Dual boot ROM support, and the first proof that cellular ram works!

[fpgaboy.git] / Uart.v

`define IN_CLK 8388608
`define OUT_CLK 57600
`define CLK_DIV `IN_CLK / `OUT_CLK
`define MMAP_ADDR 16'hFF50

module UART(
	input clk,
	input wr,
	input rd,
	input [15:0] addr,
	inout [7:0] data,
	output reg serial = 1);

	reg rdlatch = 0;
	wire decode = (addr == `MMAP_ADDR);
	
	wire [7:0] odata;
	assign data = rdlatch ? odata : 8'bzzzzzzzz;

	reg [7:0] data_stor = 0;
	reg [15:0] clkdiv = 0;
	reg have_data = 0;
	reg [3:0] diqing = 4'b0000;
	
	wire newdata = (wr) && (!have_data) && decode;
	
	assign odata = have_data ? 8'b1 : 8'b0;

	always @ (posedge clk)
	begin
		rdlatch <= rd && decode;
		/* deal with diqing */
		if(newdata) begin
			data_stor <= data;
			have_data <= 1;
			diqing <= 4'b0000;
		end else if (clkdiv == 0) begin
			diqing <= diqing + 1;
			if (have_data)
				case (diqing)
				4'b0000: serial <= 0;
				4'b0001: serial <= data_stor[0];
				4'b0010: serial <= data_stor[1];
				4'b0011: serial <= data_stor[2];
				4'b0100: serial <= data_stor[3];
				4'b0101: serial <= data_stor[4];
				4'b0110: serial <= data_stor[5];
				4'b0111: serial <= data_stor[6];
				4'b1000: serial <= data_stor[7];
				4'b1001: serial <= 1;
				4'b1010: have_data <= 0;
				default: $stop;
			endcase
		end

		/* deal with clkdiv */
		if((newdata && !have_data) || clkdiv == `CLK_DIV)
			clkdiv <= 0;
		else
			clkdiv <= clkdiv + 1;
	end
endmodule