UART
What is UART?
UART (Universal Asynchronous Receiver Transmitter) is a type of asynchronous serial communication that relies on predefined start and stop signals to ensure data is transmitted accurately. The start signal is called the start bit and end signal the stop bit, with data transmitted between these bits referred to as data bits. A parity bit may be included after the data bits to verify the integrity of the transmitted data.
A typical transmission frame consists of 1 start bit, 8 data bits, 1 parity bit (optional), and 1 or 2 stop bits, resulting in a total transmission length of 11 or 12 bits per communication cycle.
Since the addition of start, stop, and (optional) parity bits slightly reduces data transfer efficiency compared to synchronous methods, but UART remains a widely used, simple, reliable communication method due to its ease of implementation and minimal hardware requirements.
What is a Parity Bit?
A parity bit is used for error detection in data transmission and comes in two types: odd and even.
For odd parity, a 0 or 1 is appended to the data bits so that the total number of 1s is odd, and for even parity, 0 or 1 is added to the end of the data bits to ensure that the total number of 1s is even.
For example, with 8 bit data 11000101, the number of 1s is 4, which is even. For odd parity, the parity bit is set to 1 to make the number of 1s odd. While in the case of even parity, the parity bit is set to 0 to keep the number of 1s even.
By adding a parity bit, any single-bit change during transmission will alter the expected parity, allowing the receiver to detect an error in the data.
What is Baud Rate?
In UART communication, each device generates its own internal clock to regulate data transmission. This clock is referred to as the baud rate clock. For successful communication, both the transmitting and receiving devices must be set to the same baud rate, ensuring that data is exchanged in sync.
Baud Rate Error
If the baud rate clocks of the transmitter and receiver are not precisely synchronized, timing discrepancies can occur, leading to communication errors.
In 8-bit communication, the typical allowable baud rate error is approx. ±1% for both the transmitter and receiver.
