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////////////////////////////////////////////////////////////////////////////////

//

// Filename:    rxuartlite.v

//

// Project:     wbuart32, a full featured UART with simulator

//

// Purpose:     Receive and decode inputs from a single UART line.

//

//

//      To interface with this module, connect it to your system clock,

//      and a UART input.  Set the parameter to the number of clocks per

//      baud.  When data becomes available, the o_wr line will be asserted

//      for one clock cycle.

//

//      This interface only handles 8N1 serial port communications.  It does

//      not handle the break, parity, or frame error conditions.

//

//

// 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 RXU_BIT_ZERO            4'h0

`define RXU_BIT_ONE             4'h1

`define RXU_BIT_TWO             4'h2

`define RXU_BIT_THREE           4'h3

`define RXU_BIT_FOUR            4'h4

`define RXU_BIT_FIVE            4'h5

`define RXU_BIT_SIX             4'h6

`define RXU_BIT_SEVEN           4'h7

// `define      RXU_PARITY              4'h8    // Unused in RXUARTLITE

`define RXU_STOP                4'h8

// `define      RXU_SECOND_STOP         4'ha    // Unused in RXUARTLITE

// Unused 4'hb

// Unused 4'hc

// `define      RXU_BREAK               4'hd    // Unused in RXUARTLITE

// `define      RXU_RESET_IDLE          4'he    // Unused in RXUARTLITE

`define RXU_IDLE                4'hf

module rxuartlite(i_clk, i_uart_rx, o_wr, o_data);

        parameter [23:0] CLOCKS_PER_BAUD = 24'd868;

        input                   i_clk;

        input                   i_uart_rx;

        output  reg             o_wr;

        output  reg     [7:0]    o_data;

        wire    [23:0]   clocks_per_baud, half_baud;

        reg     [3:0]    state;

        assign  half_baud = { 1'b0, CLOCKS_PER_BAUD[23:1] } - 24'h1;

        reg     [23:0]   baud_counter;

        reg             zero_baud_counter;

        // Since this is an asynchronous receiver, we need to register our

        // input a couple of clocks over to avoid any problems with 

        // metastability.  We do that here, and then ignore all but the

        // ck_uart wire.

        reg     q_uart, qq_uart, ck_uart;

        initial q_uart  = 1'b0;

        initial qq_uart = 1'b0;

        initial ck_uart = 1'b0;

        always @(posedge i_clk)

        begin

                q_uart <= i_uart_rx;

                qq_uart <= q_uart;

                ck_uart <= qq_uart;

        end

        // Keep track of the number of clocks since the last change.

        //

        // This is used to determine if we are in either a break or an idle

        // condition, as discussed further below.

        reg     [23:0]   chg_counter;

        initial chg_counter = 24'h00;

        always @(posedge i_clk)

                if (qq_uart != ck_uart)

                        chg_counter <= 24'h00;

                else

                        chg_counter <= chg_counter + 1;

        // Are we in the middle of a baud iterval?  Specifically, are we

        // in the middle of a start bit?  Set this to high if so.  We'll use

        // this within our state machine to transition out of the IDLE

        // state.

        reg     half_baud_time;

        initial half_baud_time = 0;

        always @(posedge i_clk)

                half_baud_time <= (~ck_uart)&&(chg_counter >= half_baud);

        initial state = `RXU_IDLE;

        always @(posedge i_clk)

        begin

                if (state == `RXU_IDLE)

                begin // Idle state, independent of baud counter

                        // By default, just stay in the IDLE state

                        state <= `RXU_IDLE;

                        if ((~ck_uart)&&(half_baud_time))

                                // UNLESS: We are in the center of a valid

                                // start bit

                                state <= `RXU_BIT_ZERO;

                end else if (zero_baud_counter)

                begin

                        if (state < `RXU_STOP)

                                // Data arrives least significant bit first.

                                // By the time this is clocked in, it's what

                                // you'll have.

                                state <= state + 1;

                        else // Wait for the next character

                                state <= `RXU_IDLE;

                end

        end

        // Data bit capture logic.

        //

        // This is drastically simplified from the state machine above, based

        // upon: 1) it doesn't matter what it is until the end of a captured

        // byte, and 2) the data register will flush itself of any invalid

        // data in all other cases.  Hence, let's keep it real simple.

        reg     [7:0]    data_reg;

        always @(posedge i_clk)

                if (zero_baud_counter)

                        data_reg <= { ck_uart, data_reg[7:1] };

        // Our data bit logic doesn't need nearly the complexity of all that

        // work above.  Indeed, we only need to know if we are at the end of

        // a stop bit, in which case we copy the data_reg into our output

        // data register, o_data, and tell others (for one clock) that data is

        // available.

        //

        initial o_data = 8'h00;

        reg     pre_wr;

        initial pre_wr = 1'b0;

        always @(posedge i_clk)

                if ((zero_baud_counter)&&(state == `RXU_STOP))

                begin

                        o_wr   <= 1'b1;

                        o_data <= data_reg;

                end else

                        o_wr   <= 1'b0;

        // The baud counter

        //

        // This is used as a "clock divider" if you will, but the clock needs

        // to be reset before any byte can be decoded.  In all other respects,

        // we set ourselves up for clocks_per_baud counts between baud

        // intervals.

        always @(posedge i_clk)

                if ((zero_baud_counter)|||(state == `RXU_IDLE))

                        baud_counter <= CLOCKS_PER_BAUD-1'b1;

                else

                        baud_counter <= baud_counter-1'b1;

        // zero_baud_counter

        //

        // Rather than testing whether or not (baud_counter == 0) within our

        // (already too complicated) state transition tables, we use

        // zero_baud_counter to pre-charge that test on the clock

        // before--cleaning up some otherwise difficult timing dependencies.

        initial zero_baud_counter = 1'b0;

        always @(posedge i_clk)

                if (state == `RXU_IDLE)

                        zero_baud_counter <= 1'b0;

                else

                        zero_baud_counter <= (baud_counter == 24'h01);

endmodule