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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[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) 2015-2016, 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,o_uart_tx, o_busy);

        parameter       INITIAL_SETUP = 30'd868;

        input                   i_clk, i_reset;

        input           [29:0]   i_setup;

        input                   i_break;

        input                   i_wr;

        input           [7:0]    i_data;

        output  reg             o_uart_tx;

        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;

        reg     [29:0]   r_setup;

        assign  clocks_per_baud = { 4'h0, r_setup[23:0] };

        assign  break_condition = { r_setup[23:0], 4'h0 };

        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;

        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 <= 0;

                        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