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

//

// Filename:    wbuart.v

//

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

//

// Purpose:     Unlilke wbuart-insert.v, this is a full blown wishbone core

//              with integrated FIFO support to support the UART transmitter

//      and receiver found within here.  As a result, it's usage may be

//      heavier on the bus than the insert, but it may also be more useful.

//

// 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 UART_SETUP      2'b00

`define UART_FIFO       2'b01

`define UART_RXREG      2'b10

`define UART_TXREG      2'b11

module  wbuart(i_clk, i_rst,

                //

                i_wb_cyc, i_wb_stb, i_wb_we, i_wb_addr, i_wb_data,

                        o_wb_ack, o_wb_stall, o_wb_data,

                //

                i_uart_rx, o_uart_tx, i_cts_n, o_rts_n,

                //

                o_uart_rx_int, o_uart_tx_int,

                o_uart_rxfifo_int, o_uart_txfifo_int);

        parameter [30:0] INITIAL_SETUP = 31'd25; // 4MB 8N1, when using 100MHz clock

        parameter [3:0]  LGFLEN = 4;

        parameter [0:0]   HARDWARE_FLOW_CONTROL_PRESENT = 1'b1;

        // Perform a simple/quick bounds check on the log FIFO length, to make

        // sure its within the bounds we can support with our current

        // interface.

        localparam [3:0] LCLLGFLEN = (LGFLEN > 4'ha)? 4'ha

                                        : ((LGFLEN < 4'h2) ? 4'h2 : LGFLEN);

        //

        input   i_clk, i_rst;

        // Wishbone inputs

        input                   i_wb_cyc, i_wb_stb, i_wb_we;

        input           [1:0]    i_wb_addr;

        input           [31:0]   i_wb_data;

        output  reg             o_wb_ack;

        output  wire            o_wb_stall;

        output  reg     [31:0]   o_wb_data;

        //

        input                   i_uart_rx;

        output  wire            o_uart_tx;

        // RTS is used for hardware flow control.  According to Wikipedia, it

        // should probably be renamed RTR for "ready to receive".  It tell us

        // whether or not the receiving hardware is ready to accept another

        // byte.  If low, the transmitter will pause.

        //

        // If you don't wish to use hardware flow control, just set i_cts_n to

        // 1'b0 and let the optimizer simply remove this logic.

        input                   i_cts_n;

        // CTS is the "Clear-to-send" signal.  We set it anytime our FIFO

        // isn't full.  Feel free to ignore this output if you do not wish to

        // use flow control.

        output  reg             o_rts_n;

        output  wire            o_uart_rx_int, o_uart_tx_int,

                                o_uart_rxfifo_int, o_uart_txfifo_int;

        wire    tx_busy;

        //

        // The UART setup parameters: bits per byte, stop bits, parity, and

        // baud rate are all captured within this uart_setup register.

        //

        reg     [30:0]   uart_setup;

        initial uart_setup = INITIAL_SETUP;

        always @(posedge i_clk)

                // Under wishbone rules, a write takes place any time i_wb_stb

                // is high.  If that's the case, and if the write was to the

                // setup address, then set us up for the new parameters.

                if ((i_wb_stb)&&(i_wb_addr == `UART_SETUP)&&(i_wb_we))

                        uart_setup <= {

                                (i_wb_data[30])

                                        ||(!HARDWARE_FLOW_CONTROL_PRESENT),

                                i_wb_data[29:0] };

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

        //

        //

        // First, the UART receiver

        //

        //

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

        // First the wires/registers this receiver depends upon

        wire            rx_stb, rx_break, rx_perr, rx_ferr, ck_uart;

        wire    [7:0]    rx_uart_data;

        reg             rx_uart_reset;

        // Here's our UART receiver.  Basically, it accepts our setup wires, 

        // the UART input, a clock, and a reset line, and produces outputs:

        // a stb (true when new data is ready), and an 8-bit data out value

        // valid when stb is high.

`ifdef  USE_LITE_UART

        rxuartlite      #(INITIAL_SETUP[23:0])

                rx(i_clk, (i_rst), i_uart_rx, rx_stb, rx_uart_data);

        assign  rx_break = 1'b0;

        assign  rx_perr  = 1'b0;

        assign  rx_ferr  = 1'b0;

        assign  ck_uart  = 1'b0;

`else

        // The full receiver also produces a break value (true during a break

        // cond.), and parity/framing error flags--also valid when stb is true.

        rxuart  #(INITIAL_SETUP) rx(i_clk, (i_rst)||(rx_uart_reset),

                        uart_setup, i_uart_rx,

                        rx_stb, rx_uart_data, rx_break,

                        rx_perr, rx_ferr, ck_uart);

        // The real trick is ... now that we have this extra data, what do we do

        // with it?

`endif

        // We place it into a receiver FIFO.

        //

        // Here's the declarations for the wires it needs.

        wire            rx_empty_n, rx_fifo_err;

        wire    [7:0]    rxf_wb_data;

        wire    [15:0]   rxf_status;

        reg             rxf_wb_read;

        //

        // And here's the FIFO proper.

        //

        // Note that the FIFO will be cleared upon any reset: either if there's

        // a UART break condition on the line, the receiver is in reset, or an

        // external reset is issued.

        //

        // The FIFO accepts strobe and data from the receiver.

        // We issue another wire to it (rxf_wb_read), true when we wish to read

        // from the FIFO, and we get our data in rxf_wb_data.  The FIFO outputs

        // four status-type values: 1) is it non-empty, 2) is the FIFO over half

        // full, 3) a 16-bit status register, containing info regarding how full

        // the FIFO truly is, and 4) an error indicator.

        ufifo   #(.LGFLEN(LCLLGFLEN), .RXFIFO(1))

                rxfifo(i_clk, (i_rst)||(rx_break)||(rx_uart_reset),

                        rx_stb, rx_uart_data,

                        rx_empty_n,

                        rxf_wb_read, rxf_wb_data,

                        rxf_status, rx_fifo_err);

        assign  o_uart_rxfifo_int = rxf_status[1];

        // We produce four interrupts.  One of the receive interrupts indicates

        // whether or not the receive FIFO is non-empty.  This should wake up

        // the CPU.

        assign  o_uart_rx_int = rxf_status[0];

        // The clear to send line, which may be ignored, but which we set here

        // to be true any time the FIFO has fewer than N-2 items in it.

        // Why N-1?  Because at N-1 we are totally full, but already so full

        // that if the transmit end starts sending we won't have a location to

        // receive it.  (Transmit might've started on the next character by the

        // time we set this--need to set it to one character before necessary

        always @(posedge i_clk)

                o_rts_n = ((HARDWARE_FLOW_CONTROL_PRESENT)

                        &&(!uart_setup[30])

                        &&(rxf_status[(LCLLGFLEN+1):4]=={(LCLLGFLEN-2){1'b1}}));

        // If the bus requests that we read from the receive FIFO, we need to

        // tell this to the receive FIFO.  Note that because we are using a 

        // clock here, the output from the receive FIFO will necessarily be

        // delayed by an extra clock.

        initial rxf_wb_read = 1'b0;

        always @(posedge i_clk)

                rxf_wb_read <= (i_wb_stb)&&(i_wb_addr[1:0]==`UART_RXREG)

                                &&(!i_wb_we);

        // Now, let's deal with those RX UART errors: both the parity and frame

        // errors.  As you may recall, these are valid only when rx_stb is

        // valid, so we need to hold on to them until the user reads them via

        // a UART read request..

        reg     r_rx_perr, r_rx_ferr;

        initial r_rx_perr = 1'b0;

        initial r_rx_ferr = 1'b0;

        always @(posedge i_clk)

                if ((rx_uart_reset)||(rx_break))

                begin

                        // Clear the error

                        r_rx_perr <= 1'b0;

                        r_rx_ferr <= 1'b0;

                end else if ((i_wb_stb)

                                &&(i_wb_addr[1:0]==`UART_RXREG)&&(i_wb_we))

                begin

                        // Reset the error lines if a '1' is ever written to

                        // them, otherwise leave them alone.

                        //

                        r_rx_perr <= (r_rx_perr)&&(~i_wb_data[9]);

                        r_rx_ferr <= (r_rx_ferr)&&(~i_wb_data[10]);

                end else if (rx_stb)

                begin

                        // On an rx_stb, capture any parity or framing error

                        // indications.  These aren't kept with the data rcvd,

                        // but rather kept external to the FIFO.  As a result,

                        // if you get a parity or framing error, you will never

                        // know which data byte it was associated with.

                        // For now ... that'll work.

                        r_rx_perr <= (r_rx_perr)||(rx_perr);

                        r_rx_ferr <= (r_rx_ferr)||(rx_ferr);

                end

        initial rx_uart_reset = 1'b1;

        always @(posedge i_clk)

                if ((i_rst)||((i_wb_stb)&&(i_wb_addr[1:0]==`UART_SETUP)&&(i_wb_we)))

                        // The receiver reset, always set on a master reset

                        // request.

                        rx_uart_reset <= 1'b1;

                else if ((i_wb_stb)&&(i_wb_addr[1:0]==`UART_RXREG)&&(i_wb_we))

                        // Writes to the receive register will command a receive

                        // reset anytime bit[12] is set.

                        rx_uart_reset <= i_wb_data[12];

                else

                        rx_uart_reset <= 1'b0;

        // Finally, we'll construct a 32-bit value from these various wires,

        // to be returned over the bus on any read.  These include the data

        // that would be read from the FIFO, an error indicator set upon

        // reading from an empty FIFO, a break indicator, and the frame and

        // parity error signals.

        wire    [31:0]   wb_rx_data;

        assign  wb_rx_data = { 16'h00,

                                3'h0, rx_fifo_err,

                                rx_break, rx_ferr, r_rx_perr, !rx_empty_n,

                                rxf_wb_data};

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

        //

        //

        // Then the UART transmitter

        //

        //

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

        wire            tx_empty_n, txf_err, tx_break;

        wire    [7:0]    tx_data;

        wire    [15:0]   txf_status;

        reg             txf_wb_write, tx_uart_reset;

        reg     [7:0]    txf_wb_data;

        // Unlike the receiver which goes from RXUART -> UFIFO -> WB, the

        // transmitter basically goes WB -> UFIFO -> TXUART.  Hence, to build

        // support for the transmitter, we start with the command to write data

        // into the FIFO.  In this case, we use the act of writing to the 

        // UART_TXREG address as our indication that we wish to write to the 

        // FIFO.  Here, we create a write command line, and latch the data for

        // the extra clock that it'll take so that the command and data can be

        // both true on the same clock.

        initial txf_wb_write = 1'b0;

        always @(posedge i_clk)

        begin

                txf_wb_write <= (i_wb_stb)&&(i_wb_addr == `UART_TXREG)

                                        &&(i_wb_we);

                txf_wb_data  <= i_wb_data[7:0];

        end

        // Transmit FIFO

        //

        // Most of this is just wire management.  The TX FIFO is identical in

        // implementation to the RX FIFO (theyre both UFIFOs), but the TX

        // FIFO is fed from the WB and read by the transmitter.  Some key

        // differences to note: we reset the transmitter on any request for a

        // break.  We read from the FIFO any time the UART transmitter is idle.

        // and ... we just set the values (above) for controlling writing into

        // this.

        ufifo   #(.LGFLEN(LGFLEN), .RXFIFO(0))

                txfifo(i_clk, (tx_break)||(tx_uart_reset),

                        txf_wb_write, txf_wb_data,

                        tx_empty_n,

                        (!tx_busy)&&(tx_empty_n), tx_data,

                        txf_status, txf_err);

        // Let's create two transmit based interrupts from the FIFO for the CPU.

        //      The first will be true any time the FIFO has at least one open

        //      position within it.

        assign  o_uart_tx_int = txf_status[0];

        //      The second will be true any time the FIFO is less than half

        //      full, allowing us a change to always keep it (near) fully 

        //      charged.

        assign  o_uart_txfifo_int = txf_status[1];

`ifndef USE_LITE_UART

        // Break logic

        //

        // A break in a UART controller is any time the UART holds the line

        // low for an extended period of time.  Here, we capture the wb_data[9]

        // wire, on writes, as an indication we wish to break.  As long as you

        // write unsigned characters to the interface, this will never be true

        // unless you wish it to be true.  Be aware, though, writing a valid

        // value to the interface will bring it out of the break condition.

        reg     r_tx_break;

        initial r_tx_break = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        r_tx_break <= 1'b0;

                else if ((i_wb_stb)&&(i_wb_addr[1:0]==`UART_TXREG)&&(i_wb_we))

                        r_tx_break <= i_wb_data[9];

        assign  tx_break = r_tx_break;

`else

        assign  tx_break = 1'b0;

`endif

        // TX-Reset logic

        //

        // This is nearly identical to the RX reset logic above.  Basically,

        // any time someone writes to bit [12] the transmitter will go through

        // a reset cycle.  Keep bit [12] low, and everything will proceed as

        // normal.

        initial tx_uart_reset = 1'b1;

        always @(posedge i_clk)

                if((i_rst)||((i_wb_stb)&&(i_wb_addr == `UART_SETUP)&&(i_wb_we)))

                        tx_uart_reset <= 1'b1;

                else if ((i_wb_stb)&&(i_wb_addr[1:0]==`UART_TXREG)&&(i_wb_we))

                        tx_uart_reset <= i_wb_data[12];

                else

                        tx_uart_reset <= 1'b0;

`ifdef  USE_LITE_UART

        txuart  #(INITIAL_SETUP[23:0]) tx(i_clk, (tx_empty_n), tx_data,

                        o_uart_tx, tx_busy);

`else

        wire    cts_n;

        assign  cts_n = (HARDWARE_FLOW_CONTROL_PRESENT)&&(i_cts_n);

        // Finally, the UART transmitter module itself.  Note that we haven't

        // connected the reset wire.  Transmitting is as simple as setting

        // the stb value (here set to tx_empty_n) and the data.  When these

        // are both set on the same clock that tx_busy is low, the transmitter

        // will move on to the next data byte.  Really, the only thing magical

        // here is that tx_empty_n wire--thus, if there's anything in the FIFO,

        // we read it here.  (You might notice above, we register a read any

        // time (tx_empty_n) and (!tx_busy) are both true---the condition for

        // starting to transmit a new byte.)

        txuart  #(INITIAL_SETUP) tx(i_clk, 1'b0, uart_setup,

                        r_tx_break, (tx_empty_n), tx_data,

                        cts_n, o_uart_tx, tx_busy);

`endif

        // Now that we are done with the chain, pick some wires for the user

        // to read on any read of the transmit port.

        //

        // This port is different from reading from the receive port, since

        // there are no side effects.  (Reading from the receive port advances

        // the receive FIFO, here only writing to the transmit port advances the

        // transmit FIFO--hence the read values are free for ... whatever.)  

        // We choose here to provide information about the transmit FIFO

        // (txf_err, txf_half_full, txf_full_n), information about the current

        // voltage on the line (o_uart_tx)--and even the voltage on the receive

        // line (ck_uart), as well as our current setting of the break and

        // whether or not we are actively transmitting.

        wire    [31:0]   wb_tx_data;

        assign  wb_tx_data = { 16'h00,

                                i_cts_n, txf_status[1:0], txf_err,

                                ck_uart, o_uart_tx, tx_break, (tx_busy|txf_status[0]),

                                (tx_busy|txf_status[0])?txf_wb_data:8'b00};

        // Each of the FIFO's returns a 16 bit status value.  This value tells

        // us both how big the FIFO is, as well as how much of the FIFO is in 

        // use.  Let's merge those two status words together into a word we

        // can use when reading about the FIFO.

        wire    [31:0]   wb_fifo_data;

        assign  wb_fifo_data = { txf_status, rxf_status };

        // You may recall from above that reads take two clocks.  Hence, we

        // need to delay the address decoding for a clock until the data is 

        // ready.  We do that here.

        reg     [1:0]    r_wb_addr;

        always @(posedge i_clk)

                r_wb_addr <= i_wb_addr;

        // Likewise, the acknowledgement is delayed by one clock.

        reg     r_wb_ack;

        always @(posedge i_clk) // We'll ACK in two clocks

                r_wb_ack <= i_wb_stb;

        always @(posedge i_clk) // Okay, time to set the ACK

                o_wb_ack <= r_wb_ack;

        // Finally, set the return data.  This data must be valid on the same

        // clock o_wb_ack is high.  On all other clocks, it is irrelelant--since

        // no one cares, no one is reading it, it gets lost in the mux in the

        // interconnect, etc.  For this reason, we can just simplify our logic.

        always @(posedge i_clk)

                casez(r_wb_addr)

                `UART_SETUP: o_wb_data <= { 1'b0, uart_setup };

                `UART_FIFO:  o_wb_data <= wb_fifo_data;

                `UART_RXREG: o_wb_data <= wb_rx_data;

                `UART_TXREG: o_wb_data <= wb_tx_data;

                endcase

        // This device never stalls.  Sure, it takes two clocks, but they are

        // pipelined, and nothing stalls that pipeline.  (Creates FIFO errors,

        // perhaps, but doesn't stall the pipeline.)  Hence, we can just

        // set this value to zero.

        assign  o_wb_stall = 1'b0;

endmodule