-It uses just two data signals and a ground (GND) signal:
- - RX/RXD: Receive signal
- - TX/TXD: Transmit signal
-
-The protocol is asynchronous, i.e., there is no dedicated clock signal.
-Rather, both devices have to agree on a baudrate (number of bits to be
-transmitted per second) beforehand. Baudrates can be arbitrary in theory,
-but usually the choice is limited by the hardware UARTs that are used.
-Common values are 9600 or 115200.
-
-The protocol allows full-duplex transmission, i.e. both devices can send
-data at the same time. However, unlike SPI (which is always full-duplex,
-i.e., each send operation is automatically also a receive operation), UART
-allows one-way communication, too. In such a case only one signal (and GND)
-is required.
-
-The data is sent over the TX line in so-called 'frames', which consist of:
- - Exactly one start bit (always 0/low).
- - Between 5 and 9 data bits.
- - An (optional) parity bit.
- - One or more stop bit(s).
-
-The idle state of the RX/TX line is 1/high. As the start bit is 0/low, the
-receiver can continually monitor its RX line for a falling edge, in order
-to detect the start bit.
-
-Once detected, it can (due to the agreed-upon baudrate and thus the known
-width/duration of one UART bit) sample the state of the RX line "in the
-middle" of each (start/data/parity/stop) bit it wants to analyze.
-
-It is configurable whether there is a parity bit in a frame, and if yes,
-which type of parity is used:
- - None: No parity bit is included.
- - Odd: The number of 1 bits in the data (and parity bit itself) is odd.
- - Even: The number of 1 bits in the data (and parity bit itself) is even.
- - Mark/one: The parity bit is always 1/high (also called 'mark state').
- - Space/zero: The parity bit is always 0/low (also called 'space state').
-
-It is also configurable how many stop bits are to be used:
- - 1 stop bit (most common case)
- - 2 stop bits
- - 1.5 stop bits (i.e., one stop bit, but 1.5 times the UART bit width)
- - 0.5 stop bits (i.e., one stop bit, but 0.5 times the UART bit width)