Design of Low Power Universal Asynchronous Receiver and Transmitter

DOI : 10.17577/IJERTV11IS090097

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Design of Low Power Universal Asynchronous Receiver and Transmitter

Pranay Anand Tiwari Dr. Rajani Bisht

Department of Electronics Engineering Department of Electronics Engineering

Harcourt Butler Technical University Harcourt Butler Technical University Kanpur , India Kanpur , India

Abstract – Power dissipation is one of the key challenges in Electronic Circuit design and their performance in portable applications. Among high performance and high-density chips for example processors, high power dissipation restricts the amount of on chip transistors and increases the demand of essential heat removal, and that lessen the performance and increases the cost and size of the system. Consequently, analysis, power estimation along with optimization are vital challenges for CMOS Circuit design. Dynamic power dissipation is occurred when there is switching activity at some nodes in a CMOS circuit. Dynamic power dissipation is directly proportional to activity switching rate. In this paper, precise and simple method has been proposed to minimize the dynamic power dissipation in UART.

Keywords Dynamic power, switching activity rate, counter, protocol, data communication


Speed, power consumption and area are some of the major quality matrix of any digital system. [1]. In CMOS circuit there are three kinds of power consumptions. Dynamic power consumption which occurs because of charging and discharging of the circuit capacitance, short circuit power intake results from the direct current flows from supply to ground while gate switching, lastly static power consumption which happens due to leakage current[2,3,4]. However dynamic power analysis of UART has not been given much attention by researchers [5,6,7,8]. UART which stands for Universal Asynchronous Receiver and Transmitter is the electronic circuit which has been used for serial data communication using serial data communication protocol. The most important characteristics of UART are that it is used for short distance and consumes very low amount of energy[7]. There are multiple transmission modes in UART such as Simplex mode in which one way transmission is done ,Half Duplex where at a time either it transmits or it receives and Full Duplex where both transmission and reception works simultaneously .This paper presents practical analysis technique of gate switching activity factor power consumption in UART. [2]

UART Design comprises three major block as shown in figure 1- transmitter, receiver and baud Rate generator. In UART transmission, transmitter takes the data in parallel form and transfers the data in serial form and on the other end receiver process the data serially and converts it into parallel from. In order to get successful transmission in UART, baud rate for both transmission and receiving should be the same[7].

Fig. 1 UART Block Diagram [11]

In UART starting bit is fixed to LOW and that initiates bit synchronization of the word and at the receiver end it is followed by data word which signifies the data that will be transmitted. The least significant bit (LSB) is sent out first followed by next bit till the most significant bit (MSB) is transmitted .If parity is enabled it could be either odd parity or even parity. Stop bit is fixed to HIGH to provide word- framing which is used in bit synchronization at the receiver

.Transmission is initiated with start bit 0 tailed by data bit and an optional parity bit which finally finishes with stop bit 1.[14]

Fig.2 UART Packet Configuration [9]

. Baud rate generator is a Pulse generator that generates a tick pulse or also known as frequency divider circuit. The standard baud rates at which UART operates are 1200, 4800, 9600, 19200, 38400 bps. Baud rate of receiver and transmitter must be same in order to ensure that the transmitter and receiver are synchronized and excluding the necessity of the clock. This module has a clock signal, reset signal as Input and tick signal as Output.[14]


Since the value of input and output constantly changes, and circuit which comprises of capacitive loads at different points are charged and discharged, leads to power dissipation. This phenomenon is called switching power dissipation. A model of dynamic power dissipation analysis is shown in figure 3[13].

Fig 3. Dynamic power dissipation model [12]

For the complex logic gate, average power dissipation is [13]


Where, f is a clock frequency and n( N) is the number of 0- to-Vdd output transitions in the time interval [0,N], CL is the total output load capacitance VDD is the supply voltage.[13]

Where Cload is the total output load capacitance and VDD is the supply voltage, f is a clock frequency. Power dissipation in CMOS inverter is

Pd = Cload*VDD2*f (1)

We know that dynamic power expression of an inverter whose load capacitance is Cload can be calculated with expression Cload* VDD2*f . In current scenario we have assumed that output is switching in rail to rail format and input is also switching in each clock cycle [12].

But in case of a complex circuit, for example in NAND gate apart from the output load we also have to consider the capacitance present at the other node of the gate.

Fig 4. Schematic of NAND gate[15]

Now this capacitance in the nodes is getting charged and discharged simultaneously and its also possible that while switching the clock the capacitance node of the gate is not switching so in this scenario concept of switching activity is introduced. switching activity factor determines actual number of power-consuming voltage transitions faced by the output capacitance in each clock cycle


is the switching activity factor This provides the expected (average) value of the number of transitions in each clock cycle, which is known as the switching activity.

Dynamic power consumption in complex logic gates is


where i is the switching activity at the ith internal node and 0 is the switching activity on the output node, f is clock frequency . Here, it is presumed that there are k internal nodes. Algorithmic optimization can be used to minimize the switching activity in CMOS digital integrated circuits along with circuit level optimization. Due to transient switching activity of the CMOS device, dynamic component of power dissipation arises.


As we are aware that Baud rate Generator works as frequency divider circuit which consists of binary counter. One major characteristic of binary counter is that half of all sequential binary increment action requires that two or more counter bits must change. But this way of functioning has huge effect on switching activity at architectural level. We have applications where data modifies sequentially.

In order to lessen the number of transitions we have used Gray code counter instead of binary counter in the baud rate generator. We can see the grey counter logic in figure 4 .

Grey to Binary


Binary to Grey




Fig 4 . RTL Design of Grey Co


Simulation and synthesis has been done using Xilinx Vivado. In this simulation the baud rate generator which is

one of the block of an UART protocol has been designed by both binary counter and Gray counter as shown in figure 5 and 6 .

Fig. 5. Proposed Design of Grey Counter

Fig.6 RTL Schematic of Baud Rate Generator using Grey Counter

The total on chip power , dynamic power and static power of UART which we have designed from biary counter is illustrated in Table 1 below.

Total On Chip Power(W)

Dynamic Power(W)

Static Power(W)

Baud Rate Generator









UART Receiver




TABLE . Result Of UART Blocks Using Binary Counter

Total On Chip Power(W)

Dynamic Power(W)

Static Power(W)

Baud Rate Generator














TABLE . Result Of UART Blocks Using Grey Counter

And the total on chip power , dynamic power and static power of UART which we have designed from grey counter is illustrated in Table 2 below.


As per the simulation results comparing the binary coding with Gray coding we can come to the conclusion that it helps in reducing the state switching considerably and Power optimization is up to 36.1 % for transmitter and 61.6

% for Receiver .


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