commit 9b875061ded24fbfb38b7820e7d680189abeaa0c
author Shareef Jalloq <shareef@jalloq.co.uk> Wed Jan 27 09:38:29 2021 +0000
committer Philipp Wagner <mail@philipp-wagner.com> Wed Feb 03 12:06:45 2021 +0000
tree 7cd3b81405814b3fe602868fbf423bf208ba26fb
parent 4a015f9e1e316e5ce4741214263a4ebcd485e0b9

Updates to uartdpi.sv

 - fixed bug in cycle counters where (CYCLES_PER_SYMBOL-1) should be
   used as counter end value.  Only visible when running UART at max
   baud rate for simulation test, i.e. CYCLES_PER_SYMBOL==2.
 - added initial block to set 'seen_reset' qualifier for UART receive
   start bit. This prevents spurious characters being printed when the
   rx_i pin toggles before reset.

Signed-off-by: Shareef Jalloq <shareef@jalloq.co.uk>

hw/dv/dpi/uartdpi/uartdpi.sv

diff --git a/hw/dv/dpi/uartdpi/uartdpi.sv b/hw/dv/dpi/uartdpi/uartdpi.sv
index 38a7649..a293bc4 100644
--- a/hw/dv/dpi/uartdpi/uartdpi.sv
+++ b/hw/dv/dpi/uartdpi/uartdpi.sv
@@ -16,6 +16,7 @@
   // Path to a log file. Used if none is specified through the `UARTDPI_LOG_<name>` plusarg.
   localparam string DEFAULT_LOG_FILE = {NAME, ".log"};
 
+  // Min cycles is 2 for fast test mode
   localparam int CYCLES_PER_SYMBOL = FREQ / BAUD;
 
   import "DPI-C" function
@@ -51,6 +52,7 @@
   int  txcount;
   int  txcyccount;
   reg [9:0] txsymbol;
+  reg seen_reset;
 
   always_ff @(negedge clk_i or negedge rst_ni) begin
     if (!rst_ni) begin
@@ -69,7 +71,7 @@
       end else begin
         txcyccount <= txcyccount + 1;
         tx_o <= txsymbol[txcount];
-        if (txcyccount == CYCLES_PER_SYMBOL) begin
+        if (txcyccount == CYCLES_PER_SYMBOL - 1) begin
           txcyccount <= 0;
           if (txcount == 9)
             txactive <= 0;
@@ -80,6 +82,12 @@
     end
   end
 
+
+  initial begin
+    // Prevent falling edges of rx_i before reset causing spurious characters
+    seen_reset = 0;
+  end
+
   // RX
   reg rxactive;
   int rxcount;
@@ -91,16 +99,17 @@
 
     if (!rst_ni) begin
       rxactive <= 0;
+      seen_reset <= 1;
     end else begin
       if (!rxactive) begin
-        if (!rx_i) begin
+        if (!rx_i && seen_reset) begin
           rxactive <= 1;
           rxcount <= 0;
           rxcyccount <= 0;
         end
       end else begin
         if (rxcount == 0) begin
-          if (rxcyccount == CYCLES_PER_SYMBOL/2) begin
+          if (rxcyccount == CYCLES_PER_SYMBOL/2 - 1) begin
             if (rx_i) begin
               rxactive <= 0;
             end else begin
@@ -109,13 +118,13 @@
             end
           end
         end else if (rxcount <= 8) begin
-          if (rxcyccount == CYCLES_PER_SYMBOL) begin
+          if (rxcyccount == CYCLES_PER_SYMBOL - 1) begin
             rxsymbol[rxcount-1] <= rx_i;
             rxcount <= rxcount + 1;
             rxcyccount <= 0;
           end
         end else begin
-          if (rxcyccount == CYCLES_PER_SYMBOL) begin
+          if (rxcyccount == CYCLES_PER_SYMBOL - 1) begin
             rxactive <= 0;
             if (rx_i) begin
               uartdpi_write(ctx, rxsymbol);