parallel: update builtin documentation

Update the decoder's doc string to catch up with recent extensions.
Rephrase how clock and data lines interact. Sparse assignment of data
lines is supported (has been for a while). Discuss the optional reset
signal and its enable/select use.

1 files changed, 19 insertions, 8 deletions

diff --git a/decoders/parallel/__init__.py b/decoders/parallel/__init__.py
index 100523e..e7ed36c 100644
--- a/decoders/parallel/__init__.py
+++ b/decoders/parallel/__init__.py
@@ -19,16 +19,27 @@
 
 '''
 This protocol decoder can decode synchronous parallel buses with various
-number of data bits/channels and one (optional) clock line.
+data bits/channels counts, an (optional) clock line, and an (optional)
+select/enable/reset line.
 
-If no clock line is supplied, the decoder works slightly differently in
-that it interprets every transition on any of the supplied data channels
-like there had been a clock transition.
+Data bits are taken from the decoder's lowest connected input pins. The
+input signal's data lines count need not span the full amount of the
+decoder's maximum supported data lines count. Not connected data lines
+are assumed to be low.
 
-It is required to use the lowest data channels, and use consecutive ones.
-For example, for a 4-bit sync parallel bus, channels D0/D1/D2/D3 (and CLK)
-should be used. Using combinations like D7/D12/D3/D15 is not supported.
-For an 8-bit bus you should use D0-D7, for a 16-bit bus use D0-D15 and so on.
+Example use cases are: Connect D3/D2/D1/D0 (and CLK) to a 4-bit bus.
+Connect D7 and D6 to inspect the two most significant bits of an 8-bit
+bus (and have 8-bit values shown instead of just 2-bit values).
+
+When provided, the specified clock edge determines when data lines get
+sampled. Without a clock spec, each transition on any of the data lines
+will be shown, which can become busy/noisy depending on the input data.
+
+Another signal optionally can control the period of time within which
+the data lines' bit pattern gets interpreted. Typical use cases would be
+reset, or select, or enable signals that are related to the bus' data
+communication. This optional signal can also improve synchronization to
+wider payload data which spans several bus cycles (multiplexing).
 '''
 
 from .pd import Decoder