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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