phy/rmii/LiteEthPHYRMIIRX: Avoid FSM, simplify and add comments.

liteeth/phy/rmii.py

@@ -40,6 +40,7 @@ class LiteEthPHYRMIITX(LiteXModule):
         for i in range(2):
             self.specials += SDROutput(i=converter.source.data[i], o=pads.tx_data[i])
 
+
 # LiteEth PHY RMII RX ------------------------------------------------------------------------------
 
 class LiteEthPHYRMIIRX(LiteXModule):
@@ -62,6 +63,8 @@ class LiteEthPHYRMIIRX(LiteXModule):
 
         # Delay.
         # ------
+        # Add a delay to align the data with the frame boundaries since the end-of-frame condition
+        # (2 consecutive `crs_dv` signals low) is detected with a few cycles delay.
         self.delay = delay = stream.Delay(layout=[("data", 8)], n=2)
 
         # Frame Delimitation.
@@ -69,31 +72,30 @@ class LiteEthPHYRMIIRX(LiteXModule):
         crs_dv_d  = Signal()
         crs_first = Signal()
         crs_last  = Signal()
+        crs_run   = Signal()
         self.sync += crs_dv_d.eq(crs_dv)
         self.comb += [
-            crs_first.eq(crs_dv & (rx_data != 0b00)), # Start of frame on crs_dv at 1 and non-null data.
-            crs_last.eq(~crs_dv & ~crs_dv_d),         # End of frame on 2 consecutives crs_dv at 0.
+            crs_first.eq(crs_dv & (rx_data != 0b00)), # Start of frame on crs_dv high and non-null data.
+            crs_last.eq(~crs_dv & ~crs_dv_d),         # End of frame on 2 consecutive crs_dv low.
+        ]
+        self.sync += [
+            If(crs_first, crs_run.eq(1)),
+            If(crs_last,  crs_run.eq(0)),
         ]
 
-        self.fsm = fsm = FSM(reset_state="IDLE")
-        fsm.act("IDLE",
-            delay.source.ready.eq(1),
-            If(crs_first,
-                delay.sink.valid.eq(1),
+        # Datapath: Input -> Delay -> Converter -> Source.
+        # ------------------------------------------------
+        self.comb += [
+            delay.source.ready.eq(1), # Ready by default to flush pipeline.
+            delay.sink.valid.eq(crs_first | crs_run),
             delay.sink.data.eq(rx_data),
-                NextState("RECEIVE")
-            )
-        )
-        fsm.act("RECEIVE",
-            delay.sink.valid.eq(1),
-            delay.sink.data.eq(rx_data),
-            delay.source.connect(converter.sink),
-            If(crs_last,
-                converter.sink.last.eq(1),
-                NextState("IDLE")
-            )
-        )
-        self.comb += converter.source.connect(source)
+            If(crs_run,
+                converter.sink.last.eq(crs_last),
+                delay.source.connect(converter.sink, keep={"valid", "ready", "data"})
+            ),
+            converter.source.connect(source),
+        ]
+
 
 # LiteEth PHY RMII CRG -----------------------------------------------------------------------------