ps2: end data byte at rising clock edge of the stop bit

Data byte transmission is complete when the STOP bit was communicated.
End the STOP bit at the rising CLOCK edge of the 11th bit time, do not
rely on the host's clock inhibit after data transmission. This avoids
the unexpected expansion of a STOP bit well into the next data byte.

This fixes bug #1460, where absence of host activity after the first
data byte loses synchronization to the input stream.

Rephrase pin level sampling while we are here. Reflect that the clock
line is used for .wait() conditions, but its level is not evaluated.
Only data line levels get processed.

Comment on the implementation's assumption of specific input data (the
device as transmitter, host transmit may not be supported, or can result
in unexpected output). Which may be acceptable yet maintainers need to
remain aware.

diff --git a/decoders/ps2/pd.py b/decoders/ps2/pd.py
index 194b0b1f9b1f0069666e98e82508368288fd7632..a9d0a986daf0356b265c61f7e1fd6a24a09cfdd2 100644 (file)
--- a/decoders/ps2/pd.py
+++ b/decoders/ps2/pd.py
@@ -116,6 +116,11 @@ class Decoder(srd.Decoder):
 
     def decode(self):
         while True:
-            # Sample data bits on falling clock edge.
-            clock_pin, data_pin = self.wait({0: 'f'})
+            # Sample data bits on the falling clock edge (assume the device
+            # is the transmitter). Expect the data byte transmission to end
+            # at the rising clock edge. Cope with the absence of host activity.
+            _, data_pin = self.wait({0: 'f'})
             self.handle_bits(data_pin)
+            if self.bitcount == 1 + 8 + 1 + 1:
+                _, data_pin = self.wait({0: 'r'})
+                self.handle_bits(data_pin)