module POSLink(
	input xtal,
	input [3:0] tos_inputs_e2,
	input serial_e2,
	input [1:0] buttons,
	output tos_output,
	output reg data_output,
	output reg [3:0] anode,
	output reg [7:0] cathode);
	
	reg [3:0] tos_inputs_e;
	reg [3:0] tos_inputs;
	reg serial_e;
	reg serial;

	wire [3:0] tos_good;

	/* Synchronize inputs */
	always @(posedge xtal) begin
		tos_inputs_e <= tos_inputs_e2;
		tos_inputs <= tos_inputs_e;
		serial_e <= serial_e2;
		serial <= serial_e;
	end

	wire [11:0] data;
	wire data_good;

	reg [1:0] seg;

	always @(*)
		case (seg)
		2'b00:	anode = 4'b0111;
		2'b01:	anode = 4'b1011;
		2'b10:	anode = 4'b1101;
		2'b11:	anode = 4'b1110;
		endcase

	reg [1:0] tos_select;

	wire [3:0] current_bit;

	assign tos_output = tos_inputs_e2[tos_select];

	always @(*) begin
		cathode = data[7:0];
		seg = data[9:8];
		tos_select = data[11:10];
	end
	
	wire [5:0] output_stuff = { buttons, tos_good };

	always @(*)
		data_output = output_stuff[current_bit[2:0]];

	TOS_Detect detect[3:0](.xtal(xtal), .tos_input(tos_inputs), .tos_good(tos_good));
	POS_Serial serinput(.xtal(xtal), .serial(serial), .data_reg(data), .current_bit(current_bit), .data_good(data_good));
endmodule


module POS_Serial(
	input xtal,
	input serial,
	output reg [11:0] data_reg = 0,
	output reg [3:0] current_bit = 0,
	output reg data_good = 0);

	reg serial_1a;

	always @(posedge xtal)
		serial_1a = serial;

	wire edge_detect = serial ^ serial_1a;

	reg [4:0] edge_counter = 0;

	always @(posedge xtal) begin
		data_good = 0;

		if (edge_detect) begin
			if (edge_counter == 31) begin
				current_bit = 0;
				data_reg = 0;
			end else begin
		//		data_reg[11:1] = data_reg[10:0];
		//		data_reg[0] = (edge_counter > 20);
				data_reg[current_bit] = (edge_counter > 20);
				if (current_bit == 11) begin
					data_good = 1;
					current_bit = 0;
				end else
					current_bit = current_bit + 1;
			end

			edge_counter = 0;
		end else begin
			if (edge_counter != 31)
				edge_counter = edge_counter + 1;
		end
	end
endmodule


/* xtal: 25MHz (==40ns)
 * Minimum: 100ns
 *          2 cycles (80 ns)
 * Maximum: 1000ns
 *          25 cycles (we'll allow 30 = 1200ns for good measure)
 */
module TOS_Detect(
	input xtal,
	input tos_input,
	output reg tos_good = 0);
	
	reg tos_input_1a = 0;
	always @(posedge xtal)
		tos_input_1a <= tos_input;
	wire transition = tos_input ^ tos_input_1a;
	
	reg [3:0] lasttx = 0;
	always @(posedge xtal) begin
		if (transition) begin
			if (lasttx < 2) /* Too soon! */
				tos_good <= 0;
			else if (lasttx > 30) /* Too late! */
				tos_good <= 0;
			else /* OK by me. */
				tos_good <= 1;
			lasttx <= 0;
		end else begin
			if (lasttx != 31)
				lasttx <= lasttx + 1;
			else
				tos_good <= 0;
		end
	end
endmodule