build/gowin/common: Add AsyncResetSynchronizer implementation.

litex/build/gowin/common.py

@@ -1,8 +1,40 @@
 #
 # This file is part of LiteX.
 #
-# Copyright (c) 2020 Pepijn de Vos <pepijndevos@gmail.com>
-# Copyright (c) 2015-2018 Florent Kermarrec <florent@enjoy-digital.fr>
+# Copyright (c) 2021 Florent Kermarrec <florent@enjoy-digital.fr>
 # SPDX-License-Identifier: BSD-2-Clause
 
-gowin_special_overrides = {}
+from migen.fhdl.module import Module
+from migen.genlib.resetsync import AsyncResetSynchronizer
+
+from litex.build.io import *
+
+# Gowin AsyncResetSynchronizer ---------------------------------------------------------------------
+
+class GowinAsyncResetSynchronizerImpl(Module):
+    def __init__(self, cd, async_reset):
+        rst1 = Signal()
+        self.specials += [
+            Instance("DFFP",
+                i_D      = 0,
+                i_PRESET = async_reset,
+                i_CLK    = cd.clk,
+                o_Q      = rst1),
+            Instance("DFFP",
+                i_D      = rst1,
+                i_PRESET = async_reset,
+                i_CLK    = cd.clk,
+                o_Q      = cd.rst)
+        ]
+
+
+class GowinAsyncResetSynchronizer:
+    @staticmethod
+    def lower(dr):
+        return GowinAsyncResetSynchronizerImpl(dr.cd, dr.async_reset)
+
+# Gowin Special Overrides --------------------------------------------------------------------------
+
+gowin_special_overrides = {
+    AsyncResetSynchronizer: GowinAsyncResetSynchronizer,
+}