max80_fw/fpga/max80.sv

//
// Top level module for the FPGA on the MAX80 board by
// Per Mårtensson and H. Peter Anvin
//
// This is for MAX80 as slave on the ABC-bus.
//

// Sharing JTAG pins (via JTAGEN)
`undef SHARED_JTAG

module max80 (
	      // Clock oscillator
	      input 	    clock_48, // 48 MHz

	      // ABC-bus
	      input 	    abc_clk, // ABC-bus 3 MHz clock
	      input [15:0]  abc_a, // ABC address bus
	      inout [7:0]   abc_d, // ABC data bus
	      output 	    abc_d_oe, // Data bus output enable
	      input 	    abc_rst_n, // ABC bus reset strobe
	      input 	    abc_cs_n, // ABC card select strobe
	      input [4:0]   abc_out_n, // OUT, C1-C4 strobe
	      input [1:0]   abc_inp_n, // INP, STATUS strobe
	      input 	    abc_xmemfl_n, // Memory read strobe
	      input 	    abc_xmemw800_n, // Memory write strobe (ABC800)
	      input 	    abc_xmemw80_n, // Memory write strobe (ABC80)
	      input 	    abc_xinpstb_n, // I/O read strobe (ABC800)
	      input 	    abc_xoutpstb_n, // I/O write strobe (ABC80)
	      // The following are inverted versus the bus IF
	      // the corresponding MOSFETs are installed
	      output 	    abc_rdy_x, // RDY = WAIT#
	      output 	    abc_resin_x, // System reset request
	      output 	    abc_int80_x, // System INT request (ABC80)
	      output 	    abc_int800_x, // System INT request (ABC800)
	      output 	    abc_nmi_x, // System NMI request (ABC800)
	      output 	    abc_xm_x, // System memory override (ABC800)
	      // Master/slave control
	      output 	    abc_master, // 1 = master, 0 = slave
	      output 	    abc_a_oe,
	      // Bus isolation
	      output 	    abc_d_ce_n,

	      // ABC-bus extension header
	      // (Note: cannot use an array here because HC and HH are
	      // input only.)
	      inout 	    exth_ha,
	      inout 	    exth_hb,
	      input 	    exth_hc,
	      inout 	    exth_hd,
	      inout 	    exth_he,
	      inout 	    exth_hf,
	      inout 	    exth_hg,
	      input 	    exth_hh,

	      // SDRAM bus
	      output 	    sr_clk,
	      output 	    sr_cke,
	      output [1:0]  sr_ba, // Bank address
	      output [12:0] sr_a, // Address within bank
	      inout [15:0]  sr_dq, // Also known as D or IO
	      output [1:0]  sr_dqm, // DQML and DQMH
	      output 	    sr_cs_n,
	      output 	    sr_we_n,
	      output 	    sr_cas_n,
	      output 	    sr_ras_n,

	      // SD card
	      output 	    sd_clk,
	      output 	    sd_cmd,
	      inout [3:0]   sd_dat,

	      // USB serial (naming is FPGA as DCE)
	      input 	    tty_txd,
	      output 	    tty_rxd,
	      input 	    tty_rts,
	      output 	    tty_cts,
	      input 	    tty_dtr,

	      // SPI flash memory (also configuration)
	      output 	    flash_cs_n,
	      output 	    flash_sck,
	      inout [1:0]   flash_io,

	      // SPI bus (connected to ESP32 so can be bidirectional)
	      inout 	    spi_clk,
	      inout 	    spi_miso,
	      inout 	    spi_mosi,
	      inout 	    spi_cs_esp_n, // ESP32 IO10
	      inout 	    spi_cs_flash_n, // ESP32 IO01

	      // Other ESP32 connections
	      inout 	    esp_io0, // ESP32 IO00
	      inout 	    esp_int, // ESP32 IO09

	      // I2C bus (RTC and external)
	      inout 	    i2c_scl,
	      inout 	    i2c_sda,
	      input 	    rtc_32khz,
	      input 	    rtc_int_n,

	      // LED (2 = D23/G, 1 = D22/R, 0 = D17/B)
	      output [2:0]  led,

	      // GPIO pins
	      inout [5:0]   gpio,

	      // HDMI
	      output [2:0]  hdmi_d,
	      output 	    hdmi_clk,
	      inout 	    hdmi_scl,
	      inout 	    hdmi_sda,
	      inout 	    hdmi_hpd
	      );

   // Set if MOSFETs Q1-Q6 are installed rather than the corresponding
   // resistors.
   parameter [6:1] mosfet_installed = 6'b000_000;

   // PLL and reset
   parameter reset_pow2 = 12; // Assert internal reset for 4096 cycles after PLL lock
   reg [reset_pow2-1:0]     rst_ctr = 1'b0;
   reg			    rst_n   = 1'b0;	// Internal reset
   wire [1:0]		    pll_locked;

   // Clocks
   wire	    sdram_clk;		// SDRAM clock
   wire	    sdram_out_clk;	// SDRAM clock, phase shifted
   wire	    sys_clk;		// System clock
   wire	    vid_clk;		// Video pixel clock
   wire	    vid_hdmiclk;	// D:o in the HDMI clock domain
   wire     flash_clk;		// Serial flash ROM clock

   reg 	    reset_cmd_q = 1'b0;
   wire     reset_cmd;

   pll pll (
	    .areset ( reset_cmd_q ),
	    .inclk0 ( clock_48 ),
	    .c0 ( sdram_out_clk ),	// SDRAM external clock (168 MHz)
	    .c1 ( sys_clk ),		// System clock (84 MHz)
	    .c2 ( vid_clk ),		// Video pixel clock (48 MHz)
	    .c3 ( flash_clk ),		// Serial flash ROM clock (134 MHz)
	    .c4 ( sdram_clk ),		// SDRAM internal clock (168 MHz)
	    .locked ( pll_locked[0] ),
	    .phasestep ( 1'b0 ),
	    .phasecounterselect ( 3'b0 ),
	    .phaseupdown ( 1'b1 ),
	    .scanclk ( 1'b0 ),
	    .phasedone ( )
	    );

   wire all_plls_locked = &pll_locked;

   always @(negedge all_plls_locked or posedge sys_clk)
     if (~&all_plls_locked)
       begin
	  rst_ctr     <= 1'b0;
	  rst_n       <= 1'b0;
	  reset_cmd_q <= 1'b0;
       end
     else
       begin
	  reset_cmd_q <= rst_n & (reset_cmd_q | reset_cmd);
	  if (~rst_n)
	    { rst_n, rst_ctr } <= rst_ctr + 1'b1;
       end

   // Unused device stubs - remove when used

   // Reset in the video clock domain
   reg vid_rst_n;
   always @(negedge all_plls_locked or posedge vid_clk)
     if (~all_plls_locked)
       vid_rst_n <= 1'b0;
     else
       vid_rst_n <= rst_n;

   // HDMI - generate random data to give Quartus something to do
   reg [23:0] dummydata = 30'hc8_fb87;

   always @(posedge vid_clk)
     dummydata <= { dummydata[22:0], dummydata[23] };

   wire [7:0] hdmi_data[3];
   wire [9:0] hdmi_tmds[3];
   wire [29:0] hdmi_to_tx;

   assign hdmi_data[0] = dummydata[7:0];
   assign hdmi_data[1] = dummydata[15:8];
   assign hdmi_data[2] = dummydata[23:16];

   generate
      genvar   i;
      for (i = 0; i < 3; i = i + 1)
	begin : hdmitmds
	   tmdsenc enc (
		    .rst_n ( vid_rst_n ),
		    .clk ( vid_clk ),
		    .den ( 1'b1 ),
		    .d ( hdmi_data[i] ),
		    .c ( 2'b00 ),
		    .q ( hdmi_tmds[i] )
		    );
	end
   endgenerate

   assign hdmi_scl = 1'bz;
   assign hdmi_sda = 1'bz;
   assign hdmi_hpd = 1'bz;

   //
   // The ALTLVDS_TX megafunctions is MSB-first and in time-major order.
   // However, TMDS is LSB-first, and we have three TMDS words that
   // concatenate in word(channel)-major order.
   //
   transpose #(.words(3), .bits(10), .reverse_b(1),
	       .reg_d(0), .reg_q(0)) hdmitranspose
     (
      .clk ( vid_clk ),
      .d ( { hdmi_tmds[2], hdmi_tmds[1], hdmi_tmds[0] } ),
      .q ( hdmi_to_tx )
      );

   hdmitx hdmitx (
		  .pll_areset ( ~pll_locked[0] ),
		  .tx_in ( hdmi_to_tx ),
		  .tx_inclock ( vid_clk ),
		  .tx_coreclock ( vid_hdmiclk ), // Pixel clock in HDMI domain
		  .tx_locked ( pll_locked[1] ),
		  .tx_out ( hdmi_d ),
		  .tx_outclock ( hdmi_clk )
		  );

   // ABC bus

   // On ABC800, only one of XINPSTB# or XOUTPSTB# will be active;
   // on ABC80 they will either be 00 or ZZ; in the latter case pulled
   // low by external resistors.
   wire abc800 = abc_xinpstb_n | abc_xoutpstb_n;
   wire abc80  = ~abc800;

   // Memory read/write strobes
   wire abc_xmemrd = ~abc_xmemfl_n; // For consistency
   wire abc_xmemwr = abc800 ? ~abc_xmemw800_n : ~abc_xmemw80_n;

   // I/O read/write strobes
   wire abc_iord = (abc800 & ~abc_xinpstb_n)  | ~(|abc_inp_n);
   wire abc_iowr = (abc800 & ~abc_xoutpstb_n) | ~(|abc_out_n);

   reg [7:0] abc_do;
   reg [7:0] abc_di;
   assign abc_d_oe = abc_xmemrd;
   assign abc_d    = abc_d_oe ? abc_do : 8'hzz;

   // Open drain signals with optional MOSFETs
   wire abc_wait;
   wire abc_resin;
   wire abc_int;
   wire abc_nmi;
   wire abc_xm;

   function reg opt_mosfet(input signal, input mosfet);
      if (mosfet)
	opt_mosfet = signal;
      else
	opt_mosfet = signal ? 1'b0 : 1'bz;
   endfunction // opt_mosfet

   assign abc_int80_x  = opt_mosfet(abc_int & abc80, mosfet_installed[1]);
   assign abc_rdy_x    = opt_mosfet(abc_wait, mosfet_installed[2]);
   assign abc_nmi_x    = opt_mosfet(abc_nmi, mosfet_installed[3]);
   assign abc_resin_x  = opt_mosfet(abc_resin, mosfet_installed[4]);
   assign abc_int800_x = opt_mosfet(abc_int & abc800, mosfet_installed[5]);
   assign abc_xm_x     = opt_mosfet(abc_xm, mosfet_installed[6]);

   // ABC-bus extension header (exth_c and exth_h are input only)
   // The naming of pins is kind of nonsensical:
   //
   //       +3V3 -  1  2 - +3V3
   //         HA -  3  4 - HE
   //         HB -  5  6 - HG
   //         HC -  7  8 - HH
   //         HD -  9 10 - HF
   //        GND - 11 12 - GND
   //
   // This layout allows the header to be connected on either side
   // of the board. This logic assigns the following names to the pins;
   // if the ext_reversed is set to 1 then the left and right sides
   // are flipped.
   //
   //      +3V3  -  1  2 - +3V3
   //    exth[0] -  3  4 - exth[1]
   //    exth[2] -  5  6 - exth[3]
   //    exth[6] -  7  8 - exth[7]
   //    exth[4] -  9 10 - exth[5]
   //        GND - 11 12 - GND

   wire exth_reversed = 1'b0;
   wire [7:0] exth_d;	// Input data
   wire [5:0] exth_q;	// Output data
   wire [5:0] exth_oe;	// Output enable

   assign exth_d[0]    = exth_reversed ? exth_he : exth_ha;
   assign exth_d[1]    = exth_reversed ? exth_ha : exth_he;
   assign exth_d[2]    = exth_reversed ? exth_hg : exth_hb;
   assign exth_d[3]    = exth_reversed ? exth_hb : exth_hg;
   assign exth_d[4]    = exth_reversed ? exth_hf : exth_hd;
   assign exth_d[5]    = exth_reversed ? exth_hd : exth_hf;
   assign exth_d[6]    = exth_reversed ? exth_hh : exth_hc;
   assign exth_d[7]    = exth_reversed ? exth_hc : exth_hh;

   wire [2:0] erx      = { 2'b00, exth_reversed };
   assign exth_ha      = exth_oe[3'd0 ^ erx] ? exth_q[3'd0 ^ erx] : 1'bz;
   assign exth_he      = exth_oe[3'd1 ^ erx] ? exth_q[3'd1 ^ erx] : 1'bz;
   assign exth_hb      = exth_oe[3'd2 ^ erx] ? exth_q[3'd2 ^ erx] : 1'bz;
   assign exth_hg      = exth_oe[3'd3 ^ erx] ? exth_q[3'd3 ^ erx] : 1'bz;
   assign exth_hd      = exth_oe[3'd4 ^ erx] ? exth_q[3'd4 ^ erx] : 1'bz;
   assign exth_hf      = exth_oe[3'd5 ^ erx] ? exth_q[3'd5 ^ erx] : 1'bz;

   assign exth_q  = 6'b0;
   assign exth_oe = 6'b0;

   // SDRAM controller
   reg	      abc_rrq;
   reg	      abc_wrq;
   reg	      abc_xmemrd_q;
   reg	      abc_xmemwr_q;
   reg	      abc_xmem_done;
   reg [9:0]  abc_mempg;

   wire       abc_rack;
   wire       abc_wack;
   wire       abc_rready;
   wire [7:0] abc_sr_rd;

   always @(posedge sdram_clk or negedge rst_n)
     if (~rst_n)
       begin
	  abc_rrq       <= 1'b0;
	  abc_wrq       <= 1'b0;
	  abc_xmemrd_q  <= 1'b0;
	  abc_xmemwr_q  <= 1'b0;
	  abc_xmem_done <= 1'b0;
	  abc_mempg     <= 0;
       end
     else
       begin
	  abc_di        <= abc_d;
	  abc_xmemrd_q  <= abc_xmemrd;
	  abc_xmemwr_q  <= abc_xmemwr;
	  abc_xmem_done <= (abc_xmemrd_q & (abc_xmem_done | abc_rack))
	    | (abc_xmemwr_q & (abc_xmem_done | abc_wack));

	  abc_rrq <= abc_xmemrd_q & ~(abc_xmem_done | abc_rack);
	  abc_wrq <= abc_xmemwr_q & ~(abc_xmem_done | abc_wack);

	  if (abc_rack & abc_rready)
	    abc_do <= abc_sr_rd;

	  // HACK FOR TESTING ONLY
	  if (abc_iowr)
	    abc_mempg <= { abc_a[1:0], abc_di };
       end // else: !if(~rst_n)

   //
   // Internal CPU bus
   //
   wire			      cpu_mem_valid;
   wire			      cpu_mem_instr;
   wire [ 3:0]		      cpu_mem_wstrb;
   wire [31:0]                cpu_mem_addr;
   wire [31:0]                cpu_mem_wdata;
   reg  [31:0]		      cpu_mem_rdata;
   wire			      cpu_mem_ready;

   wire			      cpu_mem_read = cpu_mem_valid & ~|cpu_mem_wstrb;

   wire                       cpu_la_read;
   wire                       cpu_la_write;
   wire [31:0]                cpu_la_addr;
   wire [31:0]		      cpu_la_wdata;
   wire [ 3:0]		      cpu_la_wstrb;

   // cpu_mem_valid by address quadrant
   wire [ 3:0] cpu_mem_quad = cpu_mem_valid << cpu_mem_addr[31:30];

   // Decode for small devices; use address space within range of
   // negative offsets from the zero register [-1K,0)
   //
   // Device map:
   //  0 - LED
   //  1 - Reset
   //  2 - SPI->SDRAM downloader
   //  3 - Serial port
   //
   // A device has IRQ (devno)+16 if it needs an interrupt.
   //

   wire [15:0] iodev = cpu_mem_quad[3] << cpu_mem_addr[9:6];
   tri0 [15:0] iodev_irq;	// tri0: if nothing is driving, value is 0

   //
   // SDRAM
   //
   wire [31:0] sdram_rd;
   wire        sdram_rack;
   wire        sdram_rready;
   wire        sdram_wack;
   reg	       sdram_acked;

   wire [15:0] sdram_rom_wd;
   wire [24:1] sdram_rom_waddr;
   wire [ 1:0] sdram_rom_wrq;
   wire        sdram_rom_wacc;

   always @(posedge sdram_clk)
     sdram_acked <= cpu_mem_quad[1] & (sdram_acked | sdram_rack | sdram_wack);

   wire        sdram_req = cpu_mem_quad[1] & ~sdram_acked;

   sdram sdram (
		.rst_n    ( rst_n & ~iodev[12] ),
		.clk      ( sdram_clk ), // Internal clock
		.out_clk  ( sdram_out_clk ), // External clock (phase shifted)

		.sr_clk   ( sr_clk ),    // Output clock buffer
		.sr_cke   ( sr_cke ),
		.sr_cs_n  ( sr_cs_n ),
		.sr_ras_n ( sr_ras_n ),
		.sr_cas_n ( sr_cas_n ),
		.sr_we_n  ( sr_we_n ),
		.sr_dqm   ( sr_dqm ),
		.sr_ba    ( sr_ba ),
		.sr_a     ( sr_a ),
		.sr_dq    ( sr_dq ),

		.a0       ( { abc_mempg, abc_a } ),
		.rd0      ( abc_sr_rd ),
		.rrq0     ( abc_rrq ),
		.rack0    ( abc_rack ),
		.rready0  ( abc_rready ),
		.wd0      ( abc_d ),
		.wrq0     ( abc_wrq ),
		.wack0    ( abc_wack ),

		.a1       ( cpu_mem_addr[24:2] ),
		.rd1      ( sdram_rd ),
		.rrq1     ( sdram_req & ~|cpu_mem_wstrb ),
		.rack1    ( sdram_rack ),
		.rready1  ( sdram_rready ),
		.wd1      ( cpu_mem_wdata ),
		.wstrb1   ( {4{sdram_req}} & cpu_mem_wstrb ),
		.wack1    ( sdram_wack ),

		.a2       ( sdram_rom_waddr ),
		.wd2      ( sdram_rom_wd ),
		.wrq2     ( sdram_rom_wrq ),
		.wacc2    ( sdram_rom_wacc )
		);

   // SD card
   assign sd_clk    = 1'b1;
   assign sd_cmd    = 1'b1;
   assign sd_dat    = 4'hz;

   // SPI bus (free for ESP32)
   assign spi_clk        = 1'bz;
   assign spi_miso       = 1'bz;
   assign spi_mosi       = 1'bz;
   assign spi_cs_esp_n   = 1'bz;
   assign spi_cs_flash_n = 1'bz;

   // ESP32
   assign esp_io0  = 1'bz;
   assign esp_int  = 1'bz;

   // I2C
   assign i2c_scl = 1'bz;
   assign i2c_sda = 1'bz;

   // GPIO
   assign gpio    = 6'bzzzzzz;

   // Embedded RISC-V CPU
   parameter cpu_fast_mem_bits = 13; /* 2^[this] * 4 bytes */
   picorv32 #(
	      .ENABLE_COUNTERS ( 1 ),
	      .ENABLE_COUNTERS64 ( 1 ),
	      .ENABLE_REGS_16_31 ( 1 ),
	      .ENABLE_REGS_DUALPORT ( 1 ),
	      .LATCHED_MEM_RDATA ( 1 ),
	      .BARREL_SHIFTER ( 1 ),
	      .TWO_CYCLE_COMPARE ( 0 ),
	      .TWO_CYCLE_ALU ( 0 ),
	      .COMPRESSED_ISA ( 1 ),
	      .CATCH_MISALIGN ( 1 ),
	      .CATCH_ILLINSN ( 1 ),
	      .ENABLE_FAST_MUL ( 1 ),
	      .ENABLE_DIV ( 1 ),
	      .ENABLE_IRQ ( 1 ),
	      .ENABLE_IRQ_QREGS ( 1 ),
	      .ENABLE_IRQ_TIMER ( 1 ),
	      .REGS_INIT_ZERO ( 1 ),
	      .STACKADDR ( 32'h4 << cpu_fast_mem_bits )
	      )

   cpu (
	.clk ( sys_clk ),
	.resetn ( rst_n ),
	.trap ( ),

	.mem_instr ( cpu_mem_instr ),
	.mem_ready ( cpu_mem_ready ),
	.mem_valid ( cpu_mem_valid ),
	.mem_wstrb ( cpu_mem_wstrb ),
	.mem_addr  ( cpu_mem_addr ),
	.mem_wdata ( cpu_mem_wdata ),
	.mem_rdata ( cpu_mem_rdata ),

	.mem_la_read  ( cpu_la_read ),
	.mem_la_write ( cpu_la_write ),
	.mem_la_wdata ( cpu_la_wdata ),
	.mem_la_addr  ( cpu_la_addr ),
	.mem_la_wstrb ( cpu_la_wstrb ),

	.irq ( { iodev_irq, 16'b0 } ),
	.eoi ( )
	);

   // cpu_mem_ready is always true for fast memory; for SDRAM we have to
   // wait either for a write ack or a low-high transition on the
   // read ready signal.
   reg sdram_rready_q;
   reg sdram_mem_ready;
   reg [31:0] sdram_rdata;
   always @(posedge sys_clk)
     begin
	sdram_rready_q <= sdram_rready;
	if (cpu_mem_quad[1])
	  sdram_mem_ready <= sdram_mem_ready | sdram_wack |
			     (sdram_rready & ~sdram_rready_q);
	else
	  sdram_mem_ready <= 1'b0;

	sdram_rdata <= sdram_rd;
     end

   always @(*)
     case ( cpu_mem_quad )
       4'b0000: cpu_mem_ready = 1'b0;
       4'b0001: cpu_mem_ready = 1'b1;
       4'b0010: cpu_mem_ready = sdram_mem_ready;
       4'b0100: cpu_mem_ready = 1'b1;
       4'b1000: cpu_mem_ready = 1'b1;
       default: cpu_mem_ready = 1'bx;
     endcase // case ( mem_quad )

   //
   // Fast memory. This runs on the SDRAM clock, i.e. 2x the speed
   // of the CPU. The .bits parameter gives the number of dwords
   // as a power of 2, i.e. 11 = 2^11 * 4 = 8K.
   //
   wire [31:0] fast_mem_rdata;

   fast_mem // #(.bits(cpu_fast_mem_bits), .mif("../fw/boot"))
   fast_mem(
	    .rst_n ( rst_n ),
	    .clk   ( sys_clk ),
	    .read  ( cpu_la_read  & cpu_la_addr[31:30] == 2'b00 ),
	    .write ( cpu_la_write & cpu_la_addr[31:30] == 2'b00 ),
	    .wstrb ( cpu_la_wstrb ),
	    .addr  ( cpu_la_addr[14:2] ),
	    .wdata ( cpu_la_wdata ),
	    .rdata ( fast_mem_rdata )
	    );

       
   // Input data MUX
   wire [31:0] iodev_rdata;

   always @(*)
     case ( cpu_mem_quad )
       4'b0001: cpu_mem_rdata = fast_mem_rdata;
       4'b0010: cpu_mem_rdata = sdram_rdata;
       4'b1000: cpu_mem_rdata = iodev_rdata;
       default: cpu_mem_rdata = 32'hxxxx_xxxx;
     endcase

   // Hard system reset under program control
   assign reset_cmd = rst_n & iodev[1] & cpu_mem_wstrb[0] & cpu_mem_wdata[0];

   // LED indication from the CPU
   reg [2:0]   led_q;
   always @(negedge rst_n or posedge sys_clk)
     if (~rst_n)
       led_q <= 3'b000;
     else
       if ( iodev[0] & cpu_mem_wstrb[0] )
	 led_q <= cpu_mem_wdata[2:0];

   assign led = led_q;
   
   //
   // Serial ROM (also configuration ROM.) Fast hardwired data download
   // unit to SDRAM.
   //
   wire        rom_done;
   reg 	       rom_done_q;
   
   spirom ddu (
	       .rst_n    ( rst_n ),
	       .rom_clk  ( flash_clk ),
	       .ram_clk  ( sdram_clk ),

	       .spi_sck  ( flash_sck ),
	       .spi_io   ( flash_io ),
	       .spi_cs_n ( flash_cs_n ),

	       .wd       ( sdram_rom_wd ),
	       .waddr    ( sdram_rom_waddr ),
	       .wrq      ( sdram_rom_wrq ),
	       .wacc     ( sdram_rom_wacc ),
	       .done     ( rom_done )
	       );

   always @(posedge sys_clk)
     rom_done_q <= rom_done;
   
   //
   // Serial port. Direct to the CP2102N for reworked
   // boards or to GPIO for non-reworked boards, depending on
   // whether DTR# is asserted on either.
   //
   // The GPIO numbering matches the order of pins for FT[2]232H.
   // gpio[0] - TxD
   // gpio[1] - RxD
   // gpio[2] - RTS#
   // gpio[3] - CTS#
   // gpio[4] - DTR#
   //
   wire        tty_data_out;	// Output data
   wire        tty_data_in;	// Input data
   wire        tty_cts_out;	// Assert CTS# externally
   wire        tty_rts_in;	// RTS# received from outside
   wire [31:0] tty_rdata;

   assign tty_cts_out  = 1'b0;	// Assert CTS#
   tty tty (
	    .rst_n ( rst_n ),
	    .clk ( sys_clk ),

	    .valid ( iodev[3] ),
	    .wstrb ( cpu_mem_wstrb ),
	    .wdata ( cpu_mem_wdata ),
	    .rdata ( tty_rdata ),
	    .addr ( cpu_mem_addr[3:2] ),
	    .irq  ( iodev_irq[3] ),

	    .tty_txd ( tty_data_out ) // DTE -> DCE
	    );

   reg [1:0]   tty_dtr_q;
   always @(posedge sys_clk)
     begin
	tty_dtr_q[0] <= tty_dtr;
	tty_dtr_q[1] <= gpio[4];
     end

   //
   // Route data to the two output ports
   //

   // tty_rxd because pins are DCE named
   assign tty_data_in = (tty_txd | tty_dtr_q[0]) &
			(gpio[0] | tty_dtr_q[1]);

   assign tty_rxd = tty_dtr_q[0] ? 1'bz : tty_data_out;
   assign gpio[1] = tty_dtr_q[1] ? 1'bz : tty_data_out;

   assign tty_rts_in = (tty_rts | tty_dtr_q[0]) &
		       (gpio[2] | tty_dtr_q[1]);

   assign tty_cts = tty_dtr_q[0] ? 1'bz : tty_cts_out;
   assign gpio[3] = tty_dtr_q[1] ? 1'bz : tty_cts_out;

   //
   // I/O device input data MUX
   //
   always @(*)
     case ( cpu_mem_addr[9:6] )
       4'h0:    iodev_rdata = { 29'b0, led_q };
       4'h2:    iodev_rdata = { 31'b0, rom_done_q };
       4'h3:	iodev_rdata = tty_rdata;
       default: iodev_rdata = 32'hffff_ffff;
     endcase
   
endmodule