i2c: Fix corner-case that can occur with LA triggers.

Until now the I2C PD was basically ignoring the very first sample, and
using that as the initial 'oldscl'/'oldsda' value.

However, if your logic analyzers trigger on, say, SDA=low that will
result in a file where the first sample is really important since it
is the one which the PD will need to know that there's a falling edge
on SDA.

Thus, assume both SCL and SDA are high/1 when the PD starts. This is
a good assumption since both pins have pullups on them in practice
and are thus high/1 when the bus is idle.

Later on we might want to have config options to let the PD assume
other states of SDA/SCL initially.

1 files changed, 3 insertions, 9 deletions

diff --git a/decoders/i2c/pd.py b/decoders/i2c/pd.py
index 0b85c66..c2eb524 100644
--- a/decoders/i2c/pd.py
+++ b/decoders/i2c/pd.py
@@ -86,9 +86,9 @@ class Decoder(srd.Decoder):
         self.wr = -1
         self.is_repeat_start = 0
         self.state = 'FIND START'
-        self.oldscl = None
-        self.oldsda = None
-        self.oldpins = None
+        self.oldscl = 1
+        self.oldsda = 1
+        self.oldpins = (1, 1)
 
     def start(self, metadata):
         self.out_proto = self.add(srd.OUTPUT_PROTO, 'i2c')
@@ -206,12 +206,6 @@ class Decoder(srd.Decoder):
                 continue
             self.oldpins, (scl, sda) = pins, pins
 
-            # First sample: Save SCL/SDA value.
-            if self.oldscl == None:
-                self.oldscl = scl
-                self.oldsda = sda
-                continue
-
             # TODO: Wait until the bus is idle (SDA = SCL = 1) first?
 
             # State machine.