Debug a microcontroller-to-FPGA interface from the FPGA side

module virtual_jtag_fifo(clk,wr_en,data_in);

parameter data_width  = 32,

          fifo_depth  = 256,

          addr_width  = 8,

          al_full_val = 255,

          al_empt_val = 0;

input clk;

input wr_en;

input [data_width-1:0] data_in;

wire tdi, tck, cdr, cir, e1dr, e2dr, pdr, sdr, udr, uir;

reg  tdo;

reg  [addr_width-1:0] usedw_instr_reg;

reg  reset_instr_reg;

reg  [data_width-1:0] read_instr_reg;

reg  bypass_reg;

wire [1:0] ir_in;

wire usedw_instr  = ~ir_in[1] &  ir_in[0]; // 1

wire reset_instr  =  ir_in[1] & ~ir_in[0]; // 2

wire read_instr   =  ir_in[1] &  ir_in[0]; // 3

wire reset = reset_instr && e1dr;

wire [addr_width-1:0] usedw;

wire [data_width-1:0] data_out;

wire full;

wire al_full;

reg read_instr_d1;

reg read_instr_d2;

reg read_instr_d3;

wire rd_en = read_instr_d2 & !read_instr_d3;

always @(posedge clk or posedge reset)

begin

  if (reset)

  begin

    read_instr_d1 <= 1'b0;

    read_instr_d2 <= 1'b0;

    read_instr_d3 <= 1'b0;

  end

  else

  begin

    read_instr_d1 <= read_instr;

    read_instr_d2 <= read_instr_d1;

    read_instr_d3 <= read_instr_d2;

  end

end

scfifo  jtag_fifo (

        .aclr (reset),

        .clock (clk),

        .wrreq (wr_en & !al_full),

        .data (data_in),

        .rdreq (rd_en),

        .q (data_out),

        .full (full),

        .almost_full (al_full),

        .empty (),

        .almost_empty (),

        .usedw (usedw),

        .sclr ());

    defparam

        jtag_fifo.lpm_width = data_width,

        jtag_fifo.lpm_numwords = fifo_depth,

        jtag_fifo.lpm_widthu = addr_width,

        jtag_fifo.intended_device_family = "Stratix II",

        jtag_fifo.almost_full_value = al_full_val,

        jtag_fifo.almost_empty_value = al_empt_val,

        jtag_fifo.lpm_type = "scfifo",

        jtag_fifo.lpm_showahead = "OFF",

        jtag_fifo.overflow_checking = "ON",

        jtag_fifo.underflow_checking = "ON",

        jtag_fifo.use_eab = "ON",

        jtag_fifo.add_ram_output_register = "ON";

/* usedw_instr Instruction Handler */

always @ (posedge tck)

  if ( usedw_instr && cdr )

    usedw_instr_reg <= usedw;

  else if ( usedw_instr && sdr )

    usedw_instr_reg <= {tdi, usedw_instr_reg[addr_width-1:1]};

/* reset_instr Instruction Handler */

always @ (posedge tck)

  if ( reset_instr && sdr )

    reset_instr_reg <= tdi;//{tdi, reset_instr_reg[data_width-1:1]};

/* read_instr Instruction Handler */

always @ (posedge tck)

  if ( read_instr && cdr )

    read_instr_reg <= data_out;

  else if ( read_instr && sdr )

    read_instr_reg <= {tdi, read_instr_reg[data_width-1:1]};

/* Bypass register */

always @ (posedge tck)

  bypass_reg = tdi;

/* Node TDO Output */

always @ ( usedw_instr, reset_instr, read_instr, usedw_instr_reg[0], reset_instr_reg/*[0]*/, read_instr_reg[0], bypass_reg )

begin

  if (usedw_instr)

    tdo <= usedw_instr_reg[0];

  else if (reset_instr)

    tdo <= reset_instr_reg/*[0]*/;

  else if (read_instr)

    tdo <= read_instr_reg[0];

  else

    tdo <= bypass_reg; // Used to maintain the continuity of the scan chain.

end

sld_virtual_jtag sld_virtual_jtag_component (

.ir_in (ir_in),

.ir_out (2'b0),

.tdo (tdo),

.tdi (tdi),

.tms (),

.tck (tck),

.virtual_state_cir (cir),

.virtual_state_pdr (pdr),

.virtual_state_uir (uir),

.virtual_state_sdr (sdr),

.virtual_state_cdr (cdr),

.virtual_state_udr (udr),

.virtual_state_e1dr (e1dr),

.virtual_state_e2dr (e2dr),

.jtag_state_rti (),

.jtag_state_e1dr (),

.jtag_state_e2dr (),

.jtag_state_pir (),

.jtag_state_tlr (),

.jtag_state_sir (),

.jtag_state_cir (),

.jtag_state_uir (),

.jtag_state_pdr (),

.jtag_state_sdrs (),

.jtag_state_sdr (),

.jtag_state_cdr (),

.jtag_state_udr (),

.jtag_state_sirs (),

.jtag_state_e1ir (),

.jtag_state_e2ir ());

defparam

sld_virtual_jtag_component.sld_auto_instance_index = "NO",

sld_virtual_jtag_component.sld_instance_index = 0,

sld_virtual_jtag_component.sld_ir_width = 2,

sld_virtual_jtag_component.sld_sim_action = "((1,1,1,2))",

sld_virtual_jtag_component.sld_sim_n_scan = 1,

sld_virtual_jtag_component.sld_sim_total_length = 2;

endmodule