path: root/hdl/rs232.sv

`include "defs.svh"

module rs232_tx
    #(  PARITY      = 0
    ,   STOP_BITS   = 2
    ,   RTS_BITS    = 3
    )
    (   input   bit         clock
    ,   input   bit         reset

    ,   output  bit         clock_busy

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

    ,   output  bit         rxd
    ,   input   bit         rts
    );

    bit         hold_valid;
    uart_byte_t hold;
    bit         parity;

    assign      clock_busy = hold_valid;

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

    bit [$clog2($bits(uart_byte_t)):0]  data_bits;
    bit [$clog2(STOP_BITS):0]           stop_bits;

    bit [RTS_BITS-1:0]    rts_samples;

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

            rts_samples = rts_samples << 1;
            rts_samples[0] = rts;

            if (hold_valid) begin
                case (state)

                    START: begin
                        if (rts_samples == 0) begin
                            rxd = 0;
                            state = state.next;
                        end
                    end

                    DATA: begin
                        rxd = hold[0];
                        hold = hold >> 1;
                        parity = parity ^ rxd;
                        if (data_bits == `UART_BYTE_BITS-1)
                            state = state.next;
                        else
                            ++data_bits;
                    end

                    PARITY_BIT: begin
                        rxd = parity;
                        state = state.next;
                    end

                    STOP: begin
                        rxd = 1;
                        if (stop_bits == STOP_BITS-1) begin
                            hold_valid = 0;
                            state = state.next;
                        end else begin
                            ++stop_bits;
                        end
                    end

                endcase
            end else begin
                rxd = 1;
            end

            out_ready = !hold_valid;
        end
    end

endmodule

module rs232_rx
    #(  PARITY      = 0
    ,   OVERSAMPLE  = 0
    )
    (   input   bit         clock
    ,   input   bit         reset

    ,   input   bit         clock_busy
    ,   output  bit         clock_out

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

    ,   input   bit         txd
    ,   output  bit         cts
    );

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

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

    bit         [$clog2(OVERSAMPLE+1):0]    sample;
    bit         [$clog2(OVERSAMPLE+1):0]    clock_counter;
    bit         [$clog2(OVERSAMPLE+1):0]    next_clock_counter;

    always @(posedge clock, posedge reset) begin
        if (reset) begin
            clock_out = 0;
            in_valid = 0;
            cts = 1;
            state = state.first;
            buffer = 0;
            data_bits = 0;
            parity = PARITY;
            sample = 0;
            clock_counter = 0;
            next_clock_counter = 0;
        end else begin
            automatic bit ok = 0;

            if (in_ready && in_valid)
                in_valid = 0;

            if (state == state.first || state == state.last) begin
                sample = 0;
                ok = 1;
            end else begin
                ++sample;
                if (sample > OVERSAMPLE) begin
                    sample = 0;
                    ok = 1;
                end
            end

            ++clock_counter;
            if (clock_counter > OVERSAMPLE) begin
                clock_out = 1;
                clock_counter = 0;
            end else begin
                clock_out = 0;
            end

            ++next_clock_counter;
            if (next_clock_counter > OVERSAMPLE) begin
                next_clock_counter = 0;
                if (!clock_busy) begin
                    clock_out = 1;
                    clock_counter = 0;
                end
            end

            if (ok) begin
                case (state)

                    START: begin
                        if (txd == 0) begin
                            state = state.next;
                            if (OVERSAMPLE == 0)
                                state = state.next;
                            sample = (OVERSAMPLE+1) / 2;
                            buffer = 0;
                            data_bits = 0;
                            parity = PARITY;
                        end
                    end

                    ALIGN: state = state.next;

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

                    PARITY_BIT: begin
                        parity = parity ^ txd;
                        state = state.next;
                    end

                    STOP: begin
                        if (!in_valid && txd == 1 && parity == 0) begin
                            in_valid = 1;
                            in_data = buffer;
                        end
                        if (txd == 1 && parity == 0) begin
                            next_clock_counter = (OVERSAMPLE+1)/2;
                        end
                        if (txd == 1)
                            state = state.first;
                        else
                            state = state.next;
                    end

                    REALSTOP: begin
                        if (txd == 1)
                            state = state.first;
                    end

                endcase
            end

            cts = !(state == state.first && !in_valid);
        end
    end

endmodule