hdl/rs232.sv


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`include "defs.svh"

module rs232_tx
    (   input   bit         clock
    ,   input   bit         reset

    ,   output  bit         out_ready
    ,   input   bit         out_valid
    ,   input   uart_byte_t out_data

    ,   output  bit         tx
    );

    bit         hold_valid;
    uart_byte_t hold;
    bit         parity;

    (* syn_encoding = "one-hot" *) enum int unsigned
        {   START
        ,   DATA
        ,   PARITY
        ,   STOP1
        ,   STOP2
        } state;

    bit [$clog2(`UART_BYTE_BITS):0] data_bits;

    always @(posedge clock) begin
        if (reset) begin
            out_ready = 0;
            tx = 1;
            hold_valid = 0;
            parity = 0;
            state = state.first;
            data_bits = 0;
        end else begin
            if (out_ready && out_valid) begin
                hold_valid = 1;
                hold = out_data;
                parity = 0;
                state = state.first;
                data_bits = 0;
            end

            if (hold_valid) begin
                case (state)

                    START: begin
                        tx = 0;
                        state = state.next;
                    end

                    DATA: begin
                        tx = hold[data_bits];
                        parity = parity ^ tx;
                        if (data_bits == `UART_BYTE_BITS-1)
                            state = state.next;
                        else
                            ++data_bits;
                    end

                    PARITY: begin
                        tx = parity;
                        state = state.next;
                    end

                    STOP1: begin
                        tx = 1;
                        state = state.next;
                    end

                    STOP2: begin
                        hold_valid = 0;
                        tx = 1;
                        state = state.next;
                    end

                endcase
            end else begin
                tx = 1;
            end

            out_ready = !hold_valid;
        end
    end

endmodule

module rs232_rx
    (   input   bit         clock
    ,   input   bit         reset

    ,   input   bit         in_ready
    ,   output  bit         in_valid
    ,   output  uart_byte_t in_data

    ,   input   bit         rx
    );

    (* syn_encoding = "one-hot" *) enum int unsigned
        {   START
        ,   DATA
        ,   PARITY
        ,   STOP
        } state;

    uart_byte_t                             buffer;
    bit         [$clog2(`UART_BYTE_BITS):0] data_bits;
    bit                                     parity;

    always @(posedge clock) begin
        if (reset) begin
            in_valid = 0;
            state = state.first;
            buffer = 0;
            data_bits = 0;
            parity = 0;
        end else begin
            if (in_ready && in_valid)
                in_valid = 0;

            case (state)

                START: begin
                    if (rx == 0) begin
                        state = state.next;
                        buffer = 0;
                        data_bits = 0;
                        parity = 0;
                    end
                end

                DATA: begin
                    buffer[data_bits] = rx;
                    parity = parity ^ rx;
                    if (data_bits == `UART_BYTE_BITS-1)
                        state = state.next;
                    else
                        ++data_bits;
                end

                PARITY: begin
                    parity = parity ^ rx;
                    if (parity == 0)
                        state = state.next;
                    else
                        state = state.first;
                end

                STOP: begin
                    if (!in_valid && rx == 1) begin
                        in_valid = 1;
                        in_data = buffer;
                    end
                    state = state.next;
                end

            endcase
        end
    end

endmodule