sdq: simplify decode routine, no state machine required

Move the initial synchronization to the input data out of the main loop,
and handle the BREAK symbol when it was seen. Turns out that no state
machine is required to decode the SDQ protocol.

[ best viewed as a whitespace ignoring diff ]

1 files changed, 17 insertions, 25 deletions

diff --git a/decoders/sdq/pd.py b/decoders/sdq/pd.py
index d8df89f..c65faa8 100644
--- a/decoders/sdq/pd.py
+++ b/decoders/sdq/pd.py
@@ -64,7 +64,6 @@ class Decoder(srd.Decoder):
 
     def reset(self):
         self.samplerate = None
-        self.state = 'INIT'
         self.startsample = 0
         self.bits = []
         self.bytepos = 0
@@ -98,30 +97,23 @@ class Decoder(srd.Decoder):
         self.bit_width = float(self.samplerate) / float(self.options['bitrate'])
         self.half_bit_width = self.bit_width / 2.0
         # BREAK if the line is low for longer than this.
-        self.break_threshold = self.bit_width * 1.2
+        break_threshold = self.bit_width * 1.2
 
+        # Wait until the line is high before inspecting input data.
+        sdq, = self.wait({0: 'h'})
         while True:
-            if self.state == 'INIT':
-                sdq, = self.wait({0: 'h'}) # Wait until the line is high before starting
-                self.state = 'DATA'
-
-            elif self.state == 'DATA':
-                sdq, = self.wait({0: 'f'}) # Falling edge
-
-                self.startsample = self.samplenum
-                if self.bytepos == 0:
-                    self.bytepos = self.samplenum
-
-                sdq, = self.wait({0: 'r'}) # Rising edge
-
-                delta = self.samplenum - self.startsample
-                if delta > self.break_threshold:
-                    self.state = 'BREAK'
-                elif delta > self.half_bit_width:
-                    self.handle_bit(0)
-                else:
-                    self.handle_bit(1)
-
-            elif self.state == 'BREAK':
+            # Get the length of a low pulse (falling to rising edge).
+            sdq, = self.wait({0: 'f'})
+            self.startsample = self.samplenum
+            if self.bytepos == 0:
+                self.bytepos = self.samplenum
+            sdq, = self.wait({0: 'r'})
+
+            # Check for 0 or 1 data bits, or the BREAK symbol.
+            delta = self.samplenum - self.startsample
+            if delta > break_threshold:
                 self.handle_break()
-                self.state = 'DATA'
+            elif delta > self.half_bit_width:
+                self.handle_bit(0)
+            else:
+                self.handle_bit(1)