module async_receiver(
	input clk,
	input RxD,
	output reg RxD_data_ready = 0,
	output reg [7:0] RxD_data = 0,  // data received, valid only (for one clock cycle) when RxD_data_ready is asserted

	// We also detect if a gap occurs in the received stream of characters
	// That can be useful if multiple characters are sent in burst
	//  so that multiple characters can be treated as a "packet"
	output RxD_idle,  // asserted when no data has been received for a while
	output reg RxD_endofpacket = 0  // asserted for one clock cycle when a packet has been detected (i.e. RxD_idle is going high)
);

parameter ClkFrequency = 50000000; // 25MHz
parameter Baud = 115200;

parameter Oversampling = 8;  // needs to be a power of 2
// we oversample the RxD line at a fixed rate to capture each RxD data bit at the "right" time
// 8 times oversampling by default, use 16 for higher quality reception

generate
	if(ClkFrequency<Baud*Oversampling) ASSERTION_ERROR PARAMETER_OUT_OF_RANGE("Frequency too low for current Baud rate and oversampling");
	if(Oversampling<8 || ((Oversampling & (Oversampling-1))!=0)) ASSERTION_ERROR PARAMETER_OUT_OF_RANGE("Invalid oversampling value");
endgenerate

////////////////////////////////
reg [3:0] RxD_state = 0;

`ifdef SIMULATION
wire RxD_bit = RxD;
wire sampleNow = 1'b1;  // receive one bit per clock cycle

`else
wire OversamplingTick;
BaudTickGen #(ClkFrequency, Baud, Oversampling) tickgen(.clk(clk), .enable(1'b1), .tick(OversamplingTick));

// synchronize RxD to our clk domain
reg [1:0] RxD_sync = 2'b11;
always @(posedge clk) if(OversamplingTick) RxD_sync <= {RxD_sync[0], RxD};

// and filter it
reg [1:0] Filter_cnt = 2'b11;
reg RxD_bit = 1'b1;

always @(posedge clk)
if(OversamplingTick)
begin
	if(RxD_sync[1]==1'b1 && Filter_cnt!=2'b11) Filter_cnt <= Filter_cnt + 1'd1;
	else 
	if(RxD_sync[1]==1'b0 && Filter_cnt!=2'b00) Filter_cnt <= Filter_cnt - 1'd1;

	if(Filter_cnt==2'b11) RxD_bit <= 1'b1;
	else
	if(Filter_cnt==2'b00) RxD_bit <= 1'b0;
end

// and decide when is the good time to sample the RxD line
function integer log2(input integer v); begin log2=0; while(v>>log2) log2=log2+1; end endfunction
localparam l2o = log2(Oversampling);
reg [l2o-2:0] OversamplingCnt = 0;
always @(posedge clk) if(OversamplingTick) OversamplingCnt <= (RxD_state==0) ? 1'd0 : OversamplingCnt + 1'd1;
wire sampleNow = OversamplingTick && (OversamplingCnt==Oversampling/2-1);
`endif

// now we can accumulate the RxD bits in a shift-register
always @(posedge clk)
case(RxD_state)
	4'b0000: if(~RxD_bit) RxD_state <= `ifdef SIMULATION 4'b1000 `else 4'b0001 `endif;  // start bit found?
	4'b0001: if(sampleNow) RxD_state <= 4'b1000;  // sync start bit to sampleNow
	4'b1000: if(sampleNow) RxD_state <= 4'b1001;  // bit 0
	4'b1001: if(sampleNow) RxD_state <= 4'b1010;  // bit 1
	4'b1010: if(sampleNow) RxD_state <= 4'b1011;  // bit 2
	4'b1011: if(sampleNow) RxD_state <= 4'b1100;  // bit 3
	4'b1100: if(sampleNow) RxD_state <= 4'b1101;  // bit 4
	4'b1101: if(sampleNow) RxD_state <= 4'b1110;  // bit 5
	4'b1110: if(sampleNow) RxD_state <= 4'b1111;  // bit 6
	4'b1111: if(sampleNow) RxD_state <= 4'b0010;  // bit 7
	4'b0010: if(sampleNow) RxD_state <= 4'b0000;  // stop bit
	default: RxD_state <= 4'b0000;
endcase

always @(posedge clk)
if(sampleNow && RxD_state[3]) RxD_data <= {RxD_bit, RxD_data[7:1]};

//reg RxD_data_error = 0;
always @(posedge clk)
begin
	RxD_data_ready <= (sampleNow && RxD_state==4'b0010 && RxD_bit);  // make sure a stop bit is received
	//RxD_data_error <= (sampleNow && RxD_state==4'b0010 && ~RxD_bit);  // error if a stop bit is not received
end

`ifdef SIMULATION
assign RxD_idle = 0;
`else
reg [l2o+1:0] GapCnt = 0;
always @(posedge clk) if (RxD_state!=0) GapCnt<=0; else if(OversamplingTick & ~GapCnt[log2(Oversampling)+1]) GapCnt <= GapCnt + 1'h1;
assign RxD_idle = GapCnt[l2o+1];
always @(posedge clk) RxD_endofpacket <= OversamplingTick & ~GapCnt[l2o+1] & &GapCnt[l2o:0];
`endif

endmodule