Gameboy APU basis

gbapu.v

module pulse_t(
  input CLK12,
  input TICK,
  input [10:0] PERIOD,
  output BIT);

  reg [10:0] counter;

  always @(posedge CLK12) begin
    if (TICK) begin
      counter <= (counter == 'd0) ? PERIOD : (counter - 'd1);
    end
  end

endmodule

module gameboy_apu_t(
  input CLK12,
  input [5:0] ADDR,
  input [7:0] DATA,
  input WR,
  output [15:0] OUTPUT);

  wire pulse0_out;
  pulse_t pulse0(CLK12, tick, pulse0_period, pulse0_out);

  // apu register file
  reg [7:0] apu_reg[48];
  wire [10:0] pulse0_period = { apu_reg[  4][2:0], apu_reg[  3][7:0] };
  wire [10:0] pulse1_period = { apu_reg[  9][2:0], apu_reg[  8][7:0] };
  wire [10:0] wave_period   = { apu_reg['he][2:0], apu_reg['hd][7:0] };

  // divide 12Mhz to ~1Mhz (12Mhz / 11)
  // target is 1048576
  // actual is 1090909
  reg [3:0] clk_div;
  wire tick = (clk_div == 0);
  always @(posedge CLK12) begin
    clk_div <= (clk_div == 0) ? 10 : clk_div - 'd1;
  end

  // update register file
  always @(posedge CLK12) begin
    if (WR && ADDR < 'd48) begin
      apu_reg[ ADDR ] = DATA;
    end
  end

endmodule