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// Filename: txuartlite.v
// Project: dbgbus, a collection of 8b channel to WB bus debugging protocols
// Purpose: Transmit outputs over a single UART line. This particular UART
// implementation has been extremely simplified: it does not handle
// generating break conditions, nor does it handle anything other than the
// 8N1 (8 data bits, no parity, 1 stop bit) UART sub-protocol.
// To interface with this module, connect it to your system clock, and
// pass it the byte of data you wish to transmit. Strobe the i_wr line
// high for one cycle, and your data will be off. Wait until the 'o_busy'
// line is low before strobing the i_wr line again--this implementation
// has NO BUFFER, so strobing i_wr while the core is busy will just
// get ignored. The output will be placed on the o_txuart output line.
// (I often set both data and strobe on the same clock, and then just leave
// them set until the busy line is low. Then I move on to the next piece
// of data.)
// 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 TXUL_BIT_ZERO 4'h0
`define TXUL_BIT_ONE 4'h1
`define TXUL_BIT_TWO 4'h2
`define TXUL_BIT_THREE 4'h3
`define TXUL_BIT_FOUR 4'h4
`define TXUL_BIT_FIVE 4'h5
`define TXUL_BIT_SIX 4'h6
`define TXUL_BIT_SEVEN 4'h7
`define TXUL_STOP 4'h8
`define TXUL_IDLE 4'hf
module txuartlite(i_clk, i_wr, i_data, o_uart_tx, o_busy);
parameter [23:0] CLOCKS_PER_BAUD = 24'd868;
input wire i_clk;
input wire i_wr;
input wire [7:0] i_data;
// And the UART input line itself
output reg o_uart_tx;
// A line to tell others when we are ready to accept data. If
// (i_wr)&&(!o_busy) is ever true, then the core has accepted a byte
// for transmission.
output wire o_busy;
reg [23:0] baud_counter;
reg [3:0] state;
reg [7:0] lcl_data;
reg r_busy, zero_baud_counter;
initial r_busy = 1'b1;
initial state = `TXUL_IDLE;
always @(posedge i_clk)
if (!zero_baud_counter)
// r_busy needs to be set coming into here
r_busy <= 1'b1;
else if (state == `TXUL_IDLE) // STATE_IDLE
r_busy <= 1'b0;
if ((i_wr)&&(!r_busy))
begin // Immediately start us off with a start bit
r_busy <= 1'b1;
state <= `TXUL_BIT_ZERO;
end else begin
// One clock tick in each of these states ...
r_busy <= 1'b1;
if (state <=`TXUL_STOP) // start bit, 8-d bits, stop-b
state <= state + 1;
state <= `TXUL_IDLE;
// o_busy
// This is a wire, designed to be true is we are ever busy above.
// originally, this was going to be true if we were ever not in the
// idle state. The logic has since become more complex, hence we have
// a register dedicated to this and just copy out that registers value.
assign o_busy = (r_busy);
// lcl_data
// This is our working copy of the i_data register which we use
// when transmitting. It is only of interest during transmit, and is
// allowed to be whatever at any other time. Hence, if r_busy isn't
// true, we can always set it. On the one clock where r_busy isn't
// true and i_wr is, we set it and r_busy is true thereafter.
// Then, on any zero_baud_counter (i.e. change between baud intervals)
// we simple logically shift the register right to grab the next bit.
initial lcl_data = 8'hff;
always @(posedge i_clk)
if ((i_wr)&&(!r_busy))
lcl_data <= i_data;
else if (zero_baud_counter)
lcl_data <= { 1'b1, lcl_data[7:1] };
// o_uart_tx
// This is the final result/output desired of this core. It's all
// centered about o_uart_tx. This is what finally needs to follow
// the UART protocol.
initial o_uart_tx = 1'b1;
always @(posedge i_clk)
if ((i_wr)&&(!r_busy))
o_uart_tx <= 1'b0; // Set the start bit on writes
else if (zero_baud_counter) // Set the data bit.
o_uart_tx <= lcl_data[0];
// All of the above logic is driven by the baud counter. Bits must last
// CLOCKS_PER_BAUD in length, and this baud counter is what we use to
// make certain of that.
// The basic logic is this: at the beginning of a bit interval, start
// the baud counter and set it to count CLOCKS_PER_BAUD. When it gets
// to zero, restart it.
// However, comparing a 28'bit number to zero can be rather complex--
// especially if we wish to do anything else on that same clock. For
// that reason, we create "zero_baud_counter". zero_baud_counter is
// nothing more than a flag that is true anytime baud_counter is zero.
// It's true when the logic (above) needs to step to the next bit.
// Simple enough?
// I wish we could stop there, but there are some other (ugly)
// conditions to deal with that offer exceptions to this basic logic.
// 1. When the user has commanded a BREAK across the line, we need to
// wait several baud intervals following the break before we start
// transmitting, to give any receiver a chance to recognize that we are
// out of the break condition, and to know that the next bit will be
// a stop bit.
// 2. A reset is similar to a break condition--on both we wait several
// baud intervals before allowing a start bit.
// 3. In the idle state, we stop our counter--so that upon a request
// to transmit when idle we can start transmitting immediately, rather
// than waiting for the end of the next (fictitious and arbitrary) baud
// interval.
// When (i_wr)&&(!r_busy)&&(state == `TXUL_IDLE) then we're not only in
// the idle state, but we also just accepted a command to start writing
// the next word. At this point, the baud counter needs to be reset
// to the number of CLOCKS_PER_BAUD, and zero_baud_counter set to zero.
// The logic is a bit twisted here, in that it will only check for the
// above condition when zero_baud_counter is false--so as to make
// certain the STOP bit is complete.
initial zero_baud_counter = 1'b0;
initial baud_counter = 24'h05;
always @(posedge i_clk)
zero_baud_counter <= (baud_counter == 24'h01);
if (state == `TXUL_IDLE)
baud_counter <= 24'h0;
zero_baud_counter <= 1'b1;
if ((i_wr)&&(!r_busy))
baud_counter <= CLOCKS_PER_BAUD - 24'h01;
zero_baud_counter <= 1'b0;
end else if (!zero_baud_counter)
baud_counter <= baud_counter - 24'h01;
baud_counter <= CLOCKS_PER_BAUD - 24'h01;