Science, Technology and Development ISSN : 0950-0707

SoC UNIT FOR IOT APPLICATIONS

Annesha Dasgupta*, Manasa Potta*, Vaishaka N. Raj*, Meghana D. R*, Dr. Kendaganna Swamy S+ *UG Students, {name}[email protected] +Assistant Professor, {[email protected]}

Department of Electronics & Instrumentation RV College of Engineering, Bangalore, India

ABSTRACT Power management in IoT devices is a challenging task because these devices are powered on constantly and often, they are located remotely and must manage the power efficiently. Thus, managing power is one among the chief factors to be considered while developing a custom SOC. Considering the importance of power management in IOT applications, the authors have proposed a design of a power management unit that can be used in SOCs to manage power efficiently. The system consists of LDO, DC-DC Converters and Voltage reference in the design. Power management unit is a flexible system that can be integrated in any SOC’s for power management and additional blocks can be added to improve the power performance of SOC.

Keywords: Power Management Unit, Low Power Regulator, DC-DC Converter, Converter, Serial Interface, Voltage Reference, IoT

1. Introduction

Internet of Things (IoT) refers to the billions of devices that are embedded with multiple sensors and other top end electronics for the main purpose of automating tasks via the Internet. IoT devices are expected to grow at a lightning speed to a mass 1.3 trillion dollars by the end of this decade. This impact is because IoT has found use in daily objects; smart fridge, smart TV, smart homes etc. [1] Many of these new IoT applications will need an implementation on a single chip known as System on Chip (SoC). As there is a huge demand in the industry, many companies begin to face challenges with the SoC development.[2] Power consumption in IoT is an important topic, primarily due its portable nature and the requirements of the device to function for a longer period of time without being charged. There are three factors that propel the need for power management; the complexity of the System on chip, mobility and process technology. To increase the efficiency of a power management system in an IoT device it is vital to reduce the power usage and area of each segment [3]. Fig 1 shows how SoC chips are used for IoT Applications. And for the chip to have high efficiency and have better performance in the application used it is important to manage the power in the SoC.

Volume X Issue VI JUNE 2021 Page No : 169 Science, Technology and Development ISSN : 0950-0707

Fig. 1 SoC Chip for IoT Application

This paper is organized as follows: Section 2 describes the Power Management Unit, Section 3 briefly describes Low Power Regulators, section 4 describes DC- DC Converters, section 5 describes Voltage References, section 6 describes Serial Interfaces and the paper is concluded in section 7.

2. Power Management Unit The power management unit (PMU) is made up of several blocks for governing the power functions in IoT Devices. Fig. 2 gives the block diagram of the power management unit. The PMU consists of LDO used for supplying various voltage level rails used in SoC [3], DC-DC Converter to withstand high voltage drop in the SOC, battery chargers to provide internal power supply and voltage reference is used as external power supply, clock generator is used for generating timing sequence for synchronization, and the overall communication within the unit takes place through serial interface.[4]

Fig. 2 PMU Block Diagram

Volume X Issue VI JUNE 2021 Page No : 170 Science, Technology and Development ISSN : 0950-0707

3. Low Power Regulator

A low dropout regulator is essentially a voltage regulator, which is essential in providing a balanced power voltage which is not dependent on load impedance. These LDO regulators are known for their capacity to maintain a control with negligible difference between the voltage supplied and load voltage [5].

With an increase in IoT enabled devices, has propelled an increase in such devices being portable and requiring a stable voltage supplied regardless of the state of the battery. An IoT device works in a discontinuous manner, with its respective ON and OFF duty cycling, during the sleep mode, power is saved and, in this state, the LDO is not in force but because of its unique characteristics is able to maintain a constant output voltage [4]. There is only quiescent current that flows through the LDO in the sleeping state but it is done to preserve the internal circuitry of the device [6].

An LDO structure resembles that of a linear regulator but has replaced the N type MOSFET with a P type MOSFET, thereby becoming a current source [7]. LDO regulators require the application of a capacitor, it is primarily done for two reasons; a capacitor aids in attaining loop stability because LDO follows a closed loop design. The other reason being, a capacitor assists the regulator in maintaining better noise performance.

The newer generation LDO regulators are highly efficient with a lower magnitude of dropout voltage which makes it more reliable to be used in IoT devices. Initially analog regulators were heavily used, but because of a lack of adequate gain at lower voltage and poor noise parameters, there has been significant research in digital LDO regulators [7]. Therefore, digital LDO regulators are preferred over analog, because of its higher stability and having significantly lesser area requirements. In other words, the newer LDO regulators have a lower quiescent current, better transient response and a more stabilized load loop gain [8].

Therefore, with a more substantial presence of IoT, it has become imperative to design systems that have higher efficiency in both sleep and active state and the newer LDO regulators are making it possible by bridging the gap in efficiencies and simultaneously improving the noise parameters. Moreover, because of its cost effectiveness and good area considerations it is considered strongly for IoT applications [9].

Volume X Issue VI JUNE 2021 Page No : 171 Science, Technology and Development ISSN : 0950-0707

4. DC - DC Converter

DC to DC Converters is a type of SMPS (switched mode power supply) that stores the input DC supply and outputs the stored energy at different voltages [10]. They are made up of two semiconductors (diode and transistor) or two transistors, as well as two storage devices (capacitor and inductor) [11]. When compared to traditional voltage regulators, they are noted for their high efficiency, which reach nearly 95% as their switching conversion is much more efficient than linear voltage regulators which dissipates heat.[12] Standard inductor-based dc-to-dc converters, which require an external inductor and capacitor to create the dc output level, and switched- capacitor dc-to-dc converters, which can be implemented with on-die capacitors but cannot supply as much current as inductor-based solutions, are the two types of dc-to-dc converter technologies.[13]

DC to DC Converters is mainly of three types:

Buck Converters: As the name suggests, buck converters/step down converter is a DC-DC converter used to step down voltage i.e., the output voltage is much lower than the input voltage [14].

Boost Converters: These converters are mainly used to step the voltage. i.e., the output voltage is much higher than the input voltage [15].

Buck Boost Converters: In these converters, the voltage output can be either increased or decreased compared to voltage input., i.e., it can act as both buck or boost converter. This type of switching mechanism of reversing the polarity is mostly used in battery applications [16].

5. Voltage Reference

Voltage reference is used to provide a continuous voltage supply to the SoC in spite of loads in the circuit, varying physical parameters and varying time [18]. They provide supply to various power management unit blocks like comparators, LDOs and other analog functions. In addition, it makes sure that the system is consuming low power and provides a good power supply rejection ratio.[19]

Volume X Issue VI JUNE 2021 Page No : 172 Science, Technology and Development ISSN : 0950-0707

6. Serial Interface

Serial interface is used to communicate within a system effectively. Advantages of serial interface is communication can be performed with lower number of pins and most of the embedded systems support serial interface. It can also communicate without the use of shared memory and semaphores, hence eliminating the problems caused by them. Thus, in a power management unit serial interface is used by the microcontroller/ to control the logic state machines. The commonly used serial communication are I2C, SPI and UART [20].

6.1. I2C I2C which translates to Inter- is a serial, half-duplex, synchronous, short distance communication protocol. I2C uses only 2 pins for communication, i.e., SCL and SDA. The clock signal is sent through SCL for synchronization and data is sent through the SDA line. I2C supports multi-master and multi-slave to communicate with each other. I2C uses slave addressable protocol where the master sends a 7-bit address to access a slave. And when the slave acknowledges the master, communication is established [21].

Fig 3. I2C Master-Slave Connection

6.2. SPI Serial Peripheral Interface (SPI), is a full-duplex, serial, synchronous communication protocol which uses 4 pins for data transmission. SPI is used where extremely high-speed data transmission is required. SPI supports a single master to communicate with multiple slaves. The selection of slaves by the master takes place through pulling the active low chip select (CS) pin from high to low. The master contains multiple CS pins which are connected to each individual slave, whichever CS is asserted that respective slave responds and transmission is initiated [22].

Volume X Issue VI JUNE 2021 Page No : 173 Science, Technology and Development ISSN : 0950-0707

Fig 4. SPI Master slave Connection

Pin descriptions: MISO - Master in Slave Out (used during read operation by master) MOSI - Master Out Slave In (used during write operation by master) SCL - Serial Clock for synchronization CS - Chip select

6.3. UART Universal Asynchronous Receiver Transmitter (UART) communication is a full duplex, device to device communication protocol. UART supports single master and single slave to communicate with each other. It uses 2 pins for data transmission i.e., Receiver and transmitter.

Fig 5. UART Data transmission

In UART the parallel data is converted to serial at the transmission end and the data is transmitted serially to the receiver. At the receiver end the serial data is collected and converted to parallel data. The data transmission takes place in the form of packets [23].

Volume X Issue VI JUNE 2021 Page No : 174 Science, Technology and Development ISSN : 0950-0707

Table 1 shows the comparison among the 3-serial interface

I2C SPI UART

Speed I2C has several modes The maximum speed of Supports up to 5MHz. of speed - 100kHz, SPI is not specified. It Usually, the baud rate is 400kHz, 3.4Mhz and supports up to 20Mhz 9600 is used. 5Mhz.

Communication Synchronous, Synchronous, full- Asynchronous, full-duplex type half duplex duplex

Number of multiple single single masters

Number of multiple multiple single slaves

Advantages Multiple masters can Transferring of data is Clock is not required. be used in the design not limited to 8-bits. SPI Lesser number of pins can transfer Addressing in I2C is simpler

Disadvantages I2C supports half More pins are required. Master and slave should duplex. As number of slaves support the same baud rate. increases, Chip select pin also increases Table 1. Comparison between I2C, SPI and UART [20]

7. Conclusion

Using these different power management blocks in the SoC for IoT development which will help with the performance and efficiency of the total application [24]. This can be achieved by utilizing an efficient Power management unit comprises of a Low dropout regulator, which is integrated with the circuit and does not require an external power source, a DC- DC converter which can be scaled according to the needs of the load, serial interfaces that effectively communicate within and outside the system, and finally a voltage reference which provides a continuous voltage supply to the entire circuit [25].Noticing the impressive growth of wireless networks, a wired system is slowly becoming outdated so an efficient power transmission is of vital importance. The applications primarily focus on efficient management of power to enhance system functionality and minimize the overheads of the unit and result in a balanced performance in IoT systems [26].

Volume X Issue VI JUNE 2021 Page No : 175 Science, Technology and Development ISSN : 0950-0707

References Conference on Applied System Invention (ICASI). IEEE, 2018. 1.Gazis, Vangelis, et al. "Short paper: IoT: Challenges, projects, architectures." 2015 9. Hoon, Siew Kuok, et al. "A low noise, high 18th International Conference on power supply rejection low dropout regulator Intelligence in Next Generation Networks. for wireless system-on-chip applications." IEEE, 2015. Proceedings of the IEEE 2005 Custom Integrated Circuits Conference, 2005.. IEEE, 2.Wang, Pan, et al. "Introduction: Advances 2005. in IoT research and applications." Information Systems Frontiers 17.2 (2015): 10.Hwu KI, Peng TJ. A novel buck–boost 239-241. converter combining KY and buck converters. IEEE Transactions on power 3.Wang, Pan, et al. "Introduction: Advances electronics. 2011 Dec 30;27(5):2236-41. in IoT research and applications." Information Systems Frontiers 17.2 (2015): 11.G. Spiazzi, "Analysis of buck converters 239-241. used as power factor preregulators," PESC97. Record 28th Annual IEEE Power 4.Stephen M. Nolan, “Power Management Electronics Specialists Conference. Formerly for Internet of Things (IoT) Power Conditioning Specialists Conference (SoC) Development”, Vidatronic, Inc., 2017 1970-71. Power Processing and Electronic Specialists Conference 1972, 1997, pp. 564- 5.Asif, Muhammad, et al. "A High 570 vol.1, doi: 10.1109/PESC.1997.616778. Performance Adaptive Digital LDO Regulator with Dithering and Dynamic 12. Fossas E, Olivar G. Study of chaos in the Frequency Scaling for IoT Applications." buck converter. IEEE Transactions on IEEE Access 8 (2020): 132200-132211. Circuits and Systems I: Fundamental Theory and Applications. 1996 Jan;43(1):13-25. 6.Lau, Sai Kit, Philip KT Mok, and Ka Nang Leung. "A low-dropout regulator for SoC 13.Rosas-Caro, Julio C., et al. "A DC–DC with Q -reduction." IEEE Journal of Solid- multilevel boost converter." IET Power State Circuits 42.3 (2007): 658-664 Electronics 3.1 (2010): 129-137. 7.Patoux, Jerome. "Ask The Applications Engineer—37, Low-Dropout Regulators." 13.Gaboriault, Mark, and Andrew Notman. Analog Dialogue (2007): 41-2. "A high efficiency, non-inverting, buck- boost DC-DC converter." Nineteenth Annual 8.Wu, Chien-Yi, Jen-Chung Lou, and Zhi- IEEE Applied Power Electronics Conference Bing Deng. "An ultra-low power capacitor- and Exposition, 2004. APEC'04.. Vol. 3. less LDO for an always-on domain in NB- IEEE, 2004 IoT applications." 2018 IEEE International

Volume X Issue VI JUNE 2021 Page No : 176 Science, Technology and Development ISSN : 0950-0707

14. Ejury J. Buck converter design. Infineon Technologies North America (TFNA) Corn 21. O. G. Ç. Arlı, "Inter Integrated Circuit Design Note. 2013 Jan;1. (I2C) Serial Communication in VHDL," in State Machine using VHDL, Springer, 2021. 15.Carreon-Bautista S, Huang L, Sanchez- Sinencio E. An autonomous energy 22.Leens, Frédéric. "An introduction to I 2 C harvesting power management unit with and SPI protocols." IEEE Instrumentation & digital regulation for IoT applications. IEEE Measurement Magazine 12.1 (2009): 8-13. Journal of Solid-State Circuits. 2016 Apr 20;51(6):1457-74. 23.North Uzaimin, J., and H. H. Maimun. "The design of high speed UART." 2005 16.Liou, Wan-Rone, Mei-Ling Yeh, and Asia-Pacific Conference on Applied Yueh Lung Kuo. "A high efficiency dual- Electromagnetics. IEEE, 2005. mode buck converter IC for portable applications." IEEE Transactions on Power 24.Das, Sajal Kumar. "Battery and Power Electronics 23.2 (2008): 667-677. Management Unit Design." (2017): 317-336.

17.Yuan, Guohui, et al. "Border-collision 25.Shaikh, Taniya, et al. "IoT Based Power bifurcations in the buck converter." IEEE Management System (Academic Year 2019- Transactions on Circuits and Systems I: 20)." 2020 Fourth World Conference on Fundamental Theory and Applications 45.7 Smart Trends in Systems, Security and (1998): 707-716. Sustainability (WorldS4). IEEE, 2020.

18.Wikipedia. “Voltage Reference.” 26.Prasad, Anupriya, and Pradeep Chawda. Wikipedia, the free encyclopedia, 2019, "Power management factors and techniques https://en.wikipedia.org/wiki/Power_Manag for IoT design devices." 2018 19th ement_Unit. International Symposium on Quality Electronic Design (ISQED). IEEE, 2018[25]. 19.Rezaei, Neda, and Mitra Mirhassani. "Ultra-Low-Power Negative DC Voltage Generator Based on a Proposed Level Shifter and Voltage Reference." Microelectronics Journal (2021): 105087.

20. RF Wireless World, “Difference between UART, SPI and I2C” RF & Wireless Vendors and Resources, 2012. https://www.rfwireless- world.com/Terminology/UART-vs-SPI-vs- I2C.html

Volume X Issue VI JUNE 2021 Page No : 177