Subversion Repositories uart6551

// ============================================================================

//        __

//   \\__/ o\    (C) 2005-2023  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

//     \/_//     robfinch@finitron.ca

//       ||

//

//

// BSD 3-Clause License

// Redistribution and use in source and binary forms, with or without

// modification, are permitted provided that the following conditions are met:

//

// 1. Redistributions of source code must retain the above copyright notice, this

//    list of conditions and the following disclaimer.

//

// 2. Redistributions in binary form must reproduce the above copyright notice,

//    this list of conditions and the following disclaimer in the documentation

//    and/or other materials provided with the distribution.

//

// 3. Neither the name of the copyright holder nor the names of its

//    contributors may be used to endorse or promote products derived from

//    this software without specific prior written permission.

//

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE

// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR

// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER

// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,

// OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

//

// 369 LUTs / 409 FFs

// ============================================================================

//

`define UART_ADDR               32'hFE000001

`define UART_TRB                2'd0    // transmit/receive buffer

`define UART_STAT               2'd1

`define UART_CMD                2'd2

`define UART_CTRL               2'd3

module uart6551pci(rst_i, clk_i, cs_config_i, cs_io_i, irq_o,

        cyc_i, stb_i, ack_o, we_i, sel_i, adr_i, dat_i, dat_o,

        cts_ni, rts_no, dsr_ni, dcd_ni, dtr_no, ri_ni,

        rxd_i, txd_o, data_present,

        rxDRQ_o, txDRQ_o,

        xclk_i, RxC_i

);

parameter pClkFreq = 100;

parameter pCounterBits = 24;

parameter pFifoSize = 1024;

parameter pClkDiv = 24'd1302;   // 9.6k baud, 200.000MHz clock

parameter HIGH = 1'b1;

parameter LOW = 1'b0;

parameter CFG_BUS = 8'd0;

parameter CFG_DEVICE = 5'd16;

parameter CFG_FUNC = 3'd0;

parameter CFG_VENDOR_ID =       16'h0;

parameter CFG_DEVICE_ID =       16'h0;

parameter CFG_SUBSYSTEM_VENDOR_ID       = 16'h0;

parameter CFG_SUBSYSTEM_ID = 16'h0;

parameter CFG_ROM_ADDR = 32'hFFFFFFF0;

parameter CFG_REVISION_ID = 8'd0;

parameter CFG_PROGIF = 8'd1;

parameter CFG_SUBCLASS = 8'h80;                                 // 80 = Other

parameter CFG_CLASS = 8'h03;                                            // 03 = display controller

parameter CFG_CACHE_LINE_SIZE = 8'd8;           // 32-bit units

parameter CFG_MIN_GRANT = 8'h00;

parameter CFG_MAX_LATENCY = 8'h00;

parameter CFG_IRQ_LINE = 8'd16;

localparam CFG_HEADER_TYPE = 8'h00;                     // 00 = a general device

parameter MSIX = 1'b0;

input rst_i;

input clk_i;                    // eg 50.000MHz

input cs_config_i;              // config region circuit select

input cs_io_i;          // IO region circuit select

// WISHBONE -------------------------------

input cyc_i;            // bus cycle valid

input stb_i;

output ack_o;

input we_i;                     // 1 = write

input [3:0] sel_i;

input [31:0] adr_i;     // register address

input [31:0] dat_i;     // data input bus

output reg [31:0] dat_o;        // data output bus

//------------------------------------------

output reg [31:0] irq_o;                // interrupt request

input cts_ni;                   // clear to send - (flow control) active low

output reg rts_no;              // request to send - (flow control) active low

input dsr_ni;   // data set ready - active low

input dcd_ni;   // data carrier detect - active low

output reg dtr_no;      // data terminal ready - active low

input ri_ni;            // ring indicator

input rxd_i;            // serial data in

output txd_o;           // serial data out

output data_present;

output rxDRQ_o; // reciever DMA request

output txDRQ_o; // transmitter DMA request

input xclk_i;           // external clock source

input RxC_i;            // external receiver clock source

reg [31:0] uart_addr;

reg accessCD;           // clock multiplier access flag

reg llb;                        // local loopback mode

reg dmaEnable;

// baud rate clock control

reg [4:0] baudRateSel;

reg selCD;                              // Use clock multiplier register

reg [pCounterBits-1:0] c;       // current count

reg [pCounterBits-1:0] ckdiv;   // baud rate clock divider

reg [pCounterBits-1:0] clkdiv;  // clock multiplier register

reg [1:0] xclks;        // synchronized external clock

reg [1:0] RxCs;         // synchronized external receiver clock

reg baud16;                     // 16x baud rate clock

wire baud16rx;          // reciever clock

reg xClkSrc;            // uart baud clock is external

reg rxClkSrc;           // receiver clock is external

// frame format registers

reg [3:0] wordLength;

reg stopBit;

reg [2:0] stopBits;

reg [2:0] parityCtrl;

wire [7:0] frameSize;

reg txBreak;            // transmit a break

wire rxFull;

wire rxEmpty;

wire txFull;

wire txEmpty;

reg hwfc;                       // hardware flow control enable

wire [7:0] lineStatusReg;

wire [7:0] modemStatusReg;

wire [7:0] irqStatusReg;

// interrupt

wire irq_en;            // global IRQ enable

reg irq;

reg rxIe;

reg txIe;

reg modemStatusChangeIe;

wire modemStatusChange;

reg lineStatusChangeIe;

wire lineStatusChange;

reg rxToutIe;           // receiver timeout interrupt enable

reg [3:0] rxThres;      // receiver threshold for interrupt

reg [3:0] txThres;      // transmitter threshold for interrupt

reg rxTout;                     // receiver timeout

wire [9:0] rxCnt;       // reciever counter value

reg [7:0] rxToutMax;

reg [2:0] irqenc;       // encoded irq cause

wire rxITrig;           // receiver interrupt trigger level

wire txITrig;           // transmitter interrupt trigger level

// reciever errors

wire parityErr;         // reciever detected a parity error

wire frameErr;          // receiver char framing error

wire overrun;           // receiver over run

wire rxBreak;           // reciever detected a break

wire rxGErr;            // global error: there is at least one error in the reciever fifo

// modem controls

reg [1:0] ctsx;         // cts_n sampling

reg [1:0] dcdx;

reg [1:0] dsrx;

reg [1:0] rix;

reg deltaCts;

reg deltaDcd;

reg deltaDsr;

reg deltaRi;

// fifo

reg rxFifoClear;

reg txFifoClear;

reg txClear;

reg fifoEnable;

wire [3:0] rxQued;

wire [3:0] txQued;

// config

reg [31:0] cfg_out;

// test

wire txd1;

assign data_present = ~rxEmpty;

assign rxITrig = rxQued >= rxThres;

assign txITrig = txQued <= txThres;

wire rxDRQ1 = (fifoEnable ? rxITrig : ~rxEmpty);

wire txDRQ1 = (fifoEnable ? txITrig : txEmpty);

assign rxDRQ_o = dmaEnable & rxDRQ1;

assign txDRQ_o = dmaEnable & txDRQ1;

wire rxIRQ = rxIe & rxDRQ1;

wire txIRQ = txIe & txDRQ1;

reg [7:0] cmd0, cmd1, cmd2, cmd3;

reg [7:0] ctrl0, ctrl1, ctrl2, ctrl3;

always_ff @(posedge clk_i)

        irq <= (

          rxIRQ

        | txIRQ

        | (rxTout & rxToutIe)

        | (lineStatusChange & lineStatusChangeIe)

        | (modemStatusChange & modemStatusChangeIe)

        ) & irq_en;

        ;

// Hold onto address and data an extra cycle.

// The extra cycle updates or reads the serial transmit / receive.

reg [31:0] dati;

always @(posedge clk_i)

        dati <= dat_i;

reg [31:0] adr_h;

always @(posedge clk_i)

        adr_h <= adr_i;

reg we;

always @(posedge clk_i)

        we <= we_i;

reg [3:0] sel;

always @(posedge clk_i)

        sel <= sel_i;

wire [7:0] rx_do;

wire rdrx = cs_io && ack_o && adr_h==`UART_TRB && ~we && !accessCD;

wire txrx = cs_io && ack_o && adr_h==`UART_TRB && !accessCD;

wire cs_config = cs_config_i & cyc_i & stb_i &&

        adr_i[27:20]==CFG_BUS &&

        adr_i[19:15]==CFG_DEVICE &&

        adr_i[14:12]==CFG_FUNC;

wire cs_io = cs_io_i & cyc_i & stb_i && adr_i[27:4]==uart_addr[27:4];

ack_gen #(

        .READ_STAGES(1),

        .WRITE_STAGES(0),

        .REGISTER_OUTPUT(1)

) uag1

(

        .rst_i(rst_i),

        .clk_i(clk_i),

        .ce_i(1'b1),

        .rid_i('d0),

        .wid_i('d0),

        .i((cs_config|cs_io) & ~we_i),

        .we_i((cs_config|cs_io) & we_i),

        .o(ack_o),

        .rid_o(),

        .wid_o()

);

pci32_config #(

        .CFG_BUS(CFG_BUS),

        .CFG_DEVICE(CFG_DEVICE),

        .CFG_FUNC(CFG_FUNC),

        .CFG_VENDOR_ID(CFG_VENDOR_ID),

        .CFG_DEVICE_ID(CFG_DEVICE_ID),

        .CFG_BAR0(`UART_ADDR),

        .CFG_SUBSYSTEM_VENDOR_ID(CFG_SUBSYSTEM_VENDOR_ID),

        .CFG_SUBSYSTEM_ID(CFG_SUBSYSTEM_ID),

        .CFG_ROM_ADDR(CFG_ROM_ADDR),

        .CFG_REVISION_ID(CFG_REVISION_ID),

        .CFG_PROGIF(CFG_PROGIF),

        .CFG_SUBCLASS(CFG_SUBCLASS),

        .CFG_CLASS(CFG_CLASS),

        .CFG_CACHE_LINE_SIZE(CFG_CACHE_LINE_SIZE),

        .CFG_MIN_GRANT(CFG_MIN_GRANT),

        .CFG_MAX_LATENCY(CFG_MAX_LATENCY),

        .CFG_IRQ_LINE(CFG_IRQ_LINE)

)

ucfg1

(

        .rst_i(rst_i),

        .clk_i(clk_i),

        .irq_i(irq),

        .irq_o(irq_o),

        .cs_config_i(cs_config),

        .we_i(we),

        .sel_i(sel),

        .adr_i(adr_h),

        .dat_i(dati),

        .dat_o(cfg_out),

        .bar0_o(uart_addr),

        .bar1_o(),

        .bar2_o(),

        .irq_en_o(irq_en)

);

uart6551Rx uart_rx0

(

        .rst(rst_i),

        .clk(clk_i),

        .cyc(cyc_i),

        .cs(rdrx),

        .wr(we),

        .dout(rx_do),

        .ack(),

        .fifoEnable(fifoEnable),

        .fifoClear(rxFifoClear),

        .clearGErr(1'b0),

        .wordLength(wordLength),

        .parityCtrl(parityCtrl),

        .frameSize(frameSize),

        .stop_bits(stopBits),

        .baud16x_ce(baud16rx),

        .clear(1'b0),

        .rxd(llb ? txd1 : rxd_i),

        .full(),

        .empty(rxEmpty),

        .frameErr(frameErr),

        .overrun(overrun),

        .parityErr(parityErr),

        .break_o(rxBreak),

        .gerr(rxGErr),

        .qcnt(rxQued),

        .cnt(rxCnt)

);

uart6551Tx uart_tx0

(

        .rst(rst_i),

        .clk(clk_i),

        .cyc(cyc_i),

        .cs(txrx),

        .wr(we),

        .din(dati[7:0]),

        .ack(),

        .fifoEnable(fifoEnable),

        .fifoClear(txFifoClear),

        .txBreak(txBreak),

        .frameSize(frameSize),  // 16 x 10 bits

        .wordLength(wordLength),// 8 bits

        .parityCtrl(parityCtrl),// no parity

        .baud16x_ce(baud16),

        .cts(ctsx[1]|~hwfc),

        .clear(txClear),

        .txd(txd1),

        .full(txFull),

        .empty(txEmpty),

        .qcnt(txQued)

);

assign txd_o = llb ? 1'b1 : txd1;

assign lineStatusReg = {rxGErr,1'b0,txFull,rxBreak,1'b0,1'b0,1'b0,1'b0};

assign modemStatusChange = deltaDcd|deltaRi|deltaDsr|deltaCts;  // modem status delta

assign modemStatusReg = {1'b0,~rix[1],1'b0,~ctsx[1],deltaDcd, deltaRi, deltaDsr, deltaCts};

assign irqStatusReg = {irq_o,2'b00,irqenc,2'b00};

// mux the reg outputs

always @(posedge clk_i)

if (cs_config)

        dat_o <= cfg_out;

else if (cs_io) begin

        case(adr_i)

        `UART_TRB:      dat_o <= accessCD ? {8'h0,clkdiv} : {24'h0,rx_do};      // receiver holding register

        `UART_STAT:     dat_o <= {irqStatusReg,modemStatusReg,lineStatusReg,irq_o,dsrx[1],dcdx[1],fifoEnable ? ~txFull : txEmpty,~rxEmpty,overrun,frameErr,parityErr};

        `UART_CMD:      dat_o <= {cmd3,cmd2,cmd1,cmd0};

        `UART_CTRL:     dat_o <= {ctrl3,ctrl2,ctrl1,ctrl0};

        endcase

end

else

        dat_o <= 'd0;

// register updates

always @(posedge clk_i)

if (rst_i) begin

        rts_no <= HIGH;

        dtr_no <= HIGH;

        // interrupts

        rxIe                            <= 1'b0;

        txIe                            <= 1'b0;

        modemStatusChangeIe     <= 1'b0;

        lineStatusChangeIe      <= 1'b0;

        hwfc                            <= 1'b0;

        modemStatusChangeIe     <= 1'b0;

        lineStatusChangeIe      <= 1'b0;

        dmaEnable                       <= 1'b0;

        // clock control

        baudRateSel <= 5'h0;

        rxClkSrc        <= 1'b0;                // ** 6551 defaults to zero (external receiver clock)

        clkdiv <= pClkDiv;

        // frame format

        wordLength      <= 4'd8;        // 8 bits

        stopBit         <= 1'b0;                // 1 stop bit

        parityCtrl      <= 3'd0;        // no parity

        txBreak         <= 1'b0;

        // Fifo control

        txFifoClear     <= 1'b1;

        rxFifoClear <= 1'b1;

        txClear <= 1'b1;

        fifoEnable      <= 1'b1;

        // Test

        llb                     <= 1'b0;

        selCD           <= 1'b0;

        accessCD   <= 1'b0;

end

else begin

        //llb <= 1'b1;

        rxFifoClear <= 1'b0;

        txFifoClear <= 1'b0;

        txClear <= 1'b0;

        ctrl2[1] <= 1'b0;

        ctrl2[2] <= 1'b0;

        if (cs_io & ack_o & we) begin

                case (adr_h)    // synopsys full_case parallel_case

                `UART_TRB:

                        if (accessCD) begin

                                clkdiv <= dati;

                                accessCD <= 1'b0;

                                ctrl3[7] <= 1'b0;

                        end

                // Writing to the status register does a software reset of some bits.

                `UART_STAT:

                        begin

                                dtr_no <= HIGH;

                                rxIe <= 1'b0;

                                rts_no <= HIGH;

                                txIe <= 1'b0;

                                txBreak <= 1'b0;

                                llb <= 1'b0;

                        end

                `UART_CMD:

      begin

        if (sel[0]) begin

                cmd0 <= dati[7:0];

                                        dtr_no <= ~dati[0];

                rxIe   <= ~dati[1];

                case(dati[3:2])

                2'd0:   begin rts_no <= 1'b1; txIe <= 1'b0; txBreak <= 1'b0; end

                2'd1: begin rts_no <= 1'b0; txIe <= 1'b1; txBreak <= 1'b0; end

                2'd2: begin rts_no <= 1'b0; txIe <= 1'b0; txBreak <= 1'b0; end

                2'd3: begin rts_no <= 1'b0; txIe <= 1'b0; txBreak <= 1'b1; end

                endcase

                llb <= dati[4];

          parityCtrl <= dati[7:5];    //000=none,001=odd,011=even,101=force 1,111 = force 0

        end

        if (sel[1]) begin

                cmd1 <= dati[15:8];

                lineStatusChangeIe  <= dati[8];

                modemStatusChangeIe <= dati[9];

                rxToutIe <= dati[10];

        end

        if (sel[2])

                cmd2 <= dati[23:16];

        if (sel[3])

                cmd3 <= dati[31:24];

      end

    `UART_CTRL:

        begin

                if (sel[0]) begin

                        ctrl0 <= dati[7:0];

                baudRateSel[3:0] <= dati[3:0];

                                        rxClkSrc <= dati[4];                            // 1 = baud rate generator, 0 = external

          //11=5,10=6,01=7,00=8

          case(dati[6:5])

          2'd0: wordLength <= 6'd8;

          2'd1: wordLength <= 6'd7;

          2'd2: wordLength <= 6'd6;

          2'd3: wordLength <= 6'd5;

                endcase

          stopBit    <= dati[7];      //0=1,1=1.5 or 2

        end

                // Extended word length, values beyond 11 not supported.

        if (sel[1]) begin

                ctrl1 <= dati[15:8];

        end

        if (sel[2]) begin

                ctrl2 <= dati[23:16];

                fifoEnable <= dati[16];

                rxFifoClear <= dati[17];

                txFifoClear <= dati[18];

                case (dati[21:20])

                2'd0:   txThres <= 4'd1;                // one-byte

                2'd1:   txThres <= pFifoSize / 4;       // one-quarter full

                2'd2:   txThres <= pFifoSize / 2;       // one-half full

                2'd3:   txThres <= pFifoSize * 3 / 4;   // three-quarters full

                endcase

                case (dati[23:22])

                2'd0:   rxThres <= 4'd1;                // one-byte

                2'd1:   rxThres <= pFifoSize / 4;       // one-quarter full

                2'd2:   rxThres <= pFifoSize / 2;       // one-half full

                2'd3:   rxThres <= pFifoSize * 3 / 4;   // three quarters full

                endcase

        end

        if (sel[3]) begin

                ctrl3 <= dati[31:24];

                                        hwfc <= dati[24];

                                        dmaEnable <= dati[26];

                baudRateSel[4] <= dati[27];

                txClear <= dati[29];

                selCD <= dati[30];

                accessCD <= dati[31];

        end

      end

                default:

                        ;

                endcase

        end

end

// ----------------------------------------------------------------------------

// Baud rate control.

// ----------------------------------------------------------------------------

always @(posedge clk_i)

        xClkSrc <= baudRateSel==5'd0;

wire [pCounterBits-1:0] bclkdiv;

uart6551BaudLUT #(.pClkFreq(pClkFreq), .pCounterBits(pCounterBits)) ublt1 (.a(baudRateSel), .o(bclkdiv));

reg [pCounterBits-1:0] clkdiv2;

always @(posedge clk_i)

        clkdiv2 <= selCD ? clkdiv : bclkdiv;

always @(posedge clk_i)

if (rst_i)

        c <= 24'd1;

else begin

        c <= c + 2'd1;

        if (c >= clkdiv2)

                c <= 2'd1;

end

// for detecting an edge on the baud clock

wire ibaud16 = c == 2'd1;

// Detect an edge on the external clock

wire xclkEdge;

edge_det ed1(.rst(rst_i), .clk(clk_i), .ce(1'b1), .i(xclks[1]), .pe(xclkEdge), .ne() );

// Detect an edge on the external clock

wire rxClkEdge;

edge_det ed2(.rst(rst_i), .clk(clk_i), .ce(1'b1), .i(RxCs[1]), .pe(rxClkEdge), .ne() );

always_comb

if (xClkSrc)            // 16x external clock (xclk)

        baud16 <= xclkEdge;

else

        baud16 <= ibaud16;

assign baud16rx = rxClkSrc ? baud16 : rxClkEdge;

//------------------------------------------------------------

// external signal synchronization

//------------------------------------------------------------

// External receiver clock

always @(posedge clk_i)

        RxCs <= {RxCs[1:0],RxC_i};

// External baud clock

always @(posedge clk_i)

        xclks <= {xclks[1:0],xclk_i};

always @(posedge clk_i)

        ctsx <= {ctsx[0],llb?~rts_no:~cts_ni};

always @(posedge clk_i)

        dcdx <= {dcdx[0],~dcd_ni};

always @(posedge clk_i)

        dsrx <= {dsrx[0],llb?~dtr_no:~dsr_ni};

always @(posedge clk_i)

        rix <= {rix[0],~ri_ni};

//------------------------------------------------------------

// state change detectors

//------------------------------------------------------------

wire ne_stat;

edge_det ued3 (

        .rst(rst_i),

        .clk(clk_i),

        .ce(1'b1),

        .i(ack_o && adr_i==`UART_STAT && ~we_i && sel_i[2]),

        .pe(),

        .ne(ne_stat),

        .ee()

);

// detect a change on the dsr signal

always @(posedge clk_i)

if (rst_i)

        deltaDsr <= 1'b0;

else begin

        if (ne_stat)

                deltaDsr <= 0;

        else if (~deltaDsr)

                deltaDsr <= dsrx[1] ^ dsrx[0];

end

// detect a change on the dcd signal

always @(posedge clk_i)

if (rst_i)

        deltaDcd <= 1'b0;

else begin

        if (ne_stat)

                deltaDcd <= 0;

        else if (~deltaDcd)

                deltaDcd <= dcdx[1] ^ dcdx[0];

end

// detect a change on the cts signal

always @(posedge clk_i)

if (rst_i)

        deltaCts <= 1'b0;

else begin

        if (ne_stat)

                deltaCts <= 0;

        else if (~deltaCts)

                deltaCts <= ctsx[1] ^ ctsx[0];

end

// detect a change on the ri signal

always @(posedge clk_i)

if (rst_i)

        deltaRi <= 1'b0;

else begin

        if (ne_stat)

                deltaRi <= 0;

        else if (~deltaRi)

                deltaRi <= rix[1] ^ rix[0];

end

// detect a change in line status

reg [7:0] pLineStatusReg;

always @(posedge clk_i)

        pLineStatusReg <= lineStatusReg;

assign lineStatusChange = pLineStatusReg != lineStatusReg;

//-----------------------------------------------------

// compute recieve timeout

always @(wordLength)

        rxToutMax <= (wordLength << 2) + 6'd12;

always @(posedge clk_i)

if (rst_i)

        rxTout <= 1'b0;

else begin

        // read of receiver clears timeout counter

        if (rdrx)

                rxTout <= 1'b0;

        // Don't time out if the fifo is empty

        else if (rxCnt[9:4]==rxToutMax && ~rxEmpty)

                rxTout <= 1'b1;

end

//-----------------------------------------------------

// compute the 2x number of stop bits

always @*

if (stopBit==1'b0)          // one stop bit

        stopBits <= 3'd2;

else if (wordLength==6'd8 && parityCtrl != 3'd0)

        stopBits <= 3'd2;

else if (wordLength==6'd5 && parityCtrl == 3'd0)        // 5 bits - 1 1/2 stop bit

        stopBits <= 3'd3;

else

        stopBits <= 3'd4;          // two stop bits

// compute frame size

// frame size is one less

assign frameSize = {wordLength + 4'd1 + stopBits[2:1] + parityCtrl[0], stopBits[0],3'b0} - 1;

//-----------------------------------------------------

// encode IRQ mailbox

always @(rxDRQ_o or rxTout or txDRQ_o or lineStatusChange or modemStatusChange)

        irqenc <=

                lineStatusChange ? 3'd0 :

                ~rxDRQ_o ? 3'd1 :

                rxTout ? 3'd2 :

                ~txDRQ_o ? 3'd3 :

                modemStatusChange ? 3'd4 :

                3'd0;

endmodule