cxp downconn: cleanup & add reason for comma_det

src/gateware/cxp_downconn.py

@@ -286,8 +286,8 @@ class QPLL(Module):
 # Warning: Xilinx transceivers are LSB first, and comma needs to be flipped
 # compared to the usual 8b10b binary representation.
 class Comma_Detector(Module):
-    def __init__(self, comma, check_period=100_000, width=20):
-        self.data = Signal(width)
+    def __init__(self, comma, check_period=100_000):
+        self.data = Signal(20)
         self.rxinit_done = Signal()
 
         self.aligner_en_rxclk = Signal()
@@ -296,6 +296,12 @@ class Comma_Detector(Module):
 
         # # #
 
+
+
+        # The built-in RXBYTEISALIGNED can be falsely asserted at linerate higher than 5Gbps
+        # - UG476 (v1.12.1) p.228
+
+        # The validity of data & comma are checked externally
         aligned = Signal()
         valid_data = Signal()
         aligned_rxclk = Signal()
@@ -308,7 +314,6 @@ class Comma_Detector(Module):
         self.submodules.check_ps = check_ps = PulseSynchronizer("sys", "cxp_gtx_rx") 
 
         comma_n = ~comma & 0b1111111111
-
         self.sync.cxp_gtx_rx += [
             If(check_ps.o,
                 aligned_rxclk.eq(0),
@@ -327,7 +332,6 @@ class Comma_Detector(Module):
 
         check_counter = Signal(reset=check_period-1, max=check_period)
         check = Signal()
-
         self.sync += [
             check.eq(0),
             If(check_counter == 0,
@@ -338,12 +342,11 @@ class Comma_Detector(Module):
             )
         ]
 
-        self.submodules.fsm = fsm = FSM(reset_state="IDLE")
+        self.submodules.fsm = fsm = FSM(reset_state="WAIT_COMMA_ALIGN")
 
         aligner_en = Signal()
         self.specials += MultiReg(aligner_en, self.aligner_en_rxclk, odomain="cxp_gtx_rx")
-
-        fsm.act("IDLE",
+        fsm.act("WAIT_COMMA_ALIGN",
             aligner_en.eq(1),
             If(check,
                 check_ps.i.eq(1),
@@ -363,7 +366,7 @@ class Comma_Detector(Module):
                     NextState("READY"),
                 ).Else(
                     self.restart.eq(1),   
-                    NextState("IDLE"),
+                    NextState("WAIT_COMMA_ALIGN"),
                 )
             )
         )
@@ -374,7 +377,7 @@ class Comma_Detector(Module):
                 check_ps.i.eq(1),
                 If(~(aligned & valid_data),
                     self.restart.eq(1),   
-                    NextState("IDLE"),
+                    NextState("WAIT_COMMA_ALIGN"),
                 )
             )
         )
@@ -564,7 +567,7 @@ class GTX(Module):
                 # RX Byte and Word Alignment Attributes
                 p_ALIGN_COMMA_DOUBLE="FALSE",
                 p_ALIGN_COMMA_ENABLE=0b1111111111,
-                p_ALIGN_COMMA_WORD=2, # allow rxslide to shift 20 times
+                p_ALIGN_COMMA_WORD=2, # align comma to rxdata[:10] only
                 p_ALIGN_MCOMMA_DET="TRUE",
                 p_ALIGN_MCOMMA_VALUE=0b1010000011,
                 p_ALIGN_PCOMMA_DET="TRUE",
@@ -575,7 +578,7 @@ class GTX(Module):
                 p_RX_SIG_VALID_DLY=10,
                 i_RXPCOMMAALIGNEN=comma_aligner_en,
                 i_RXMCOMMAALIGNEN=comma_aligner_en,
-                i_RXCOMMADETEN=1, # enable auto word alignment
+                i_RXCOMMADETEN=1,
 
                 # RX 8B/10B Decoder Attributes
                 p_RX_DISPERR_SEQ_MATCH="FALSE",