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 = 13; // clock rate (1Mhz) / (baud rate (19200) * 4)
// 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] rx_clk_divider = CLOCK_DIVIDE;
reg [10:0] tx_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.
rx_clk_divider = rx_clk_divider - 1;
if (!rx_clk_divider) begin
rx_clk_divider = CLOCK_DIVIDE;
rx_countdown = rx_countdown - 1;
end
tx_clk_divider = tx_clk_divider - 1;
if (!tx_clk_divider) begin
tx_clk_divider = CLOCK_DIVIDE;
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_clk_divider = CLOCK_DIVIDE;
rx_countdown = 2;
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 = 4;
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 = 4;
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 = 8;
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.
recv_state = RX_IDLE;
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_clk_divider = CLOCK_DIVIDE;
tx_countdown = 4;
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 = 4;
tx_state = TX_SENDING;
end else begin
// Set delay to send out 2 stop bits.
tx_out = 1;
tx_countdown = 8;
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
// Documented Verilog UART
// Copyright (C) 2010 Timothy Goddard (tim@goddard.net.nz)
// Distributed under the MIT licence.