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// Filename: rxuartlite.v
// Project: dbgbus, a collection of 8b channel to WB bus debugging protocols
// Purpose: Receive and decode inputs from a single UART line.
// To interface with this module, connect it to your system clock,
// and a UART input. Set the parameter to the number of clocks per
// baud. When data becomes available, the o_wr line will be asserted
// for one clock cycle.
// This interface only handles 8N1 serial port communications. It does
// not handle the break, parity, or frame error conditions.
// Creator: Dan Gisselquist, Ph.D.
// Gisselquist Technology, LLC
// Copyright (C) 2015-2017, Gisselquist Technology, LLC
// This file is part of the debugging interface demonstration.
// The debugging interface demonstration is free software (firmware): you can
// redistribute it and/or modify it under the terms of the GNU Lesser General
// Public License as published by the Free Software Foundation, either version
// 3 of the License, or (at your option) any later version.
// This debugging interface demonstration is distributed in the hope that it
// will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
// General Public License for more details.
// You should have received a copy of the GNU Lesser General Public License
// along with this program. (It's in the $(ROOT)/doc directory. Run make
// with no target there if the PDF file isn't present.) If not, see
// <> for a copy.
// License: LGPL, v3, as defined and found on,
`default_nettype none
`define RXUL_BIT_ZERO 4'h0
`define RXUL_BIT_ONE 4'h1
`define RXUL_BIT_TWO 4'h2
`define RXUL_BIT_THREE 4'h3
`define RXUL_BIT_FOUR 4'h4
`define RXUL_BIT_FIVE 4'h5
`define RXUL_BIT_SIX 4'h6
`define RXUL_BIT_SEVEN 4'h7
`define RXUL_STOP 4'h8
`define RXUL_IDLE 4'hf
module rxuartlite(i_clk, i_uart_rx, o_wr, o_data);
parameter [23:0] CLOCKS_PER_BAUD = 24'd868;
input wire i_clk;
input wire i_uart_rx;
output reg o_wr;
output reg [7:0] o_data;
wire [23:0] half_baud;
reg [3:0] state;
assign half_baud = { 1'b0, CLOCKS_PER_BAUD[23:1] } - 24'h1;
reg [23:0] baud_counter;
reg zero_baud_counter;
// Since this is an asynchronous receiver, we need to register our
// input a couple of clocks over to avoid any problems with
// metastability. We do that here, and then ignore all but the
// ck_uart wire.
reg q_uart, qq_uart, ck_uart;
initial q_uart = 1'b0;
initial qq_uart = 1'b0;
initial ck_uart = 1'b0;
always @(posedge i_clk)
q_uart <= i_uart_rx;
qq_uart <= q_uart;
ck_uart <= qq_uart;
// Keep track of the number of clocks since the last change.
// This is used to determine if we are in either a break or an idle
// condition, as discussed further below.
reg [23:0] chg_counter;
initial chg_counter = 24'h00;
always @(posedge i_clk)
if (qq_uart != ck_uart)
chg_counter <= 24'h00;
chg_counter <= chg_counter + 1;
// Are we in the middle of a baud iterval? Specifically, are we
// in the middle of a start bit? Set this to high if so. We'll use
// this within our state machine to transition out of the IDLE
// state.
reg half_baud_time;
initial half_baud_time = 0;
always @(posedge i_clk)
half_baud_time <= (~ck_uart)&&(chg_counter >= half_baud);
initial state = `RXUL_IDLE;
always @(posedge i_clk)
if (state == `RXUL_IDLE)
begin // Idle state, independent of baud counter
// By default, just stay in the IDLE state
state <= `RXUL_IDLE;
if ((~ck_uart)&&(half_baud_time))
// UNLESS: We are in the center of a valid
// start bit
state <= `RXUL_BIT_ZERO;
end else if (zero_baud_counter)
if (state < `RXUL_STOP)
// Data arrives least significant bit first.
// By the time this is clocked in, it's what
// you'll have.
state <= state + 1;
else // Wait for the next character
state <= `RXUL_IDLE;
// Data bit capture logic.
// This is drastically simplified from the state machine above, based
// upon: 1) it doesn't matter what it is until the end of a captured
// byte, and 2) the data register will flush itself of any invalid
// data in all other cases. Hence, let's keep it real simple.
reg [7:0] data_reg;
always @(posedge i_clk)
if (zero_baud_counter)
data_reg <= { ck_uart, data_reg[7:1] };
// Our data bit logic doesn't need nearly the complexity of all that
// work above. Indeed, we only need to know if we are at the end of
// a stop bit, in which case we copy the data_reg into our output
// data register, o_data, and tell others (for one clock) that data is
// available.
initial o_data = 8'h00;
always @(posedge i_clk)
if ((zero_baud_counter)&&(state == `RXUL_STOP))
o_wr <= 1'b1;
o_data <= data_reg;
end else
o_wr <= 1'b0;
// The baud counter
// This is used as a "clock divider" if you will, but the clock needs
// to be reset before any byte can be decoded. In all other respects,
// we set ourselves up for CLOCKS_PER_BAUD counts between baud
// intervals.
always @(posedge i_clk)
if ((zero_baud_counter)|||(state == `RXUL_IDLE))
baud_counter <= CLOCKS_PER_BAUD-1'b1;
baud_counter <= baud_counter-1'b1;
// zero_baud_counter
// Rather than testing whether or not (baud_counter == 0) within our
// (already too complicated) state transition tables, we use
// zero_baud_counter to pre-charge that test on the clock
// before--cleaning up some otherwise difficult timing dependencies.
initial zero_baud_counter = 1'b0;
always @(posedge i_clk)
if (state == `RXUL_IDLE)
zero_baud_counter <= 1'b0;
zero_baud_counter <= (baud_counter == 24'h01);