usb_signalling: Track USB symbol width to compensate frequency errors

USB low/full speed allows for frequency tolerance of 1.5%/0.25%. At
maximum packet size (sync + PID + data + CRC16) of 12 bytes/1027 bytes
this amounts to 1.4 bits/20 bits, so the decoder has to lock to the
actual symbol frequency to avoid any symbol misdetections.

The signal is sampled twice, once at the symbol center and once at
the expected edge position. Comparing the symbol at both positions gives
an indication if the current bit width is too low or too high. Adjust
accordingly.

1 files changed, 24 insertions, 9 deletions

diff --git a/decoders/usb_signalling/pd.py b/decoders/usb_signalling/pd.py
index 8ba55d9..6b4c028 100644
--- a/decoders/usb_signalling/pd.py
+++ b/decoders/usb_signalling/pd.py
@@ -117,9 +117,11 @@ class Decoder(srd.Decoder):
         self.syms = []
         self.bitrate = None
         self.bitwidth = None
-        self.bitnum = 0
+        self.samplepos = None
         self.samplenum_target = None
+        self.samplenum_edge = None
         self.oldpins = None
+        self.edgepins = None
         self.consecutive_ones = 0
         self.state = 'IDLE'
 
@@ -150,16 +152,16 @@ class Decoder(srd.Decoder):
 
     def putpb(self, data):
         s, h = self.samplenum, self.halfbit
-        self.put(s - h, s + h, self.out_python, data)
+        self.put(self.samplenum_edge, s + h, self.out_python, data)
 
     def putb(self, data):
         s, h = self.samplenum, self.halfbit
-        self.put(s - h, s + h, self.out_ann, data)
+        self.put(self.samplenum_edge, s + h, self.out_ann, data)
 
     def set_new_target_samplenum(self):
-        bitpos = self.ss_sop + (self.bitwidth / 2)
-        bitpos += self.bitnum * self.bitwidth
-        self.samplenum_target = int(bitpos)
+        self.samplepos += self.bitwidth;
+        self.samplenum_target = int(self.samplepos)
+        self.samplenum_edge = int(self.samplepos - (self.bitwidth / 2))
 
     def wait_for_sop(self, sym):
         # Wait for a Start of Packet (SOP), i.e. a J->K symbol change.
@@ -167,6 +169,7 @@ class Decoder(srd.Decoder):
             self.oldsym = sym
             return
         self.ss_sop = self.samplenum
+        self.samplepos = self.ss_sop - (self.bitwidth / 2) + 0.5
         self.set_new_target_samplenum()
         self.putpx(['SOP', None])
         self.putx([4, ['SOP', 'S']])
@@ -194,15 +197,15 @@ class Decoder(srd.Decoder):
         self.syms.append(sym)
         self.putpb(['SYM', sym])
         self.putb([sym_idx[sym], ['%s' % sym, '%s' % sym[0]]])
-        self.bitnum += 1
         self.set_new_target_samplenum()
         self.oldsym = sym
         if self.syms[-2:] == ['SE0', 'J']:
             # Got an EOP.
             self.putpm(['EOP', None])
             self.putm([5, ['EOP', 'E']])
-            self.bitnum, self.syms, self.state = 0, [], 'IDLE'
+            self.syms, self.state = [], 'IDLE'
             self.consecutive_ones = 0
+            self.bitwidth = float(self.samplerate) / float(self.bitrate)
 
     def get_bit(self, sym):
         if sym == 'SE0':
@@ -214,8 +217,17 @@ class Decoder(srd.Decoder):
         self.syms.append(sym)
         self.putpb(['SYM', sym])
         b = '0' if self.oldsym != sym else '1'
+        if (self.oldsym != sym):
+            # edge
+            edgesym = symbols[self.options['signalling']][tuple(self.edgepins)]
+            if (edgesym not in ('SE0', 'SE1')):
+                if (edgesym == sym):
+                    self.bitwidth = self.bitwidth - (0.001 * self.bitwidth)
+                    self.samplepos = self.samplepos - (0.01 * self.bitwidth)
+                else:
+                    self.bitwidth = self.bitwidth + (0.001 * self.bitwidth)
+                    self.samplepos = self.samplepos + (0.01 * self.bitwidth)
         self.handle_bit(sym, b)
-        self.bitnum += 1
         self.set_new_target_samplenum()
         self.oldsym = sym
 
@@ -231,8 +243,11 @@ class Decoder(srd.Decoder):
                 self.oldpins = pins
                 sym = symbols[self.options['signalling']][tuple(pins)]
                 self.wait_for_sop(sym)
+                self.edgepins = pins
             elif self.state in ('GET BIT', 'GET EOP'):
                 # Wait until we're in the middle of the desired bit.
+                if self.samplenum == self.samplenum_edge:
+                    self.edgepins = pins
                 if self.samplenum < self.samplenum_target:
                     continue
                 sym = symbols[self.options['signalling']][tuple(pins)]