UART Communication

UART stands for universal asynchronous receiver and transmitter. UART is an asynchronous half duplex communication protocol. It is called universal because of configurable parameters such as speed, frame format and can be configured to meet the needs of a given communication requirement. There is no clock signal between transmitter and receiver. Both transmitter and receiver may work on different clock speed.  Two devices communicate with each other through three signals TX, RX and GND as shown in figure below:

Device1 Tx will connect with Device2 RX and Device1 RX will connect with Device2 TX and both devices GND will be common.  It is half duplex communication protocol which share the same register for data transmission and register. So when using UART it should be kept in minds that you should transmit and receive simultaneously. At transmitter side UART done parallel to serial conversion of the data which is received from the CPU and at receiver end UART perform serial to parallel conversion on the data received through serial communication.

Frame Structure

In UART data is transferred in a proper frame structure. UART frame consist of data bits and control bits. UART frame consists of following data:

·        Start bit

·        8 or 9 data bit

·        Parity Bit (optional)

·        1 or 2 stop bit

Figure below show the data frame with 8 data bit 1 start bit and 1 stop bit:

The parity bit in UART data frame is optional.  The parity can be even or odd. The UART peripheral calculates the number of 1s present in the transmission. If the parity is configured to even and the number of 1’s is even then the parity bit is set zero. If the number of 1s is odd, the parity bit is set to a 1 to make the count even. If the parity is configured to odd, and the number of 1s is odd, then parity bit is set to 0. Otherwise it is set to 1 to make the count odd. At receiver end UART check remove this bit from the data. START bit always will be 0 and STOP bit always will be 1.

When there is no communication TX pin remains high. When Communication starts Transmitter pulls the TX pin low (START bit). After data bit STOP bit will be transmitted. Every Bit will be of certain duration. This depends on the communication Baud rate selected.

After getting START bit (Sense LOW on RX pin) receiver samples in the middle of every period and save data to UART DATA REGISTER and also give information to CPU by setting flag in register or by interrupt or CPU if enabled.  Receiver also verifies the Parity Bit and STOP bit. If there is any framing or parity receiver notifies to CPU by setting error bits in UART STATUS register.

Figure show below the transmission of Char ‘D’ without parity bit and 1 stop bit.

Char ‘D’ Decimal equivalent is 68 and hex decimal equivalent is 0x44.

Binary equivalent will be B”01000100”.

What is BAUD RATE?

BAUD rate defines the data communication rate in UART communication. Baud rate specified in bps(bits per second). So baud defines how many bits may be transferred in one second from one device to another device. For example if Baud rate defines as 9600 bps. It means 9600 bits can be transferred from one device to another device. For proper communication receiver and transmitter should be configured to same baud rate. Otherwise receiver will receive abrupt data.

One bit period at 9600 baud rate will be = 1/9600 sec = 104.16 usec

1 Byte with 1 start bit and 1 stop bit transmission time will be = 10*104.16 usec                                                                                            = 1.04 ms

So it takes 1.04 ms to transmit one byte from one device to another device.