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////////////////////////////////////////////////////////////////////////////////
//
// Filename: 	txuart.v
//
// Project:	wbuart32, a full featured UART with simulator
//
// Purpose:	Transmit outputs over a single UART line.
//
//	To interface with this module, connect it to your system clock,
//	pass it the 32 bit setup register (defined below) and the byte
//	of data you wish to transmit.  Strobe the i_wr line high for one
//	clock 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
//	cause your data to be lost.  The output will be placed on the o_txuart
//	output line.  If you wish to set/send a break condition, assert the
//	i_break line otherwise leave it low.
//
//	There is a synchronous reset line, logic high.
//
//	Now for the setup register.  The register is 32 bits, so that this
//	UART may be set up over a 32-bit bus.
//
//	i_setup[30]	Set this to zero to use hardware flow control, and to
//		one to ignore hardware flow control.  Only works if the hardware
//		flow control has been properly wired.
//
//		If you don't want hardware flow control, fix the i_rts bit to
//		1'b1, and let the synthesys tools optimize out the logic.
//
//	i_setup[29:28]	Indicates the number of data bits per word.  This will
//		either be 2'b00 for an 8-bit word, 2'b01 for a 7-bit word, 2'b10
//		for a six bit word, or 2'b11 for a five bit word.
//
//	i_setup[27]	Indicates whether or not to use one or two stop bits.
//		Set this to one to expect two stop bits, zero for one.
//
//	i_setup[26]	Indicates whether or not a parity bit exists.  Set this
//		to 1'b1 to include parity.
//
//	i_setup[25]	Indicates whether or not the parity bit is fixed.  Set
//		to 1'b1 to include a fixed bit of parity, 1'b0 to allow the
//		parity to be set based upon data.  (Both assume the parity
//		enable value is set.)
//
//	i_setup[24]	This bit is ignored if parity is not used.  Otherwise,
//		in the case of a fixed parity bit, this bit indicates whether
//		mark (1'b1) or space (1'b0) parity is used.  Likewise if the
//		parity is not fixed, a 1'b1 selects even parity, and 1'b0
//		selects odd.
//
//	i_setup[23:0]	Indicates the speed of the UART in terms of clocks.
//		So, for example, if you have a 200 MHz clock and wish to
//		run your UART at 9600 baud, you would take 200 MHz and divide
//		by 9600 to set this value to 24'd20834.  Likewise if you wished
//		to run this serial port at 115200 baud from a 200 MHz clock,
//		you would set the value to 24'd1736
//
//	Thus, to set the UART for the common setting of an 8-bit word, 
//	one stop bit, no parity, and 115200 baud over a 200 MHz clock, you
//	would want to set the setup value to:
//
//	32'h0006c8		// For 115,200 baud, 8 bit, no parity
//	32'h005161		// For 9600 baud, 8 bit, no parity
//	
//
// 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_PARITY	4'h8	// Constant 1
`define	TXU_STOP	4'h9	// Constant 1
`define	TXU_SECOND_STOP	4'ha
// 4'hb	// Unused
// 4'hc	// Unused
// `define	TXU_START	4'hd	// An unused state
`define	TXU_BREAK	4'he
`define	TXU_IDLE	4'hf
//
//
module txuart(i_clk, i_reset, i_setup, i_break, i_wr, i_data,
		i_cts_n, o_uart_tx, o_busy);
	parameter	[30:0]	INITIAL_SETUP = 31'd868;
	input			i_clk, i_reset;
	input		[30:0]	i_setup;
	input			i_break;
	input			i_wr;
	input		[7:0]	i_data;
	// Hardware flow control Ready-To-Send bit.  Set this to one to use
	// the core without flow control.  (A more appropriate name would be
	// the Ready-To-Receive bit ...)
	input			i_cts_n;
	// 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;
 
	wire	[27:0]	clocks_per_baud, break_condition;
	wire	[1:0]	data_bits;
	wire		use_parity, parity_even, dblstop, fixd_parity,
			fixdp_value, hw_flow_control;
	reg	[30:0]	r_setup;
	assign	clocks_per_baud = { 4'h0, r_setup[23:0] };
	assign	break_condition = { r_setup[23:0], 4'h0 };
	assign	hw_flow_control = !r_setup[30];
	assign	data_bits       =  r_setup[29:28];
	assign	dblstop         =  r_setup[27];
	assign	use_parity      =  r_setup[26];
	assign	fixd_parity     =  r_setup[25];
	assign	parity_even     =  r_setup[24];
	assign	fixdp_value     =  r_setup[24];
 
	reg	[27:0]	baud_counter;
	reg	[3:0]	state;
	reg	[7:0]	lcl_data;
	reg		calc_parity, r_busy, zero_baud_counter;
 
 
	// First step ... handle any hardware flow control, if so enabled.
	//
	// Clock in the flow control data, two clocks to avoid metastability
	// Default to using hardware flow control (uart_setup[30]==0 to use it).
	// Set this high order bit off if you do not wish to use it.
	reg	q_cts_n, qq_cts_n, ck_cts;
	// While we might wish to give initial values to q_rts and ck_cts,
	// 1) it's not required since the transmitter starts in a long wait
	// state, and 2) doing so will prevent the synthesizer from optimizing
	// this pin in the case it is hard set to 1'b1 external to this
	// peripheral.
	//
	// initial	q_cts_n  = 1'b1;
	// initial	qq_cts_n = 1'b1;
	// initial	ck_cts   = 1'b0;
	always	@(posedge i_clk)
		q_cts_n <= i_cts_n;
	always	@(posedge i_clk)
		qq_cts_n <= q_cts_n;
	always	@(posedge i_clk)
		ck_cts <= (!qq_cts_n)||(!hw_flow_control);
 
	initial	o_uart_tx = 1'b1;
	initial	r_busy = 1'b1;
	initial	state  = `TXU_IDLE;
	initial	lcl_data= 8'h0;
	initial	calc_parity = 1'b0;
	// initial	baud_counter = clocks_per_baud;//ILLEGAL--not constant
	always @(posedge i_clk)
	begin
		if (i_reset)
		begin
			r_busy <= 1'b1;
			state <= `TXU_IDLE;
		end else if (i_break)
		begin
			state <= `TXU_BREAK;
			r_busy <= 1'b1;
		end else if (!zero_baud_counter)
		begin // r_busy needs to be set coming into here
			r_busy <= 1'b1;
		end else if (state == `TXU_BREAK)
		begin
			state <= `TXU_IDLE;
			r_busy <= 1'b1;
		end else if (state == `TXU_IDLE)	// STATE_IDLE
		begin
			if ((i_wr)&&(!r_busy))
			begin	// Immediately start us off with a start bit
				r_busy <= 1'b1;
				case(data_bits)
				2'b00: state <= `TXU_BIT_ZERO;
				2'b01: state <= `TXU_BIT_ONE;
				2'b10: state <= `TXU_BIT_TWO;
				2'b11: state <= `TXU_BIT_THREE;
				endcase
			end else begin // Stay in idle
				r_busy <= !ck_cts;
			end
		end else begin
			// One clock tick in each of these states ...
			// baud_counter <= clocks_per_baud - 28'h01;
			r_busy <= 1'b1;
			if (state[3] == 0) // First 8 bits
			begin
				if (state == `TXU_BIT_SEVEN)
					state <= (use_parity)?`TXU_PARITY:`TXU_STOP;
				else
					state <= state + 1;
			end else if (state == `TXU_PARITY)
			begin
				state <= `TXU_STOP;
			end else if (state == `TXU_STOP)
			begin // two stop bit(s)
				if (dblstop)
					state <= `TXU_SECOND_STOP;
				else
					state <= `TXU_IDLE;
			end else // `TXU_SECOND_STOP and default:
			begin
				state <= `TXU_IDLE; // Go back to idle
				// Still r_busy, since we need to wait
				// for the baud clock to finish counting
				// out this last bit.
			end
		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);
 
 
	// r_setup
	//
	// Our setup register.  Accept changes between any pair of transmitted
	// words.  The register itself has many fields to it.  These are
	// broken out up top, and indicate what 1) our baud rate is, 2) our
	// number of stop bits, 3) what type of parity we are using, and 4)
	// the size of our data word.
	initial	r_setup = INITIAL_SETUP;
	always @(posedge i_clk)
		if (state == `TXU_IDLE)
			r_setup <= i_setup;
 
	// 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.
	always @(posedge i_clk)
		if (!r_busy)
			lcl_data <= i_data;
		else if (zero_baud_counter)
			lcl_data <= { 1'b0, 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.
	//
	// Ok, that said, our rules are:
	//	1'b0 on any break condition
	//	1'b0 on a start bit (IDLE, write, and not busy)
	//	lcl_data[0] during any data transfer, but only at the baud
	//		change
	//	PARITY -- During the parity bit.  This depends upon whether or
	//		not the parity bit is fixed, then what it's fixed to,
	//		or changing, and hence what it's calculated value is.
	//	1'b1 at all other times (stop bits, idle, etc)
	always @(posedge i_clk)
		if (i_reset)
			o_uart_tx <= 1'b1;
		else if ((i_break)||((i_wr)&&(!r_busy)))
			o_uart_tx <= 1'b0;
		else if (zero_baud_counter)
			casez(state)
			4'b0???:	o_uart_tx <= lcl_data[0];
			`TXU_PARITY:	o_uart_tx <= calc_parity;
			default:	o_uart_tx <= 1'b1;
			endcase
 
 
	// calc_parity
	//
	// Calculate the parity to be placed into the parity bit.  If the
	// parity is fixed, then the parity bit is given by the fixed parity
	// value (r_setup[24]).  Otherwise the parity is given by the GF2
	// sum of all the data bits (plus one for even parity).
	always @(posedge i_clk)
		if (fixd_parity)
			calc_parity <= fixdp_value;
		else if (zero_baud_counter)
		begin
			if (state[3] == 0) // First 8 bits of msg
				calc_parity <= calc_parity ^ lcl_data[0];
			else
				calc_parity <= parity_even;
		end else if (!r_busy)
			calc_parity <= parity_even;
 
 
	// 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 = 28'h05;
	always @(posedge i_clk)
	begin
		zero_baud_counter <= (baud_counter == 28'h01);
		if ((i_reset)||(i_break))
		begin
			// Give ourselves 16 bauds before being ready
			baud_counter <= break_condition;
			zero_baud_counter <= 1'b0;
		end else if (!zero_baud_counter)
			baud_counter <= baud_counter - 28'h01;
		else if (state == `TXU_BREAK)
			// Give us four idle baud intervals before becoming
			// available
			baud_counter <= clocks_per_baud<<2;
		else if (state == `TXU_IDLE)
		begin
			baud_counter <= 28'h0;
			zero_baud_counter <= 1'b1;
			if ((i_wr)&&(!r_busy))
			begin
				baud_counter <= clocks_per_baud - 28'h01;
				zero_baud_counter <= 1'b0;
			end
		end else
			baud_counter <= clocks_per_baud - 28'h01;
	end
endmodule