UART

Topics: Embedded Systems

Universal Asynchronous Receiver-Transmitter (UART) is an asynchronous, serial, hardware communication protocol that uses two wires to send data between devices without a clock. Unlike SPI or I2C, it is a physical circuit in a microcontroller or a stand-alone IC. Since there is no clock signal, both UART devices must agree on a preset speed called baud rate, most commonly $115200$ bits per seconds, or $9600$.

Here is a summary of the characteristics of UART:

Interface

Each UART device has 2 pins:

• Transmitter Tx
• Receiver Rx

The data is transmitted serially on the wires, i.e. bit by bit.

Data Transmission

UART uses data packets of the following form for transmission:

• start bit: the data line is normally held high; so to start data transfer, the UART devices pulls the line low for the duration of 1 bit
• data frame: the actual data being transferred. It is $5$ to $8$ bits if a parity bit is used, or $5$ to $9$ bits if it's not.
• parity bit: bit that denotes the evenness of the packet, used for error detection
• stop bits: drive the line high for $1$ to $2$ bit durations to signal the end of the packet

Advantages

• simplicity; if the application does not need a more sophisticated topology
• low wire count
• error checking with parity bit

Disadvantages

• limited to point-to-point communication between 2 devices
• no clock synchronization, which could lead to data corruption if the baud rates of the devices drift
• slower speeds than synchronous protocols like SPI
• Limited data frame

References

1. https://www.analog.com/en/resources/analog-dialogue/articles/uart-a-hardware-communication-protocol.html
2. https://www.circuitbasics.com/basics-uart-communication/