This under-estimation of charging current takes into account that not 100% of current flowing into the batteries result in usable energy as a portion of the energy is transferred into heat in the process. 63 The system software determines the operating state of the system and adds or subtracts the corresponding current from the battery capacity counter every minute. For example, if the Baby Unit is powered by battery, operates in monitoring mode, playing lUllaby, and with 2 LEOs plus the nightlight turned on, the system software then subtracts 145 + 85 + 2 x 15 + 40 =300mA from the battery capacitor counter every minute. If the adaptor is plugged in and the batteries are now under quick charge, then instead of deducting 300mA from the counter, the system software adds 189mA to the counter every minute. This is because the hardware switch will prevent any current being consumed from the batteries during charging. 4.6 Battery Charging Control The system software switches the battery charging between a higher current quick charge and a lower current trickle charge depending on the battery capacity counter value and the measured battery voltage. In addition, four threshold and timing settings are defined and used in the charging control algorithm: • Quick Charge Stop Threshold (Vth_stop): represents the voltage threshold for which the batteries are considered to be fUlly charged. • Quick Charge Start Threshold (Vth_start): represents the voltage threshold for which the batteries are considered to be below fUlly charged level. 64 • Quick Charge Counter Threshold (Cth_start): represents the counter value for which the batteries are considered to be below fully charged level. • Quick Charge Timer: defines the maximum amount of time for the quick charge to continue once the battery capacity counter has reached its maximum value but the measured battery voltage remains under Vth_stop. Figure 23 below shows the flow chart for the control algorithm. NO YES IYES LY_ES IYES YES YES '----~------' Figure 23: Battery Charging Control Flow Chart 65 The system software switches the charging control from Trickle Charge to Quick Charge if either the measured VBAT falls below the Vth_start threshold or if the battery capacity counter value falls below the Cth_start threshold. On the other hand, the charging control is switched from Quick Charge to Trickle Charge if the measured VBAT is above the Vth_stop threshold and at the same time the battery capacity counter has reached its maximum value. This is because the battery should have been fully charged only if both conditions are met. However, the terminal voltage of fully charged NiMH battery does degrade as the battery cell ages (i.e. after the battery has been through many charge-discharge cycles). Therefore, it is possible that the battery terminal voltage might not reach the Vth_stop threshold even if the cell has already been fully charged. Furthermore, continuously over-charge NiMH cells causes the battery's internal pressure to build-up and could lead to explosion. To prevent the battery from excessive over-charging, a Quick Charge Timer sets the maximum quick charge duration after the battery capacity counter has reached its maximum value regardless of the measured battery terminal voltage. 66 4.7 Automatic System Power Off Under Low Battery Once the charge inside the NiMH cell is completely depleted, a much higher charging current (in the range of 0.5C to 1.0C) is required to bring the battery cell back up from depletion. This means that the 0.1 C (10% of the rated capacity) Quick Charge current will not be able to charge up NiMH battery that has been depleted. Therefore, the system software automatically powers off the Baby Unit when the battery is near empty to prevent the batteries from getting completely depleted. The system software monitors the measured VBAT value and shuts down the Baby Unit if the VBAT level falls below a Low Battery threshold. In addition, the system software checks the measured VBATvalue during system power up to ensure that the batteries have enough capacity left to at least allow the system to complete the power up sequence. If the measured VBAT is too low for the system to power up, the system software halts the power up sequence and continues to monitor the battery terminal voltage. 67 5 CARBON MONOXIDE DETECTION ALGORITHM The Carbon Monoxide (CO) detection implementation consists of a CO sensor, the hardware that conditions and amplifies the signal from the sensor, an ADC that converts the signal to digital value, and the software algorithm that processes the ADC measurement and controls the visual (flashing LED) and audio (speaker alarm) outputs. Figure 24 below shows the system-level block diagram of the CO detection implementation. Signal Conditioning System Controller o Figure 24: CO Detection Implementation Block Diagram While the hardware implementations are presented in Section 3.1, the following Sections present the CO detection requirements, the CO concentration measurement and calibration method, and the alarming condition calculation algorithm which determines the necessary condition to activate or deactivate the visual and audio alarm. 68 5.1 CO Detection Requirements The EN 50291 standard, defining the test methods and performance requirements for CO detection electrical apparatus in domestic premises, is used as the reference requirement for the CO detection and alarming conditions. Table 5 below lists the CO alarm conditions as outlined in EN 50291: Table 5: CO Alarm Conditions Outlined in EN50291 Ambient CO Test CO Gas No Alarm Alarm Reference Concentration Volume Ratio Before Before A 30 ppm 33 ppm ± 3 ppm 120 minutes B 50 ppm 55ppm ± 5 ppm 60 minutes 90 minutes C 100 ppm 110 ppm ± 10 ppm 10 minutes 40 minutes 0 300 ppm 330 ppm ± 30 ppm 3 minutes 5.2 CO Concentration Measurement Calculation The 8-bit ADC build-in to the system controller is used for measuring amplifier output signal. Eq 17 below lists the conversion equation. ADC Output = VAIX (V)· 256 -1 = V (V). 128 -1 Eq 17 - V AIX 2V ref From Eq 10, Eq 11, and Eq 17, the ambient CO concentration in parts per million (ppm) can be expressed in terms of the sensor sensitivity and the ADC output value as shown in Eq 18 in the next page. 69 CO(ppm) = ADC_ Output + 1 128· Sensitivity(nAI ppm) .10-9 ~. Rl(Q)· (l + R3)(V) nA R4 V Eq 18 =7.1808.10-2 ADC Output +1 Sensitivity(nAI ppm) The sensitivity value, marked on the sensor's barcode label, is scanned and stored into Baby Unit's EEPROM memory during mass production. 5.3 CO Concentration Measurement Calibration Equation 18 in the previous section describes the theoretical and ideal relationship between the measured ADC output value and the ambient CO concentration. However, there are several non-ideal factors that need to be catered in the calculation. The main non-ideal factors include: • Variation in CO sensor's sensitivity and leakage current • Variation in resistors' resistance value • Variation in amplifier characteristics • ADC Quantization error • Temperature dependency of CO sensor's sensitivity • Humidity dependency of CO sensor's sensitivity The following sub-sections analyze the error introduced by these non-ideal factors and describe the compensation method implemented. 70 5.3.1 CO Sensor Variation and Resistor Tolerance The error in ambient CO concentration calculation as a result of the variation in CO sensor characteristics and resistor tolerance is identified to be: • CO sensor's leakage current causes a baseline offset in sensor's output current. The specified baseline offset tolerance is ± 15ppm. • Variation in CO sensor's sensitivity causes proportional error in the calculated CO concentration. The specified sensitivity tolerance is ± 20%. • The resistor's 1% resistance tolerance causes a proportional error in the amplifier gain, and thus a proportional error in the calculated CO concentration. The worst case error is +(1 - 1 / (0.99 x 0.99/1.01)) or -(1 - 1 / (1.01 x 1.01 /0.99)) = ± 3%. Therefore, the overall error in calculated CO concentration caused by sensor variation and resistor tolerance is ± 15ppm plus ± 23%. Since the EN50291 standard requires the detection to accurately differentiate between the 33ppm ± 3ppm reference A test CO gas and the 55ppm ± 5ppm reference B test CO gas, production calibration is required to reduce detection tolerance. These errors are addressed by introducing a scaling factor and an offset term to the ambient CO concentration calculation as shown in Eq 19 in the next page. 71 , (-2--.-.=.'-----~--ADC Output + 1 J CO(ppm) = COOjf,et +COSca/e ' 7.1808·10 Eq 19 Sensltlvlty(nA / ppm) In Eq 19, the COOffset offset compensates for the leakage current (Le. baseline offset) of the CO sensor, while the COScale multiplication factor compensates for the tolerance in CO sensor's sensitivity and resistor's resistance. In mass production, Baby Units are subjected to two different ambient CO concentration levels inside a sealed chamber, and the system software's CO concentration measurement is recorded. The manufacturing ATE program can then take these two measurement values and solve for the COOffset and COSca1e parameters. However, this calibration method might not always be practical considering the increased production cost in utilizing sealed gas chamber in production site. Nevertheless, this calibration step is required at least during the initial mass production ramp up in order to ensure that the product meets the design target. The calibration data collected from the initial mass production can then be analyzed to determine if the calibration method can be further simplified to achieve production cost reduction. 5.3.2 Amplifier's Non-ideal Characteristics The main non-ideal characteristics of the Operational Amplifier in our application include input offset voltage, input bias current, and input offset current. Since the 72 contribution of these non-ideal characteristics results in an offsets at the amplifier's output voltage, an ADC output offset value is added for compensation. The calibration implementation is to short-circuit the sensor terminals and record the zero-input ADC value measured by the system software. Eq 20 below shows the CO concentration calculation equation with the ADC offset term. ADC - Output + ADCOffset + 1 CO(ppm),=COoff,et + COScale .(-27.1808·10 ----.-.-.----=--J Eq 20 SensltlVlty(nA / ppm) 5.3.3 ADC Quantization Error Since the 8-bit analog to digital conversion is based on a 2.0V reference voltage, each quantized step at output corresponds to 2.0V/256 = 7.8125mV error voltage at input. Considering the amplifier gain of 561, each quantization error step then corresponds to 7.8125mV/561 = 13.931JV error across the sensor terminals. Therefore, the worse case ADC quantization error, expressed in CO concentration (ppm), is calculated to be 13.93ppm as shown in Eq 21. Note that the minimum sensitivity of 1 nAlppm is used for this worst case scenario calculation. ~~; 5~1 (~llO~on 10 ~l 0 0 '( (InA r13093Ppm Eq 21 1.0 ppm 73 Since the ADC quantization error is inherent to the AID converter and limited by the finite 8-bit resolution, no practical compensation method can be applied to further improve the measurement accuracy. 5.3.4 Temperature Dependency Because the CO sensor's sensitivity is temperature dependent, a compensation scale factor, (1/CF), is applied to the sensor's sensitivity in the CO concentration calculation as shown in Eq 21 below: I [_2 ADC _ Output + ADCOjjlet + 1] CO(ppm) = COO/ftet + COSeate ' 7.1808 ·10 Eq 22 Sensitivity(nA / ppm)' _1_ CF The CF data at different sensor body temperature is available from sensor manufacturer, Figaro, and implemented in the algorithm as a lookup table. Figure 25 in the next page shows the temperature dependency of the CO sensor's sensitivity from -40°C to +70°C. In order to obtain accurate compensation factor, temperature readout from the SHT10 humidity sensor, which is located in close proximity to the CO sensor, is used to lookup the corresponding CF value. 74 Temperatuer Compensation for Sensor Sensitivity 1.20 ~ 1.10 ,/' 1.00 / ~ ~ :s / /' ~ 0.90 ...0 U nl /' LL c: 0.80 / 0 ..nl / III c: Ql 0.70 / Q. E / 0 U 0.60 / / 0.50 / / 0.40 -40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 CO Sensor Body Temperature Figure 25: Sensor Sensitivity Compensation for Temperature Dependency 5.3.5 Humidity Dependency Similar to the temperature dependency, the CO Sensor's sensitivity is also humidity dependent, but to a less extend. Based on the sensor specification, the overall change in sensitivity is less than 3% for a change of relative humidity from 20% to 100%. Because of the relatively weak dependency and the fact that no compensation data is readily available from the sensor manufacture, no compensation method is implemented to address the variations in sensor's sensitivity due to change in ambient humidity level. 75 5.4 CO Alarm Control Algorithm The CO alarm control algorithm is designed based on the requirements outlined in EN50291 which takes into account both the ambient CO concentration level and the exposure time. Based on the alarm conditions outlined in EN50291 (see Table 5 in Section 5.1), the CO concentration level and exposure time can be summarized into four different ranges as listed in Table 6 below. Table 6: CO Alarm Conditions Derived from EN50291 Constant CO Concentration Range Exposure Time Required To Trigger Alarm 1 oto 3 min 300 ppm 2 10 min to 40 min 100 ppm 3 60 min to 90 min 50 ppm 4 > 120 min 30 ppm These requirements essentially define the "safety threshold" in terms of both the exposure duration and the CO concentration level that the human body is exposed to. This implies that the risk level of shorter exposure to higher CO concentration should be considered to be the similar to the risk level of longer exposure to lower CO concentration. Therefore, it is logical to interpret the above requirements in terms of the accumulative CO concentration - the product of CO concentration and exposure time. Thus the four ranges listed in Table 6 can be rewritten into an upper bond for activating the CO alarm and a lower bond for deactivating the CO alarm shown in Table 7 in the next page. 76 Table 7: CO Alarm Conditions In Terms OfAccumulative CO Concentration .. Accumulative CO Concentration (ppm-min) Time (min) Upper Bond - Triggers Alarm Lower Bond - Resets Alarm oto 3 900 N/A 10 to 40 4,000 1,000 60 to 90 4,500 3,000 > 120 N/A 3,600 The algorithm is designed to periodically calculate and compare two variables: an accumulative CO concentration level (a) and an alarm threshold as a function of time (13). The accumulative CO concentration level, a, represents the product of CO concentration and exposure time. The time-varying alarm threshold, r3, is the decision boundary between the upper and lower bonds derived from the EN50291 requirements. The CO alarm is triggered if a > r3 and deactivated if a < r3. To derive a single threshold from the upper and lower bonds, the logarithmic mean shown in Eq 22 below is used to calculate the mid-point time. In addition, to make the decision threshold "smooth" and properly centered between the upper and lower bonds, the 4 non-consecutive ranges given in Table 7 are interpolated into 6 consecutive piecewise-linear threshold lines as listed in Table 8 in the next page. (-( Logarithmic _ Mean = (2) I()) Eq 23 (In (2 -In (, 77 Table 8: Piecewise Linear CO Alarm Threshold (13) • ° Threshold End Points y(t) = Threshold Slope Time (min) (ppm-min) (ppm-min/min) 1-3 199-597 199 4 - 22 682 - 2212 85 23 -43 2261 - 3241 49 44 -74 3257 - 3737 16 75 - 120 3752 - 4427 15 > 120 4427 0 Accumulative CO Concentration - Alarm Triggering Threshold _10,000 c ). °e 9,000 c.E 8,000 I c. ~ ~ ~ 7,000 o ~ .. 6,000 f! V I\i U ~T R GC EF A AI M AI 0'IE UP PE ~B or 0 ~ -; 5,000 (.) .J c o 4,000 - u ~ ". l...,,;oo o 3,000 ~ u ~ ."",. ~~~ ."",. ~ 2,000 ~ - ~ ------.. ~~~ -r"u ST NC T RI G<: ER AI AF M BE LO WLOWI R ~O~[ .; 1,000 iii """" E ~ :::::l 0 J (.) ~ 0 20 40 60 80 100 120 Time (min) _ppm-min Lower Bond _ppm-min Upper Bond -Decision Threshold Figure 26: Calculated CO Alarm Threshold (13) Figure 26 above shows the upper bond of which the alarm must be triggered, the lower bond of which the alarm must not be triggered, and the calculated decision 78 threshold for activating and deactivating the CO alarm. As shown in the figure, the calculated alarm threshold is positioned within the upper and lower bonds. Also note that the decision threshold, l3(t) is a function of time with a limiting range of [0, 120]. This finite time scale allows practical algorithm implementation. The software algorithm takes an ADC sample to calculate CO concentration level every 6 seconds. Every minute, an averaged value, x, is calculated from the past 10 samples. In addition, the variables 0,13, and time t are updated every minute according to the following condition: • If the average CO reading, x, is larger than or equal to 30 ppm, it is considered that significant CO concentration is detected, and: )0> Increment time t =t + 1 unless t =120 )0> Update 0 =0 + x • If the average CO reading, x, is smaller than 30 ppm, it is considered that no significant CO concentration is detected, and )0> Decrement t =t - 1 unless t =0 )0> Update 0 = 0 - 2y, where y(t) is the slope at time t defined in Table 6 • Re-calculate l3(t) with the new time t based on the piecewise-linear threshold lines defined in Table 6 The algorithm continuously updates 0 to reflect the increase or decrease in accumulative CO concentration and moves the time-dependent variable l3(t) along the time scale t. Since EN50291 defines that less than 30 ppm in CO concentration should not trigger the alarm, the value of 30 ppm is used in the 79 algorithm to determine if the accumulation should advance or retreat. Therefore, the algorithm essentially monitors both the accumulative exposure to CO and the accumulative "recovery" after the exposure. Furthermore, two limiting conditions are implemented in the algorithm to ensure that the algorithm functions properly even in exceptional conditions. First, if the CO concentration reading is lower than 30ppm for 100 consecutive samples (= 10 minutes), a will be set to the value of (3(t). This limiting condition is implemented to ensure that the alarm will be turned off within 10 minutes after the CO concentration level has dropped to a relatively safe level regardless of the CO concentration level that has triggered the alarm. One of the exceptional cases for this limiting condition would be that the Baby Unit has been exposed to high CO concentration for a prolonged period of time, thus a can increase to a very high value. Once the source of CO is removed, it will take a very long time for a to drop down. Therefore, this limiting condition ensures that the alarm will be turned off once the source of CO is removed for more than 10 minutes. Second, if the measured CO concentration level is larger than 200 ppm at any time, a will also be set to the value of (3. This is done to ensure that the alarm will be triggered quickly whenever consecutive high CO concentration levels are detected. 80 6 CONCLUSION The Philips DAP SCD51 0-540 DECT Digital Wireless Baby Monitors were designed and implemented to fulfill the small form factor industrial design and customer's feature and performance requirements. The project started with feasibility study based on customer requirements, progressed with critical component selection, baseband circuitry design, RF circuitry integration, PCB layout implementation, mechanical structure fitting, software development, and finished with regulatory approval submission followed by pilot production. High level block diagrams for both the Baby Unit and Parent Unit were presented to outline the functional blocks in the baseband hardware design. The hardware circuitry design and implementation considerations for each functional block were discussed in detail. The PCB layout implementation of the design was completed and verified although it is not covered in this project report. The charging algorithm design and implementation concept for the Baby Unit's Nicole Metal-Hydride (NiMH) rechargeable batteries were discussed in detail. The algorithm periodically calculates and maintains a battery capacity counter that represents the remaining battery capacity. Based on this battery capacity counter, the algorithm then determines the proper battery charging control and appropriate icon display for the battery status. 81 Since the Carbon Monoxide (CO) detection was a brand new feature for the Baby Monitor products, significant amount of research and development effort were spend on the requirement study and the design of the CO detection hardware and software. The CO sensor's principle of operation, the detection hardware design, and the detection tolerance were discussed in detail. Sources of error in the CO concentration measurements and the corresponding compensation methods were presented. The CO detection and alarm control algorithm was designed based on the interpretations of the requirements outlined in EN50291. Although not all four of the Baby Monitor products have completed the regulatory approval process and entered mass production stage due to the exceptional financial circumstances of the company, performance validation at the system level has been conducted as part of the product development cycle. At functional block level, the baseband hardware design, the charging algorithm, and the sensor detection algorithm has been tested and verified for the most part based on the pilot production. In addition, the design's manufacturability and reliability have been evaluated and validated for the most part during the pilot production. 82 7 REFERENCES [1] Guo, K. "B252-PS01-X-VY-PE: Product Specification", Ascalade Technologies Inc. December 2007. [2] Yao, C. "B252-PS02-X-VY-PE: Production Technical Specification", Ascalade Technologies Inc. January 2008. [3] Guo, K. "B252-PS04-X-VY-PE: Functional Specification", Ascalade Technologies Inc. August 2007. [4] Yao, C. 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