edid.py: sample SCL only every 64 clock cycles, to avoid bouncing

Possibly due to SCL rising fairly slowly (in the 0.5-1 us range),
bouncing has been observed while crossing the "forbidden" region
between Vil(max) and Vih(min).

By lowering the sample rate from once per system clock to once
every 64 clock cycles, we make sure we sample at most once during
the bounce interval and thus never see a false edge. (Although we
may see a rising edge one sample time late, which is perfectly
harmless.)

diff --git a/milkymist/dvisampler/edid.py b/milkymist/dvisampler/edid.py
index 4c9da42091483c00c2754396f06d96e006faa217..f42b9dfdf40d63b2e613419e6ec4b03ed7e96193 100644 (file)
--- a/milkymist/dvisampler/edid.py
+++ b/milkymist/dvisampler/edid.py
@@ -23,24 +23,25 @@ class EDID(Module, AutoCSR):
 
                ###
 
-               scl_i = Signal()
+               scl_raw = Signal()
                sda_i = Signal()
                sda_drv = Signal()
                _sda_drv_reg = Signal()
                _sda_i_async = Signal()
                self.sync += _sda_drv_reg.eq(sda_drv)
                self.specials += [
-                       MultiReg(pads.scl, scl_i),
+                       MultiReg(pads.scl, scl_raw),
                        Tristate(pads.sda, 0, _sda_drv_reg, _sda_i_async),
                        MultiReg(_sda_i_async, sda_i)
                ]
 
-               # FIXME: understand what is really going on here and get rid of that workaround
-               for x in range(20):
-                       new_scl = Signal()
-                       self.sync += new_scl.eq(scl_i)
-                       scl_i = new_scl
-               #
+               scl_i = Signal()
+               samp_count = Signal(6)
+               samp_carry = Signal()
+               self.sync += [
+                       Cat(samp_count, samp_carry).eq(samp_count + 1),
+                       If(samp_carry, scl_i.eq(scl_raw))
+               ]
 
                scl_r = Signal()
                sda_r = Signal()