From febc5e2a684312f9a050bcf58fd13b2b42f38047 Mon Sep 17 00:00:00 2001
From: Josh Blum <josh@joshknows.com>
Date: Mon, 16 Jul 2012 20:33:07 -0700
Subject: gpmc: tighter timing constraints and easier to route gpmc to fifo

---
 usrp2/gpmc/gpmc_to_fifo.v | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

(limited to 'usrp2/gpmc/gpmc_to_fifo.v')

diff --git a/usrp2/gpmc/gpmc_to_fifo.v b/usrp2/gpmc/gpmc_to_fifo.v
index cfc5aaa8b..043ec9b6b 100644
--- a/usrp2/gpmc/gpmc_to_fifo.v
+++ b/usrp2/gpmc/gpmc_to_fifo.v
@@ -1,5 +1,5 @@
 //
-// Copyright 2011 Ettus Research LLC
+// Copyright 2011-2012 Ettus Research LLC
 //
 // This program is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by
@@ -22,7 +22,7 @@
 // The GPMC is asynchronously alerted when a BRAM page is available.
 //
 // EM_CLK:
-// A GPMC read transaction consists of one EM_CLK cycle (idle low).
+// A GPMC write transaction consists of one EM_CLK cycle (idle low).
 //
 // EM_WE:
 // Write enable is actually the combination of ~NWE & ~NCS.
@@ -45,17 +45,20 @@ module gpmc_to_fifo
     //states for the GPMC side of things
     wire [17:0] data_i;
     reg gpmc_state;
-    reg [ADDR_WIDTH:1] last_addr;
+    reg [15:0] vita_len;
+    reg [ADDR_WIDTH:1] addr;
+    wire [ADDR_WIDTH:1] last_addr = {vita_len[ADDR_WIDTH-2:0], 1'b0} - 1'b1 + 2;
     reg [PTR_WIDTH:0] gpmc_ptr, next_gpmc_ptr;
     localparam GPMC_STATE_START = 0;
     localparam GPMC_STATE_FILL = 1;
 
     //states for the FIFO side of things
-    reg fifo_state;
+    reg [1:0] fifo_state;
     reg [ADDR_WIDTH-1:0] counter;
     reg [PTR_WIDTH:0] fifo_ptr;
     localparam FIFO_STATE_CLAIM = 0;
     localparam FIFO_STATE_EMPTY = 1;
+    localparam FIFO_STATE_PRE = 2;
 
     //------------------------------------------------------------------
     // State machine to control the data from GPMC to BRAM
@@ -65,14 +68,16 @@ module gpmc_to_fifo
             gpmc_state <= GPMC_STATE_START;
             gpmc_ptr <= 0;
             next_gpmc_ptr <= 0;
+            addr <= 0;
         end
         else if (EM_WE) begin
+            addr <= EM_A + 1;
             case(gpmc_state)
 
             GPMC_STATE_START: begin
                 if (EM_A == 2) begin
                     gpmc_state <= GPMC_STATE_FILL;
-                    last_addr <= {EM_D[ADDR_WIDTH-2:0], 1'b0} - 1'b1 + 2;
+                    vita_len <= EM_D;
                     next_gpmc_ptr <= gpmc_ptr + 1;
                 end
             end
@@ -81,6 +86,7 @@ module gpmc_to_fifo
                 if (data_i[17]) begin
                     gpmc_state <= GPMC_STATE_START;
                     gpmc_ptr <= next_gpmc_ptr;
+                    addr <= 0;
                 end
             end
 
@@ -105,6 +111,7 @@ module gpmc_to_fifo
     cross_clock_reader #(.WIDTH(PTR_WIDTH+1)) read_next_gpmc_ptr
         (.clk(clk), .rst(reset | clear), .in(next_gpmc_ptr), .out(safe_next_gpmc_ptr));
 
+    wire [PTR_WIDTH:0] fifo_ptr_next = fifo_ptr + 1;
     always @(posedge clk)
         if (reset | clear) have_space <= 0;
         else               have_space <= (fifo_ptr ^ (1 << PTR_WIDTH)) != safe_next_gpmc_ptr;
@@ -122,16 +129,22 @@ module gpmc_to_fifo
             case(fifo_state)
 
             FIFO_STATE_CLAIM: begin
-                if (bram_available_to_empty) fifo_state <= FIFO_STATE_EMPTY;
+                if (bram_available_to_empty && data_o[16]) fifo_state <= FIFO_STATE_PRE;
                 counter <= 2;
             end
 
+            FIFO_STATE_PRE: begin
+                fifo_state <= FIFO_STATE_EMPTY;
+                counter <= counter + 1;
+            end
+
             FIFO_STATE_EMPTY: begin
                 if (src_rdy_o && dst_rdy_i && data_o[17]) begin
                     fifo_state <= FIFO_STATE_CLAIM;
                     fifo_ptr <= fifo_ptr + 1;
+                    counter <= 2;
                 end
-                if (src_rdy_o && dst_rdy_i) begin
+                else if (src_rdy_o && dst_rdy_i) begin
                     counter <= counter + 1;
                 end
             end
@@ -140,18 +153,20 @@ module gpmc_to_fifo
         end
     end //always
 
+    wire enable = (fifo_state != FIFO_STATE_EMPTY) || dst_rdy_i;
+
     assign src_rdy_o = fifo_state == FIFO_STATE_EMPTY;
 
     //assign data and frame bits to bram input
     assign data_i[15:0] = EM_D;
-    assign data_i[16] = (gpmc_state == GPMC_STATE_START);
-    assign data_i[17] = (EM_A == last_addr);
+    assign data_i[16] = (addr == 2);
+    assign data_i[17] = (addr == last_addr);
 
     //instantiate dual ported bram for async read + write
     ram_2port #(.DWIDTH(18),.AWIDTH(PTR_WIDTH + ADDR_WIDTH)) async_fifo_bram
      (.clka(~EM_CLK),.ena(1'b1),.wea(EM_WE),
-      .addra({gpmc_ptr[PTR_WIDTH-1:0], EM_A}),.dia(data_i),.doa(),
-      .clkb(~clk),.enb(1'b1),.web(1'b0),
+      .addra({gpmc_ptr[PTR_WIDTH-1:0], addr}),.dia(data_i),.doa(),
+      .clkb(clk),.enb(enable),.web(1'b0),
       .addrb({fifo_ptr[PTR_WIDTH-1:0], counter}),.dib(18'h3ffff),.dob(data_o));
 
 endmodule // gpmc_to_fifo
-- 
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