genlib.cdc.MultiReg: pull in from Migen.

examples/cdc.py

@@ -0,0 +1,10 @@
+from nmigen.fhdl import *
+from nmigen.back import rtlil, verilog
+from nmigen.genlib.cdc import *
+
+
+sys  = ClockDomain()
+i, o = Signal(name="i"), Signal(name="o")
+frag = MultiReg(i, o).get_fragment(platform=None)
+# print(rtlil.convert(frag, ports=[i, o], clock_domains={"sys": sys}))
+print(verilog.convert(frag, ports=[i, o], clock_domains={"sys": sys}))

nmigen/genlib/cdc.py

@@ -0,0 +1,22 @@
+from ..fhdl import *
+
+
+__all__ = ["MultiReg"]
+
+
+class MultiReg(Module):
+    def __init__(self, i, o, odomain="sys", n=2, reset=0):
+        self.i = i
+        self.o = o
+        self.odomain = odomain
+
+        self.regs = [Signal(self.i.bits_sign(), name="cdc{}".format(i),
+                            reset=reset, reset_less=True)#, attrs=("no_retiming",))
+                     for i in range(n)]
+
+    def get_fragment(self, platform):
+        f = Module()
+        for i, o in zip((self.i, *self.regs), self.regs):
+            f.sync[self.odomain] += o.eq(i)
+        f.comb += self.o.eq(self.regs[-1])
+        return f.lower(platform)