From d6d6edfd57c3dc4db01666034d11ac84d31ec2ef Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 22 May 2022 15:42:55 -0700
Subject: Only sample RS232 signals once per clock; use a delayed flop
 internally.

This removes metastability issues on inputs and makes everything work.
---
 hdl/rs232.sv | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

(limited to 'hdl/rs232.sv')

diff --git a/hdl/rs232.sv b/hdl/rs232.sv
index 29ccb6c..d163cda 100644
--- a/hdl/rs232.sv
+++ b/hdl/rs232.sv
@@ -36,6 +36,8 @@ module rs232_tx
 
     bit [RTS_BITS-1:0]    rts_samples;
 
+    bit old_rts;
+
     always @(posedge clock, posedge reset) begin
         if (reset) begin
             out_ready = 0;
@@ -46,6 +48,7 @@ module rs232_tx
             data_bits = 0;
             stop_bits = 0;
             rts_samples = 0;
+            old_rts = 0;
         end else begin
             if (out_ready && out_valid) begin
                 hold_valid = 1;
@@ -57,7 +60,7 @@ module rs232_tx
             end
 
             rts_samples = rts_samples << 1;
-            rts_samples[0] = rts;
+            rts_samples[0] = old_rts;
 
             if (hold_valid) begin
                 case (state)
@@ -100,6 +103,8 @@ module rs232_tx
             end
 
             out_ready = !hold_valid;
+
+            old_rts = rts;
         end
     end
 
@@ -140,6 +145,8 @@ module rs232_rx
     bit         [$clog2(OVERSAMPLE+1):0]    clock_counter;
     bit         [$clog2(OVERSAMPLE+1):0]    next_clock_counter;
 
+    bit old_txd;
+
     always @(posedge clock, posedge reset) begin
         if (reset) begin
             clock_out = 0;
@@ -152,6 +159,7 @@ module rs232_rx
             sample = 0;
             clock_counter = 0;
             next_clock_counter = 0;
+            old_txd = 0;
         end else begin
             automatic bit ok = 0;
 
@@ -190,7 +198,7 @@ module rs232_rx
                 case (state)
 
                     START: begin
-                        if (txd == 0) begin
+                        if (old_txd == 0) begin
                             state = state.next;
                             if (OVERSAMPLE == 0)
                                 state = state.next;
@@ -205,8 +213,8 @@ module rs232_rx
 
                     DATA: begin
                         buffer = buffer >> 1;
-                        buffer[`UART_BYTE_BITS-1] = txd;
-                        parity = parity ^ txd;
+                        buffer[`UART_BYTE_BITS-1] = old_txd;
+                        parity = parity ^ old_txd;
                         if (data_bits == `UART_BYTE_BITS-1)
                             state = state.next;
                         else
@@ -214,26 +222,26 @@ module rs232_rx
                     end
 
                     PARITY_BIT: begin
-                        parity = parity ^ txd;
+                        parity = parity ^ old_txd;
                         state = state.next;
                     end
 
                     STOP: begin
-                        if (!in_valid && txd == 1 && parity == 0) begin
+                        if (!in_valid && old_txd == 1 && parity == 0) begin
                             in_valid = 1;
                             in_data = buffer;
                         end
-                        if (txd == 1 && parity == 0) begin
+                        if (old_txd == 1 && parity == 0) begin
                             next_clock_counter = (OVERSAMPLE+1)/2;
                         end
-                        if (txd == 1)
+                        if (old_txd == 1)
                             state = state.first;
                         else
                             state = state.next;
                     end
 
                     REALSTOP: begin
-                        if (txd == 1)
+                        if (old_txd == 1)
                             state = state.first;
                     end
 
@@ -241,6 +249,8 @@ module rs232_rx
             end
 
             cts = !(state == state.first && !in_valid);
+
+            old_txd = txd;
         end
     end
 
-- 
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