Subversion Repositories uart6551

// ============================================================================
//        __
//   \\__/ o\    (C) 2005-2023  Robert Finch, Waterloo
//    \  __ /    All rights reserved.
//     \/_//     robfinch<remove>@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