ECE 477 Digital Systems Senior Design Project Fall 2007

Homework 6: Printed Circuit Board Layout Design Narrative Due: Friday, October 5, at NOON

Team Code Name: wifiMote______Group No. 7 Team Member Completing This Homework: Anish N. Prithviraj e-mail Address of Team Member: aprithvi @ purdue.edu

NOTE: This is the third in a series of four “design component” homework assignments, each of which is to be completed by one team member. The completed homework will count for 20% of the individual component of the team member’s grade. The body of the report should be 3-5 pages, not including this cover sheet, references, attachments or appendices.

Evaluation:

SCORE DESCRIPTION Excellent – among the best papers submitted for this assignment. Very few 10 corrections needed for version submitted in Final Report. Very good – all requirements aptly met. Minor additions/corrections needed for 9 version submitted in Final Report. Good – all requirements considered and addressed. Several noteworthy 8 additions/corrections needed for version submitted in Final Report. Average – all requirements basically met, but some revisions in content should 7 be made for the version submitted in the Final Report. Marginal – all requirements met at a nominal level. Significant revisions in 6 content should be made for the version submitted in the Final Report. Below the passing threshold – major revisions required to meet report * requirements at a nominal level. Revise and resubmit. * Resubmissions are due within one week of the date of return, and will be awarded a score of “6” provided all report requirements have been met at a nominal level.

Comments: Comments from the grader will be inserted here. ECE 477 Digital Systems Senior Design Project Fall 2007

1.0 Introduction The main purpose of this embedded project is to successfully design and build a wireless TV remote. This remote will be equipped with an onboard Touch Screen LCD with the capability of displaying video and accepting touch input. The purpose of the screen is to display live video from TV channels other than what is being viewed on the TV. This allows the user to “channel surf” on his/her remote rather than on the TV. The desired channel can then viewed on the TV with the touch of a button. There are many PCB considerations that need to be taken into account to successfully create a design that performs efficiently. The analog circuits, digital circuits and power management circuitry need to be isolated from each other to ensure stable operation of the different parts. The power management system will be driven by six AA rechargeable NiMH batteries with a recharging interface. Special considerations need to be made for power management system to supply various voltages for the different components.

2.0 PCB Layout Design Considerations – Overall One of the major considerations that need to be taken for the PCB layout is the separation of the Analog, Digital and Power circuits. This will help in minimizing Electromagnetic Interference (EMI) [6]. The batteries will supply a voltage of 7.2V. This voltage needs to be converted to -11, 2.5, 5 and 14V lines. The -10V and 15V will be created using a charge pump. Special care needs to be taken to minimize the large amounts of potential EMI caused by the charge-pump circuitry. The traces used for power management circuits will have to be larger than those used for the remaining circuitry [6]. The power circuitry will be placed at the bottom of the circuit to allow easy access to the batteries. The power management circuits will also consist of two additional IC’s for recharging the NiMH batteries and to monitor the status of the batteries. The battery status monitor will be placed at the edge of the power management section to allow easy access to the microcontroller. The battery monitor also needs to be placed as far away as possible from the switching circuitry as its input is very sensitive and interference needs to be minimized [5]. The analog portion of the PCB layout will consist of the 2.4GHz Composite video receiver and the analog to digital converter. This will be placed towards the top right corner of the PCB and isolated to minimize EMI from the digital circuits. Also, it makes sense to place the receiver

-2- ECE 477 Digital Systems Senior Design Project Fall 2007 at the top of the PCB in order to allow easy access to an antenna on the circuit. The antenna being used is the ANT-2.4-JJB by Antenna Factor [9]. This antenna is small in size and can be mounted on the PCB. This antenna also needs to be facing away from the digital circuitry and the power circuit to avoid picking up unwanted EMI. The digital portion of the PCB layout will consist of the microcontroller and the touch controller. The maximum amount of current would be drawn by the microcontroller. The microcontroller can draw up to 200 mA [1] which corresponds to a required trace width of 0.046 mils. Similarly trace widths required by some of the other major components are 0.05 mils for the LCD (40 mA), 0.0306 mils for the Battery charging IC (28 mA) and 0.0101 mils for the charge pump circuit (12.5 mA). This safely allows us to use the recommended trace widths of 8 to 10 mils. All the trace width calculations were performed with the help of a PCB trace width calculator [7]. Being a handheld device, it is important that the PCB be made as small as possible with minimum loss of stability to keep the design feasible. At the same time, a lot of ICs need to be placed in the limited space in order to create a working design. It is imperative that during this extreme optimization for size, the functionality of particular components is not lost when they are soldered onto the board. This is one of the major manufacturing concerns of our design.

3.0 PCB Layout Design Considerations – Microcontroller The microcontroller requires bypass capacitors for the I/O pins in the event of an external clock being used [1]. Since the provided internal clock is being utilized, these capacitors would not be required. The frequency of the internal clock of the microcontroller is 7.37MHz. When the internal system clock is being used, the clock frequency is routed to OSC1 and OSC2 pins on the chip. The microcontroller comes with two internal oscillators, an FRC oscillator and a LPRC oscillator. For our design, we will be using the FRC oscillator with an on chip programmable frequency divider which will allow us to maximize the clock speed. Figure 1 below shows a simplified version of the onboard oscillator system in the microcontroller.

-3- ECE 477 Digital Systems Senior Design Project Fall 2007

Fig. 1 – PIC24H series Oscillator System Diagram[1]

The microcontroller requires a bypass capacitor of 10uF running from the VDD to the VSS pin. This capacitor serves the purpose of stabilizing the performance of the on-chip voltage regulator. The microcontroller can run at a nominal voltage of 2.5V but is provided with the voltage regulator to run at a typical voltage of around 3.3V [1]. Owing to the availability of spare I/O pins on our microcontroller, these will be routed to headers on the PCB in order to help us debug our design during testing and make room for any aspects of our design that might have been missed before the submission of out PCB layout.

4.0 PCB Layout Design Considerations - Power Supply The power supply portion of the wifiMote is one of the more complex pieces of the design. The power supply is required to generate voltages of 5V [4][2], 1.8V (digital and analog) [8],

3.3V [1][2] [3], -10V and 15V [2]. The incoming voltage supply for the device is six AA NiMH rechargeable batteries supplying a total of 7.2V. A charge pump is required to supply voltages of -10 and 15 within the board to drive different modules in the Touch Screen LCD. The bypass capacitors for the PCB layout need to be placed as close to the various ICs as possible [6]. Being a microcontroller based design, not more than one bulk capacitor would be required. This is because bulk capacitors can generally charge anywhere between fifteen and twenty IC’s [6]. Since the wifiMote design has less than fifteen ICs only one capacitor would be required. The design will have a well thought out ground system although there is not much room for a ground plane. Ideally, there would be a number of parallel traces leading to ground along the PCB. This ground plane would help minimize unwanted inductance created. Instead, a single point ground system will be used where all the different grounds will be traced to this point. Separate analog and digital ground points will be used. The entire power management circuitry will have to be as compact as possible for two reasons. The first reason being, to maximize the gap from the power system to the rest of the layout in order to minimize interference caused by the noise from switching circuits. The second reason is to minimize the amount of space used by the larger traces for the power lines. Placing the components together would give us the advantage of extremely short traces between the power ICs.

5.0 Summary This document talks about the major considerations of creating the PCB layout for the wifiMote design. The thought process behind the placement of different parts on the PCB layout has been visited in overall design consideration section. The reasons for having separate portions for analog, digital and power circuits have also been mentioned in the same section. Considerations made for signal routing, trace sizing and EMI minimization have also been stated. The Microcontroller design consideration section talks about the placement of bypass capacitors, tracing of power and ground to the microcontroller and the layout for the internal oscillator of the microcontroller. The power supply section talks about the placement of the

-5- ECE 477 Digital Systems Senior Design Project Fall 2007 passive components with respect to the power circuits and the placement of bypass capacitors. Trace routing and ground plane considerations have also been stated. The screen shot of a highly preliminary layout of our footprint can be seen below in Fig. 2. This layout includes most of our major components and the circuitry involved with them. Currently, the team is still in the process of completing footprints of some of our ordered components.

Figure 2 – Preliminary PCB layout

List of References

[1] Microchip Technology, “PIC24HJ256GP210”, microchip.com, [Online]. Available: http://ww1.microchip.com/downloads/en/DeviceDoc/70175F.pdf. [Accessed Oct. 5, 2007].

[2] Microtips Technology, “MTF-T035DHSLP-A”, microtipsusa.com, [Online]. Available: http://www.microtipsusa.com/product_pdfs/Color%20TFT/MTF-T035DHSLP-A %20(24BIT,%20parallel,%20external%20BL%20control,%20anti-glare, %20TP).pdf. [Accessed Oct. 5, 2007].

[3] Texas Instruments, “ADS7843”, focus.ti.com, [Online]. Available: http://focus.ti.com/docs/prod/folders/print/ads7843.html. [Accessed Oct. 5, 2007].

[4] Airwave, “AWM634RX”, saelig.com, [Online]. Available: http://www.active- robots.com/products/radio-solutions/radio-modules/datasheets/awm634.pdf [Accessed Oct. 5, 2007].

[5] Dallas Semiconductor, “DS2715”, maxim-ic.com, [Online]. Available: http://datasheets.maxim-ic.com/en/ds/DS2715.pdf. [Accessed Sept. 28, 2007].

[6] Freescale. “Motorola App Note AN1259”, freescale.com, [Online]. Available: http:/www.freescale.com/files/microcontrollers/doc/app_note/AN1259.pdf%20. [Accessed Oct. 5, 2007].

[7] CircuitCalculator.com. “The CircuitCalculator.com Blog”, circuitcalculator.com, [Online]. Available: http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width- calculator/. [Accessed Oct. 5, 2007].

[8] Texas Instruments, “TVP5150A”, focus.ti.com, [Online]. Available: http:// focus.ti.com/pdfs/vf/vidimg/tvp5150_pb.pdf. [Accessed Oct. 5, 2007].

[9] Antenna Factor, “ANT-2.4-JJB”, focus.ti.com, [Online]. Available: http://www.antennafactor.com/documents/ANT-2.4-JJB-xx_Data_Sheet.pdf. [Accessed Oct. 5, 2007].