Universal Asynchronous Receiver/ Transmitter (UART)

To transfer data, computers adopt two method: serial and parallel. In serial communication data is sent a bit at a time (1 line)while in parallel communication 8 or more lines are often used to transfer data. The UART is a form of serial communication

When distance is short, digital signal can be transferred as it is on a single wire and require no modulation. This is how PC keyboard transfer data between the keyboard and motherboard.

For long distances like telephone lines, serial data communication requires a modem to modulate.We will not go into the details of this in this lesson, this shall be left for subsequent lessons.

Synchronous vs. Asynchronous

Serial communication operate in two ways : synchronous and asynchronous. In the synchronous mode, a block of data is sent at a time while in the asynchronous mode, a single byte of data is sent at a time.

It is possible to write code to provide both the synchronous and asynchronous serial communication functions however this could turn out tedious and over complicated therefore, special chips are manufactured to perform these functions. When these chips are added to a microcontroller, they become known as Serial Communication Interface (SCI). The chip that provides the the asynchronous communication (UART) is the main theme of this lesson. The chip for synchronous communication is known as the USART and shall left for another lesson.

Asynchronous Communication Data Framing

The data is received in zeros and ones format, in order to make sense of the data, the sender and the receiver must agree on a set of rules i.e. a protocol, on how data is packed, how many bits constitute a character and when data begins and ends.

The Protocol

  1. Start and Stop bits

Asynchronous serial communication is widely used in character transfer. Each character is packed between start and stop bits. This is known as the framing.

start bit :  Always 1 bit, value is always 0

stop bit:   1 or 2 bits, value is always 1

Lets take an example. Lets say we are transferring ASCII character ‘A’

its binary code is 0100 0001 and 0x41 in hex.Furthermore, it is framed between a start bit and 2 stop bits. This is what its frame will look like.

Frame of ASCII character 'A'

Frame of ASCII character ‘A’

 2. Parity bit

Some systems require parity bit in order to maintain data integrity.The parity bit of a character byte is included in the data frame after the stop bit(s).

 3.Rate of data transfer : Baud rate/bps

We shall explain this concept with an example. Lets say we are asked to calculate the number of bits used  and time taken in transferring 50 pages of text, each with 80×25 characters. Assuming 8 bits per character and 1 stop bit.


For each character a total of 10 bits is used ( 1 start bit, 8 bits character, 1 stop bit).

Therefore total number of bits  = 80 x 25 x10 = 20,000 bits per page

50 pages  implies, 50×20,000  =  1,000,000 bits.

Therefore it will take 1 million bits to transfer  this information.

The time it takes to transfer the entire data using

a. 9600 baudrate implies,  1,000,000 / 9600 = 204 seconds

b. 57,600 baudrate implies 1,000,000 /57,600 = 17 seconds

Baudrate simply means the transfer of bits per second. There are various standardized baudrate we can choose from when we program our serial communication devices.  The key thing is, both communication devices must have the same baudrate. We shall see this later on in our example code.

Programming the ARM Cortex-M UART

ARM Cortex –  microconrtollers from different vendors come with more than 1 internal UARTs. For instance the the ARM Cortex-M4 Tiva Launchpad from Texas Instrument comes with 8 UARTs.

Two registers are used to set the baudrate:

-UART Integer Baudrate Divisor

-UART Fraction Baudrate Divisor

To get your desired baudrate, its simply  SystemClock / (16 x ClkDiv)

SystemClock can be  : XTAL,PLL, internal RC etc.

ClkDiv                          :  value we load into the divisor register

Steps to configure UART and Transmit a byte of data 

  1. Provide clock to UART by writing 1 to UART clock gating register
  2. Provide clock to the GPIO port the UART function is connected .
  3. Disable the UART by writing 0 to the UART control register.
  4. Write the integer portion of the Baudrate to the UARTIBRD register.
  5. Write the fractional portion of the Baudrate to the UARTFBRD register.
  6. Select the system clock as UART clock source .

Important Registers

Before we move on to provide example source code, there are 3 important registers we need to take a closer look at.

UART Control Register

As mentioned in the earlier section, in calculating the baudrate, the default system clock is divided by 16, however, we can decide to divide it by 8 by the setting the 1 to the HSE bit of the UART control register. The HSE implies, High-Speed-Enable.

UART Control Register

Important bits in the UART control register include:

  • UARTEN : To enable and disable UART
  • HSE : High speed enable, to run high baudrate with low frequency
  • RXE: This must be enabled in order to receive data
  • TXE: This must be enabled in order to transmit data

UART Line Control Register

This register is used to set the number bits per character i.e. data length

Also used to set the stop bit. This what the register looks like:

UART Line Register

UART Line Register

The default stop bit is 1 to set the stop bit to 2  set STP2 (D3) bit =1

FEN(D4) : It is known as the FIFO enable.

Because of the 1 byte transfer / receive limitation, the CPU keeps getting interrupted every time a single byte of data is transferred. This could create bottleneck  especially in multitasking systems. Therefore  cortex -microcontrollers provide FIFO buffers to store data for transmission. The ARM Cortex-M4 TM4C123  provides a 16-bytes FIFO buffer to store data for transmission and another 16-bytes FIFO buffer to save all received data.

When FEN is enabled, we can write up 16-bytes of data block into the transmission FIFO buffer and let it transfer a byte a time.

Also, the developer may set threshold for UART to notify the CPU when level of FIFO passes threshold.

WLEN (D6 – D5)  : Word length. this is used to set the number of bits per character.  Character data in each frame can be 5,6,7 or 8. Generally, 8 bits is used.  The default value is 5 on some cortex-microcontrollers and must be set to 8 simply by setting D5 =1 and D6=1.


UART Data Register 

UART Data Register

UART Data Register

Data to be transmitted is placed in the UART data register

Only the lower 8-bits are used

A write to this register initializes a transmission from the UART

A read  receives transmitted bytes. For read, the lower 8-bits hold the received byte, other 4 bits are used for error detection e.g. parity, framing etc.

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