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   1
   2
   3 module serdes_fc_rx
   4   #(parameter LWMARK = 64,
   5     parameter HWMARK = 320)
   6     (input clk, input rst,
   7      input [15:0] fifo_space,
   8      output reg send_xon,
   9      output reg send_xoff,
  10      input sent);
  11
  12    reg [15:0]     countdown;
  13    reg            send_xon_int, send_xoff_int;
  14
  15    always @(posedge clk)
  16      if(rst)
  17        begin
  18           send_xon_int <= 0;
  19           send_xoff_int <= 0;
  20           countdown <= 0;
  21        end
  22      else
  23        begin
  24           send_xon_int <= 0;
  25           send_xoff_int <= 0;
  26           if(countdown == 0)
  27             if(fifo_space < LWMARK)
  28               begin
  29                  send_xoff_int <= 1;
  30                  countdown <= 240;
  31               end
  32             else
  33               ;
  34           else
  35             if(fifo_space > HWMARK)
  36               begin
  37                  send_xon_int <= 1;
  38                  countdown <= 0;
  39               end
  40             else
  41               countdown <= countdown - 1;
  42        end // else: !if(rst)
  43
  44    // If we are between the high and low water marks, we let the countdown expire
  45
  46    always @(posedge clk)
  47      if(rst)
  48        send_xon <= 0;
  49      else if(send_xon_int)
  50        send_xon <= 1;
  51      else if(sent)
  52        send_xon <= 0;
  53
  54    always @(posedge clk)
  55      if(rst)
  56        send_xoff <= 0;
  57      else if(send_xoff_int)
  58        send_xoff <= 1;
  59      else if(sent)
  60        send_xoff <= 0;
  61
  62 endmodule // serdes_fc_rx