Subversion Repositories a-z80

// Simple transmit-only UART model

module uart #(

    parameter BAUD = 115200,

    parameter IN_CLOCK = 50000000)

(

    // Outputs

    output wire busy,          // Set when busy transmitting

    output reg uart_tx,        // UART transmit wire

    // Inputs

    input wire wr,             // Write a new byte to transmit

    input wire [7:0] data,     // 8-bit data

    input wire clk,

    input wire reset

);

reg [3:0] bitcount;

reg [8:0] shifter;

assign busy = |bitcount[3:1];

wire sending = |bitcount;

// Calculate UART clock based on the input clock

reg [28:0] d;

wire [28:0] inc = d[28] ? (BAUD) : (BAUD - IN_CLOCK);

wire [28:0] delta = d + inc;

always @(posedge clk)

begin

    if (reset)

    begin

        d = 0;

    end else

    begin

        d = delta;

    end

end

wire ser_clk = ~d[28]; // UART clock

always @(posedge clk) begin

    if (reset)

    begin

        uart_tx <= 1;

        bitcount <= 0;

        shifter <= 0;

    end else

    begin

        if (wr & ~busy)

        begin

            $strobe("[UART] %c", data[7:0]);

            shifter <= { data[7:0], 1'h0 };

            bitcount <= 4'd11; // 1 + 8 + 2

        end

        if (sending & ser_clk)

        begin

            { shifter, uart_tx } <= { 1'h1, shifter };

            bitcount <= bitcount - 4'd1;

        end

    end

end

endmodule