genlib: add Gray counter

examples/basic/graycounter.py

@@ -0,0 +1,19 @@
+from random import Random
+
+from migen.fhdl.module import Module
+from migen.genlib.cdc import GrayCounter
+from migen.sim.generic import Simulator
+
+class TB(Module):
+	def __init__(self, width=3):
+		self.width = width
+		self.submodules.gc = GrayCounter(self.width)
+		self.prng = Random(7345)
+
+	def do_simulation(self, s):
+		print("{0:0{1}b} CE={2}".format(s.rd(self.gc.q),
+			self.width, s.rd(self.gc.ce)))
+		s.wr(self.gc.ce, self.prng.getrandbits(1))
+
+sim = Simulator(TB())
+sim.run(35)

migen/genlib/cdc.py

@@ -1,4 +1,5 @@
 from migen.fhdl.structure import *
+from migen.fhdl.module import Module
 from migen.fhdl.specials import Special
 from migen.fhdl.tools import list_signals
 
@@ -71,3 +72,26 @@ class PulseSynchronizer:
 		return Fragment(comb, 
 			{self.idomain: sync_i, self.odomain: sync_o},
 			specials={MultiReg(toggle_i, toggle_o, self.odomain)})
+
+class GrayCounter(Module):
+	def __init__(self, width):
+		self.ce = Signal()
+		self.q = Signal(width)
+		self.q_next = Signal(width)
+
+		###
+
+		q_binary = Signal(width)
+		q_next_binary = Signal(width)
+		self.comb += [
+			If(self.ce,
+				q_next_binary.eq(q_binary + 1)
+			).Else(
+				q_next_binary.eq(q_binary)
+			),
+			self.q_next.eq(q_next_binary ^ q_next_binary[1:])
+		]
+		self.sync += [
+			q_binary.eq(q_next_binary),
+			self.q.eq(self.q_next)
+		]