srd: UART: Add some protocol documentation.

1 files changed, 72 insertions, 0 deletions

diff --git a/decoders/uart.py b/decoders/uart.py
index ee39697..efc1756 100644
--- a/decoders/uart.py
+++ b/decoders/uart.py
@@ -22,6 +22,78 @@
 # 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
+#
+
 import sigrok
 
 # States