lib.fifo: fix reset handling of asynchronous FIFOs.

Author: Jean-François Nguyen

nmigen/lib/fifo.py

@@ -355,7 +355,7 @@ def elaborate(self, platform):
         consume_enc = m.submodules.consume_enc = \
             GrayEncoder(self._ctr_bits)
         consume_cdc = m.submodules.consume_cdc = \
-            FFSynchronizer(consume_r_gry, consume_w_gry, o_domain=self._w_domain)
+            FFSynchronizer(consume_r_gry, consume_w_gry, o_domain=self._w_domain, reset_less=False)
         m.d.comb += consume_enc.i.eq(consume_r_nxt)
         m.d[self._r_domain] += consume_r_gry.eq(consume_enc.o)
 
@@ -414,9 +414,10 @@ def elaborate(self, platform):
         w_rst = ResetSignal(domain=self._w_domain, allow_reset_less=True)
         r_rst = Signal()
 
-        # Async-set-sync-release synchronizer avoids CDC hazards
+        # Async-set-sync-release synchronizer avoids CDC hazards. In the read domain, 3 clock cycles
+        # are needed to propagate the reset value of produce_w_gry to consume_r_bin.
         rst_cdc = m.submodules.rst_cdc = \
-            AsyncFFSynchronizer(w_rst, r_rst, o_domain=self._r_domain)
+            AsyncFFSynchronizer(w_rst, r_rst, o_domain=self._r_domain, stages=3)
 
         # Decode Gray code counter synchronized from write domain to overwrite binary
         # counter in read domain.
@@ -425,7 +426,7 @@ def elaborate(self, platform):
         m.d.comb += rst_dec.i.eq(produce_r_gry)
         with m.If(r_rst):
             m.d.comb += r_empty.eq(1)
-            m.d[self._r_domain] += consume_r_gry.eq(produce_r_gry)
+            m.d[self._r_domain] += consume_r_gry.eq(produce_r_gry.reset)
             m.d[self._r_domain] += consume_r_bin.eq(rst_dec.o)
             m.d[self._r_domain] += self.r_rst.eq(1)
         with m.Else():
@@ -435,6 +436,9 @@ def elaborate(self, platform):
             with m.If(Initial()):
                 m.d.comb += Assume(produce_w_gry == (produce_w_bin ^ produce_w_bin[1:]))
                 m.d.comb += Assume(consume_r_gry == (consume_r_bin ^ consume_r_bin[1:]))
+            for i in range(rst_cdc._stages):
+                with m.If(Past(Initial(), i)):
+                    m.d.comb += Assume(r_rst.implies(w_rst))
 
         return m
 
@@ -509,21 +513,25 @@ def elaborate(self, platform):
         ]
 
         r_consume_buffered = Signal()
-        m.d.comb += r_consume_buffered.eq((self.r_rdy - self.r_en) & self.r_rdy)
+        m.d.comb += r_consume_buffered.eq((self.r_rdy - self.r_en) & self.r_rdy & ~fifo.r_rst)
         m.d[self._r_domain] += self.r_level.eq(fifo.r_level + r_consume_buffered)
 
         w_consume_buffered = Signal()
-        m.submodules.consume_buffered_cdc = FFSynchronizer(r_consume_buffered, w_consume_buffered, o_domain=self._w_domain, stages=4)
+        m.submodules.consume_buffered_cdc = FFSynchronizer(r_consume_buffered, w_consume_buffered,
+            o_domain=self._w_domain, stages=4, reset_less=False)
         m.d.comb += self.w_level.eq(fifo.w_level + w_consume_buffered)
 
-        with m.If(self.r_en | ~self.r_rdy):
+        with m.If(fifo.r_rst):
+            m.d[self._r_domain] += self.r_rdy.eq(0)
+        with m.Elif(self.r_en | ~self.r_rdy):
             m.d[self._r_domain] += [
                 self.r_data.eq(fifo.r_data),
                 self.r_rdy.eq(fifo.r_rdy),
-                self.r_rst.eq(fifo.r_rst),
             ]
             m.d.comb += [
                 fifo.r_en.eq(1)
             ]
 
+        m.d[self._r_domain] += self.r_rst.eq(fifo.r_rst)
+
         return m

tests/test_lib_fifo.py

@@ -357,3 +357,79 @@ def test_async_buffered_fifo_level_full(self):
     def test_async_buffered_fifo_level_empty(self):
         fifo = AsyncFIFOBuffered(width=32, depth=9, r_domain="read", w_domain="write")
         self.check_async_fifo_level(fifo, fill_in=0, expected_level=0, read=True)
+
+    def check_async_fifo_reset(self, fifo, fill_in, r_period, w_period):
+        m = Module()
+        m.submodules.fifo = fifo
+        write_rst = Signal()
+        m.d.comb += ResetSignal("write").eq(write_rst)
+        read_por_done = Signal()
+
+        def write_process():
+            while not (yield read_por_done):
+                yield
+
+            yield fifo.w_en.eq(1)
+            for _ in range(fill_in):
+                yield
+            yield fifo.w_en.eq(0)
+            yield
+
+            while not (yield fifo.r_rdy):
+                yield
+
+            yield write_rst.eq(1)
+            yield
+            yield write_rst.eq(0)
+            yield
+            yield Passive()
+            while True:
+                self.assertEqual((yield fifo.w_level), 0)
+                yield
+
+        def read_process():
+            while not (yield fifo.r_rst):
+                yield
+            while (yield fifo.r_rst):
+                yield
+            yield read_por_done.eq(1)
+
+            while not (yield fifo.r_rst):
+                yield
+            while (yield fifo.r_rst):
+                yield
+            for _ in range(10):
+                self.assertEqual((yield fifo.r_level), 0)
+                yield
+
+        simulator = Simulator(m)
+        simulator.add_clock(w_period, domain="write")
+        simulator.add_clock(r_period, domain="read")
+        simulator.add_sync_process(write_process, domain="write")
+        simulator.add_sync_process(read_process,  domain="read")
+        with simulator.write_vcd("test.vcd"):
+            simulator.run()
+
+    def test_async_fifo_reset_r10w10(self):
+        fifo = AsyncFIFO(width=1, depth=8, r_domain="read", w_domain="write")
+        self.check_async_fifo_reset(fifo, fill_in=4, r_period=10e-9, w_period=10e-9)
+
+    def test_async_fifo_reset_r10w27(self):
+        fifo = AsyncFIFO(width=1, depth=8, r_domain="read", w_domain="write")
+        self.check_async_fifo_reset(fifo, fill_in=4, r_period=10e-9, w_period=27e-9)
+
+    def test_async_fifo_reset_r27w10(self):
+        fifo = AsyncFIFO(width=1, depth=8, r_domain="read", w_domain="write")
+        self.check_async_fifo_reset(fifo, fill_in=4, r_period=27e-9, w_period=10e-9)
+
+    def test_async_buffered_fifo_reset_r10w10(self):
+        fifo = AsyncFIFOBuffered(width=1, depth=8, r_domain="read", w_domain="write")
+        self.check_async_fifo_reset(fifo, fill_in=4, r_period=10e-9, w_period=10e-9)
+
+    def test_async_buffered_fifo_reset_r10w27(self):
+        fifo = AsyncFIFOBuffered(width=1, depth=8, r_domain="read", w_domain="write")
+        self.check_async_fifo_reset(fifo, fill_in=4, r_period=10e-9, w_period=27e-9)
+
+    def test_async_buffered_fifo_reset_r27w10(self):
+        fifo = AsyncFIFOBuffered(width=1, depth=8, r_domain="read", w_domain="write")
+        self.check_async_fifo_reset(fifo, fill_in=4, r_period=27e-9, w_period=10e-9)