diff options
Diffstat (limited to 'decoders/uart')
| -rw-r--r-- | decoders/uart/__init__.py | 80 |
1 files changed, 14 insertions, 66 deletions
diff --git a/decoders/uart/__init__.py b/decoders/uart/__init__.py index dcee3bf..a8b7303 100644 --- a/decoders/uart/__init__.py +++ b/decoders/uart/__init__.py @@ -24,72 +24,20 @@ UART protocol decoder. 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 +This decoder should work on all "UART-like" async protocols with one +start bit (0), 7-9 databits, an (optional) parity bit, and one or more +stop bits (1), in this order. + +It can be run on one signal line (RX or TX) only, or on two lines (RX + TX). + +There are various standards for the physical layer specification of the +signals, including RS232, (TTL) UART, RS485, and others. However, the logic +level of the respective pins is only relevant when acquiring the data via +a logic analyzer (you have to select the correct logic analyzer and/or +the correct place where to probe). Once the data is in digital form and +matches the "UART" description above, this protocol decoder can work with +it though, no matter whether the source was on TTL UART levels, or RS232, +or others. Protocol output format: |