First cut at timer

[fpgaboy.git] / Timer.v

`define ADDR_DIV  16'hFF04
`define ADDR_TIMA 16'hFF05
`define ADDR_TMA  16'hFF06
`define ADDR_TAC  16'hFF07

module Timer(
	input clk,
	input wr,
	input rd,
	input [15:0] addr,
	inout [7:0] data,
	output reg irq);

	reg [7:0] tima = 0, tma = 0, tac = 0, div = 0;
	reg ovf = 0;
	reg [9:0] clkdv;

	wire is_tima = addr == `ADDR_TIMA;
	wire is_tma  = addr == `ADDR_TMA;
	wire is_tac  = addr == `ADDR_TAC;

	assign data = rd ?
	                 is_tima ? tima :
	                 is_tma ? tma :
	                 is_tac ? tac :
	                 8'bzzzzzzzz :
	              8'bzzzzzzzz;

	wire cksel = tac[2] ?
	                (tac[1:0] == 2'b00) ? (clkdv == 10'b0)     :
	                (tac[1:0] == 2'b01) ? (clkdv[3:0] == 4'b0) :
	                (tac[1:0] == 2'b10) ? (clkdv[5:0] == 6'b0) :
	                (clkdv[7:0] == 8'b0) :
	             0;

	always @ (negedge clk) 
	begin
		if(wr) begin
			case(addr)
			`ADDR_DIV: div <= 8'b0;
			`ADDR_TIMA: tima <= data;
			`ADDR_TMA: tma <= data;
			`ADDR_TAC: tac <= data;
			endcase
		end
		else begin
			if(ovf) begin
				tima <= tma;
				ovf <= 0;
				irq <= 1;
			end
			else begin
				if(cksel)
					{ovf,tima} <= {1'b0,tima} + 1;
				if(irq)
					irq <= 0;
			end

			if(clkdv[7:0] == 8'b0)
				div <= div + 1;
		end
		clkdv <= clkdv + 1;
	end

endmodule