Subversion Repositories connect-6

module async_receiver(clk, RxD, RxD_data_ready, RxD_data, RxD_endofpacket, RxD_idle);

input clk, RxD;

output RxD_data_ready;  // onc clock pulse when RxD_data is valid

output [7:0] RxD_data;

//parameter ClkFrequency = 62500000; // 50MHz

parameter ClkFrequency = 20000000; // 50MHz

//parameter ClkFrequency = 27000000; // 27MHz

parameter Baud = 115200;

// 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_endofpacket;  // one clock pulse, when no more data is received (RxD_idle is going high)

output RxD_idle;  // no data is being received

// Baud generator (we use 8 times oversampling)

parameter Baud8 = Baud*8;

parameter Baud8GeneratorAccWidth = 16;

parameter Baud8GeneratorInc = ((Baud8<<(Baud8GeneratorAccWidth-7))+(ClkFrequency>>8))/(ClkFrequency>>7);

reg [Baud8GeneratorAccWidth:0] Baud8GeneratorAcc;

always @(posedge clk) Baud8GeneratorAcc <= Baud8GeneratorAcc[Baud8GeneratorAccWidth-1:0] + Baud8GeneratorInc;

wire Baud8Tick = Baud8GeneratorAcc[Baud8GeneratorAccWidth];

////////////////////////////

reg [1:0] RxD_sync_inv;

always @(posedge clk) if(Baud8Tick) RxD_sync_inv <= {RxD_sync_inv[0], ~RxD};

// we invert RxD, so that the idle becomes "0", to prevent a phantom character to be received at startup

reg [1:0] RxD_cnt_inv;

reg RxD_bit_inv;

always @(posedge clk)

if(Baud8Tick)

begin

  if( RxD_sync_inv[1] && RxD_cnt_inv!=2'b11) RxD_cnt_inv <= RxD_cnt_inv + 1;

  else

  if(~RxD_sync_inv[1] && RxD_cnt_inv!=2'b00) RxD_cnt_inv <= RxD_cnt_inv - 1;

  if(RxD_cnt_inv==2'b00) RxD_bit_inv <= 0;

  else

  if(RxD_cnt_inv==2'b11) RxD_bit_inv <= 1;

end

reg [3:0] state;

reg [3:0] bit_spacing;

// "next_bit" controls when the data sampling occurs

// depending on how noisy the RxD is, different values might work better

// with a clean connection, values from 8 to 11 work

wire next_bit = (bit_spacing==10);

always @(posedge clk)

if(state==0)

  bit_spacing <= 0;

else

if(Baud8Tick)

  bit_spacing <= {bit_spacing[2:0] + 1} | {bit_spacing[3], 3'b000};

always @(posedge clk)

if(Baud8Tick)

case(state)

  4'b0000: if(RxD_bit_inv) state <= 4'b1000;  // start bit found?

  4'b1000: if(next_bit) state <= 4'b1001;  // bit 0

  4'b1001: if(next_bit) state <= 4'b1010;  // bit 1

  4'b1010: if(next_bit) state <= 4'b1011;  // bit 2

  4'b1011: if(next_bit) state <= 4'b1100;  // bit 3

  4'b1100: if(next_bit) state <= 4'b1101;  // bit 4

  4'b1101: if(next_bit) state <= 4'b1110;  // bit 5

  4'b1110: if(next_bit) state <= 4'b1111;  // bit 6

  4'b1111: if(next_bit) state <= 4'b0001;  // bit 7

  4'b0001: if(next_bit) state <= 4'b0000;  // stop bit

  default: state <= 4'b0000;

endcase

reg [7:0] RxD_data;

always @(posedge clk)

if(Baud8Tick && next_bit && state[3]) RxD_data <= {~RxD_bit_inv, RxD_data[7:1]};

reg RxD_data_ready, RxD_data_error;

always @(posedge clk)

begin

  RxD_data_ready <= (Baud8Tick && next_bit && state==4'b0001 && ~RxD_bit_inv);  // ready only if the stop bit is received

  RxD_data_error <= (Baud8Tick && next_bit && state==4'b0001 &&  RxD_bit_inv);  // error if the stop bit is not received

end

reg [4:0] gap_count;

always @(posedge clk) if (state!=0) gap_count<=0; else if(Baud8Tick & ~gap_count[4]) gap_count <= gap_count + 1;

assign RxD_idle = gap_count[4];

reg RxD_endofpacket; always @(posedge clk) RxD_endofpacket <= Baud8Tick & (gap_count==15);

endmodule