+'''
+Universal Asynchronous Receiver Transmitter (UART) is a simple serial
+communication protocol which allows two devices to talk to each other.
+
+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)
+
+The bit order of the 5-9 data bits is LSB-first.
+
+Possible special cases:
+ - One or both data lines could be inverted, which also means that the idle
+ state of the signal line(s) is low instead of high.
+ - Only the data bits on one or both data lines (and the parity bit) could
+ be inverted (but the start/stop bits remain non-inverted).
+ - The bit order could be MSB-first instead of LSB-first.
+ - The baudrate could change in the middle of the communication. This only
+ happens in very special cases, and can only work if both devices know
+ to which baudrate they are to switch, and when.
+ - Theoretically, the baudrate on RX and the one on TX could also be
+ different, but that's a very obscure case and probably doesn't happen
+ very often in practice.
+
+Error conditions:
+ - If there is a parity bit, but it doesn't match the expected parity,
+ this is called a 'parity error'.
+ - If there are no stop bit(s), that's called a 'frame error'.
+
+More information:
+TODO: URLs
+
+Protocol output format:
+
+UART packet:
+[<packet-type>, <rxtx>, <packet-data>]
+
+This is the list of <packet-types>s and their respective <packet-data>:
+ - 'STARTBIT': The data is the (integer) value of the start bit (0 or 1).
+ - 'DATA': The data is the (integer) value of the UART data. Valid values
+ range from 0 to 512 (as the data can be up to 9 bits in size).
+ - 'PARITYBIT': The data is the (integer) value of the parity bit (0 or 1).
+ - 'STOPBIT': The data is the (integer) value of the stop bit (0 or 1).
+ - 'INVALID STARTBIT': The data is the (integer) value of the start bit
+ (0 or 1).
+ - 'INVALID STOPBIT': The data is the (integer) value of the stop bit
+ (0 or 1).
+ - 'PARITY ERROR': The data is a tuple with two entries. The first one is
+ the expected parity value, the second is the actual parity value.
+ - TODO: Frame error?
+
+The <rxtx> field is 0 for RX packets, 1 for TX packets.
+'''
+
projects / libsigrokdecode.git / blobdiff