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////////////////////////////////////////////////////////////////////////////////
//
// Filename: 	txuartlite.v
//
// Project:	wbuart32, a full featured UART with simulator
//
// 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 program is free software (firmware): you can redistribute it and/or
// modify it under the terms of  the GNU General Public License as published
// by the Free Software Foundation, either version 3 of the License, or (at
// your option) any later version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
// for more details.
//
// You should have received a copy of the GNU 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
// <http://www.gnu.org/licenses/> for a copy.
//
// License:	GPL, v3, as defined and found on www.gnu.org,
//		http://www.gnu.org/licenses/gpl.html
//
//
////////////////////////////////////////////////////////////////////////////////
//
//
`define	TXU_BIT_ZERO	4'h0
`define	TXU_BIT_ONE	4'h1
`define	TXU_BIT_TWO	4'h2
`define	TXU_BIT_THREE	4'h3
`define	TXU_BIT_FOUR	4'h4
`define	TXU_BIT_FIVE	4'h5
`define	TXU_BIT_SIX	4'h6
`define	TXU_BIT_SEVEN	4'h7
`define	TXU_STOP	4'h8
`define	TXU_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			i_clk;
	input			i_wr;
	input		[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  = `TXU_IDLE;
	initial	lcl_data= 8'h0;
	always @(posedge i_clk)
	begin
		if (!zero_baud_counter)
			// r_busy needs to be set coming into here
			r_busy <= 1'b1;
		else if (state == `TXU_IDLE)	// STATE_IDLE
		begin
			r_busy <= 1'b0;
			if ((i_wr)&&(!r_busy))
			begin	// Immediately start us off with a start bit
				r_busy <= 1'b1;
				state <= `TXU_BIT_ZERO;
			end
		end else begin
			// One clock tick in each of these states ...
			r_busy <= 1'b1;
			if (state <=`TXU_STOP) // start bit, 8-d bits, stop-b
				state <= state + 1;
			else
				state <= `TXU_IDLE;
		end 
	end
 
	// 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 == `TXU_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)
	begin
		zero_baud_counter <= (baud_counter == 24'h01);
		if (state == `TXU_IDLE)
		begin
			baud_counter <= 24'h0;
			zero_baud_counter <= 1'b1;
			if ((i_wr)&&(!r_busy))
			begin
				baud_counter <= CLOCKS_PER_BAUD - 24'h01;
				zero_baud_counter <= 1'b0;
			end
		end else if (!zero_baud_counter)
			baud_counter <= baud_counter - 24'h01;
		else
			baud_counter <= CLOCKS_PER_BAUD - 24'h01;
	end
endmodule