lib.cdc: add ResetSynchronizer.

nmigen/lib/cdc.py

@@ -1,7 +1,7 @@
 from .. import *
 
 
-__all__ = ["MultiReg"]
+__all__ = ["MultiReg", "ResetSynchronizer"]
 
 
 class MultiReg:
@@ -23,3 +23,28 @@ class MultiReg:
             m.d[self.odomain] += o.eq(i)
         m.d.comb += self.o.eq(self._regs[-1])
         return m
+
+
+class ResetSynchronizer:
+    def __init__(self, arst, domain="sync", n=2):
+        self.arst = arst
+        self.domain = domain
+
+        self._regs = [Signal(name="arst{}".format(i), reset=1,
+                             attrs={"no_retiming": True})
+                      for i in range(n)]
+
+    def elaborate(self, platform):
+        if hasattr(platform, "get_reset_sync"):
+            return platform.get_reset_sync(self)
+
+        m = Module()
+        m.domains += ClockDomain("_reset_sync", async_reset=True)
+        for i, o in zip((0, *self._regs), self._regs):
+            m.d._reset_sync += o.eq(i)
+        m.d.comb += [
+            ClockSignal("_reset_sync").eq(ClockSignal(self.domain)),
+            ResetSignal("_reset_sync").eq(self.arst),
+            ResetSignal(self.domain).eq(self._regs[-1])
+        ]
+        return m