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爱被打了一巴掌 2023-02-15 01:18 25阅读 0赞

//------------------------------------------------------------------------------
    // File       : gig_ethernet_pcs_pma_0_johnson_cntr.v
    // Author     : Xilinx Inc.
    //------------------------------------------------------------------------------
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    //
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    // WITH ALL FAULTS, AND XILINX HEREBY DISCLAIMS ALL WARRANTIES
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    // 
    // 
    //------------------------------------------------------------------------------
    //  Description:  This logic describes a standard johnson counter to
    //                create divided down clocks.  A divide by 10 clock is
    //                created.
    //
    //                The capabilities of this Johnson counter are extended
    //                with the use of the clock enables - it is only the
    //                clock-enabled cycles which are divided down.
    //
    //                The divide by 10 clock is output directly from a rising
    //                edge triggered flip-flop (clocked on the input clk).
    
    
    `timescale 1 ps/1 ps
    
    
    //------------------------------------------------------------------------------
    // Module declaration.
    //-----------------------------------------------------------------------------
    
    module gig_ethernet_pcs_pma_0_johnson_cntr
      (
        input  reset,           // Synchronous Reset
        input  clk,             // Input clock (always at 125MHz)
        input  clk_en,          // Clock enable for rising edge triggered flip flops
        output clk_div10        // (Clock, gated with clock enable) divide by 10
      );
    
    
    
      //----------------------------------------------------------------------------
      // internal signals used in this top level wrapper.
      //----------------------------------------------------------------------------
    
    
      reg reg1;            // first flip flop
      reg reg2;            // second flip flop
      reg reg3;            // third flip flop
      reg reg4;            // fourth flip flop
      reg reg5;            // fifth flip flop
    
    
    
      // Create a 5-stage shift register
      always @(posedge clk)
      begin
        if (reset == 1'b1) begin
          reg1 <= 1'b0;
          reg2 <= 1'b0;
          reg3 <= 1'b0;
          reg4 <= 1'b0;
          reg5 <= 1'b0;
        end
        else begin
          if (clk_en == 1'b1) begin
            if (reg5 == 1'b1 && reg4 == 1'b0) begin  // ensure that LFSR self corrects on every repetition
              reg1 <= 1'b0;
              reg2 <= 1'b0;
              reg3 <= 1'b0;
              reg4 <= 1'b0;
              reg5 <= 1'b0;
            end
            else begin
              reg1 <= !reg5;
              reg2 <= reg1;
              reg3 <= reg2;
              reg4 <= reg3;
              reg5 <= reg4;
            end
          end
        end
      end
    
    
    
      // The 5-stage shift register causes reg3 to toggle every 5 clock
      // enabled cycles, effectively creating a divide by 10 clock
      assign clk_div10 = reg3;
    
    
    
    endmodule