//--------------------------------------------------------------------------------
// UartRxExtreme.v
// Konstantin Pavlov, pavlovconst@gmail.com
//--------------------------------------------------------------------------------

// INFO --------------------------------------------------------------------------------
//  Extreme minimal UART receiver optimized for 20MHz/115200 data rate
//  
// CAUTION:
// optimized for 20MHz/115200
// rx_sample_cntr[7:0] does never stop, but reloads on sequense start condition
// initial rx_sample_cntr[7:0] value has been made 255 instead of 257 to save one precious counter register
// rx_busy and rx_done fall simultaneously 0,5 bit before stop bit time end

/* --- INSTANTIATION TEMPLATE BEGIN ---

UartRxExtreme UR1 (
    .clk(),
	.rx_data(),
	.rx_busy(),
	.rx_done(),
	.rxd()
    );

--- INSTANTIATION TEMPLATE END ---*/


module UartRxExtreme(clk, rx_data, rx_busy, rx_done, rxd);


input wire clk;

output reg [7:0] rx_data = 0;
reg rx_data_9th_bit = 0;	// {rx_data[7:0],rx_data_9th_bit} is actually a shift register

output reg rx_busy = 0;     // sequence control is done by rx_busy and unique high logic state of rx_data_9th_bit register
output wire rx_done;
input wire rxd;


// Falling edge detector
reg rxd_prev = 0;
always @ (posedge clk) begin
	rxd_prev <= rxd;
end
wire start_bit_strobe = ~rx_busy && (~rxd & rxd_prev);


// Sample counter
reg [7:0] rx_sample_cntr = 0;
always @ (posedge clk) begin
	if (start_bit_strobe) begin
		rx_sample_cntr[7:0] <= (86 * 3 - 1) - 2;
	end else begin
		if (rx_sample_cntr[7:0] == 0) begin
			rx_sample_cntr[7:0] <= (86 * 2 - 1);
		end else begin
			rx_sample_cntr[7:0] <= rx_sample_cntr[7:0] - 1;
		end // rx_sample_cntr
	end // start_bit_strobe
end
wire rx_do_sample = (rx_sample_cntr[7:0] == 0);

// Data shifting
always @ (posedge clk) begin
	if (start_bit_strobe) begin
		{rx_data[7:0],rx_data_9th_bit} <= 9'b100000000;
		rx_busy <= 1;
	end // start_bit_strobe

	if (rx_busy && rx_do_sample) begin
		if (rx_data_9th_bit) begin
		      rx_busy <= 0;
		end else begin
		      {rx_data[7:0],rx_data_9th_bit} <= {rxd,rx_data[7:0]};
		end
	end // (rx_busy && rx_do_sample)
end

assign
	rx_done = (rx_busy && rx_do_sample && rx_data_9th_bit) && rxd;
	
endmodule