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`timescale 1ns / 1ps

// Documented Verilog UART

// Copyright (C) 2010 Timothy Goddard (tim@goddard.net.nz)

// Distributed under the MIT licence.

//

// Permission is hereby granted, free of charge, to any person obtaining a copy

// of this software and associated documentation files (the "Software"), to deal

// in the Software without restriction, including without limitation the rights

// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

// copies of the Software, and to permit persons to whom the Software is

// furnished to do so, subject to the following conditions:

// 

// The above copyright notice and this permission notice shall be included in

// all copies or substantial portions of the Software.

// 

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN

// THE SOFTWARE.

// 

module uart(

    input clk, // The master clock for this module

    input rst, // Synchronous reset.

    input rx, // Incoming serial line

    output tx, // Outgoing serial line

    input transmit, // Signal to transmit

    input [7:0] tx_byte, // Byte to transmit

    output received, // Indicated that a byte has been received.

    output [7:0] rx_byte, // Byte received

    output is_receiving, // Low when receive line is idle.

    output is_transmitting, // Low when transmit line is idle.

    output recv_error // Indicates error in receiving packet.

    );

parameter CLOCK_DIVIDE = 325; // clock rate (50Mhz) / baud rate (9600) / 16

// States for the receiving state machine.

// These are just constants, not parameters to override.

parameter RX_IDLE = 0;

parameter RX_CHECK_START = 1;

parameter RX_READ_BITS = 2;

parameter RX_CHECK_STOP = 3;

parameter RX_DELAY_RESTART = 4;

parameter RX_ERROR = 5;

parameter RX_RECEIVED = 6;

// States for the transmitting state machine.

// Constants - do not override.

parameter TX_IDLE = 0;

parameter TX_SENDING = 1;

parameter TX_DELAY_RESTART = 2;

reg [10:0] clk_divider = CLOCK_DIVIDE;

reg [2:0] recv_state = RX_IDLE;

reg [5:0] rx_countdown;

reg [3:0] rx_bits_remaining;

reg [7:0] rx_data;

reg tx_out = 1'b1;

reg [1:0] tx_state = TX_IDLE;

reg [5:0] tx_countdown;

reg [3:0] tx_bits_remaining;

reg [7:0] tx_data;

assign received = recv_state == RX_RECEIVED;

assign recv_error = recv_state == RX_ERROR;

assign is_receiving = recv_state != RX_IDLE;

assign rx_byte = rx_data;

assign tx = tx_out;

assign is_transmitting = tx_state != TX_IDLE;

always @(posedge clk) begin

        if (rst) begin

                recv_state = RX_IDLE;

                tx_state = TX_IDLE;

        end

        // The clk_divider counter counts down from

        // the CLOCK_DIVIDE constant. Whenever it

        // reaches 0, 1/16 of the bit period has elapsed.

   // Countdown timers for the receiving and transmitting

        // state machines are decremented.

        clk_divider = clk_divider - 1;

        if (!clk_divider) begin

                clk_divider = CLOCK_DIVIDE;

                rx_countdown = rx_countdown - 1;

                tx_countdown = tx_countdown - 1;

        end

        // Receive state machine

        case (recv_state)

                RX_IDLE: begin

                        // A low pulse on the receive line indicates the

                        // start of data.

                        if (!rx) begin

                                // Wait half the period - should resume in the

                                // middle of this first pulse.

                                rx_countdown = 8;

                                recv_state = RX_CHECK_START;

                        end

                end

                RX_CHECK_START: begin

                        if (!rx_countdown) begin

                                // Check the pulse is still there

                                if (!rx) begin

                                        // Pulse still there - good

                                        // Wait the bit period to resume half-way

                                        // through the first bit.

                                        rx_countdown = 16;

                                        rx_bits_remaining = 8;

                                        recv_state = RX_READ_BITS;

                                end else begin

                                        // Pulse lasted less than half the period -

                                        // not a valid transmission.

                                        recv_state = RX_ERROR;

                                end

                        end

                end

                RX_READ_BITS: begin

                        if (!rx_countdown) begin

                                // Should be half-way through a bit pulse here.

                                // Read this bit in, wait for the next if we

                                // have more to get.

                                rx_data = {rx, rx_data[7:1]};

                                rx_countdown = 16;

                                rx_bits_remaining = rx_bits_remaining - 1;

                                recv_state = rx_bits_remaining ? RX_READ_BITS : RX_CHECK_STOP;

                        end

                end

                RX_CHECK_STOP: begin

                        if (!rx_countdown) begin

                                // Should resume half-way through the stop bit

                                // This should be high - if not, reject the

                                // transmission and signal an error.

                                recv_state = rx ? RX_RECEIVED : RX_ERROR;

                        end

                end

                RX_DELAY_RESTART: begin

                        // Waits a set number of cycles before accepting

                        // another transmission.

                        recv_state = rx_countdown ? RX_DELAY_RESTART : RX_IDLE;

                end

                RX_ERROR: begin

                        // There was an error receiving.

                        // Raises the recv_error flag for one clock

                        // cycle while in this state and then waits

                        // 2 bit periods before accepting another

                        // transmission.

                        rx_countdown = 32;

                        recv_state = RX_DELAY_RESTART;

                end

                RX_RECEIVED: begin

                        // Successfully received a byte.

                        // Raises the received flag for one clock

                        // cycle while in this state and then waits

                        // 1/4 bit period before starting back in the

                        // idle state. This only actually waits for

                        // 3/4 of the stop bit before resuming, but

                        // this should not do any harm as the stop bit

                        // is received the same as the line idle state.

                        rx_countdown = 4;

                        recv_state = RX_DELAY_RESTART;

                end

        endcase

        // Transmit state machine

        case (tx_state)

                TX_IDLE: begin

                        if (transmit) begin

                                // If the transmit flag is raised in the idle

                                // state, start transmitting the current content

                                // of the tx_byte input.

                                tx_data = tx_byte;

                                // Send the initial, low pulse of 1 bit period

                                // to signal the start, followed by the data

                                tx_countdown = 16;

                                tx_out = 0;

                                tx_bits_remaining = 8;

                                tx_state = TX_SENDING;

                        end

                end

                TX_SENDING: begin

                        if (!tx_countdown) begin

                                if (tx_bits_remaining) begin

                                        tx_bits_remaining = tx_bits_remaining - 1;

                                        tx_out = tx_data[0];

                                        tx_data = {1'b0, tx_data[7:1]};

                                        tx_countdown = 16;

                                        tx_state = TX_SENDING;

                                end else begin

                                        // Set delay to send out 2 stop bits.

                                        tx_out = 1;

                                        tx_countdown = 32;

                                        tx_state = TX_DELAY_RESTART;

                                end

                        end

                end

                TX_DELAY_RESTART: begin

                        // Wait until tx_countdown reaches the end before

                        // we send another transmission. This covers the

                        // "stop bit" delay.

                        tx_state = tx_countdown ? TX_DELAY_RESTART : TX_IDLE;

                end

        endcase

end

endmodule