Switches Led on and off according to a clock.

led_blink_toggle.v

// Module to receive USART letters from the PC and toggle
// 4 leds.
// Rafael Arce Nazario 2017

// Israel O. Dilan Pantojas
// Jose A. Reyes Zayas
// 22-Marzo-2017
// Expirento 4 Parte B

module top
(
    input wire          clk,
    input wire          ftdi_rx,
	 output reg [3:0]  	led = 4'b0000
);

wire uart_valid;

wire [7:0] uart_data_rx;

wire letter_a, letter_s;

integer n;

integer t;

integer c;

// Combinational logic 
assign letter_a = uart_valid && (uart_data_rx == 8'h61);
assign letter_s = uart_valid && (uart_data_rx == 8'h73);

// Module instantiation
uart_rx rx0 (
	.clk(clk),
	.rst(1'b0),
	.din(ftdi_rx),
	.data_out(uart_data_rx),
	.valid(uart_valid)
);

// Whenever the signals letter_a, etc pulse then 
// toggle the corresponding output.

initial begin
	n = 0;
	t = 1;
	c = 0;
end
always @(posedge clk) begin
	begin
		c <= c + t;
		if (t <= 0) 
			begin 
				t <= 1; 
			end		
	end
	begin
		if (n == 4) begin n <= 0; end
		if (c >= 32000000)
			begin 
				led[n] <= ~led[n]; 
				n <= n+1;
				c <= 0; 
			end
	end
	begin 
		if (letter_a && t > 0) 
			begin 
				t <= t/2;
			end
		if (letter_s && t < 32000000) 
			begin 
				t <= t*2;
			end
	end
end

endmodule