git.zerfleddert.de Git - fpga-games/blob - galaxian/src/mc_clock.v

   1 //---------------------------------------------------------------------
   2 // FPGA MOONCRESTA   CLOCK GEN
   3 //
   4 // Version : 1.00
   5 //
   6 // Copyright(c) 2004 Katsumi Degawa , All rights reserved
   7 //
   8 // Important !
   9 //
  10 // This program is freeware for non-commercial use.
  11 // An author does no guarantee about this program.
  12 // You can use this under your own risk.
  13 //
  14 //---------------------------------------------------------------------
  15
  16
  17
  18 module mc_clock(
  19
  20 I_CLK_36M,
  21 O_CLK_18M,
  22 O_CLK_12M,
  23 O_CLK_06M
  24
  25 );
  26
  27 input I_CLK_36M;
  28 output O_CLK_18M;
  29 output O_CLK_12M;
  30 output O_CLK_06M;
  31
  32 // 2/3 clock divider(duty 33%)
  33 //I_CLK   1010101010101010101
  34 //c_ff10  0011110011110011110
  35 //c_ff11  0011000011000011000
  36 //c_ff20  0000110000110000110
  37 //c_ff21  0110000110000110000
  38 //O_12M   0000110110110110110
  39 reg [1:0] state;
  40 reg clk_12m;
  41 initial state = 0;
  42 initial clk_12m = 0;
  43
  44 // 2/3 clock         (duty 66%)
  45 always @(negedge I_CLK_36M)
  46 begin
  47    case (state)
  48       2'd0: state <= 2'd1;
  49       2'd1: state <= 2'd2;
  50       2'd2: state <= 2'd0;
  51       2'd3: state <= 2'd0;
  52    endcase
  53
  54    if (state == 2'd2)
  55       clk_12m = 1;
  56    else
  57       clk_12m = 0;
  58 end
  59
  60 assign O_CLK_12M = clk_12m;
  61
  62 reg CLK_18M;
  63 always @(posedge I_CLK_36M)
  64 begin
  65    CLK_18M <= ~ CLK_18M;
  66 end
  67 assign O_CLK_18M = CLK_18M;
  68
  69 // 1/3 clock divider (duty 50%)
  70 reg CLK_6M;
  71 always @(posedge O_CLK_12M)
  72 begin
  73    CLK_6M  <= ~CLK_6M;
  74 end
  75 assign O_CLK_06M = CLK_6M;
  76
  77 endmodule