[SOLVED] Test bench for Seven Segment Display (Nexys 3 board)

[BojackHorseman]

I want to simulate the code for a seven segment display but I am not getting how to write the test bench for the same. I generate the test bench using ISE Xilinx but when I simulate by giving input values SEG and AN are shown XXXXXX(in red) in the simulation waveform.


Code:

module sseg(
   input clk_100MHz,               // Nexys 3 clock
   input [3:0] ones,  // ones value of the input number
    input [3:0] tens,  // tens value of the input number
    input [3:0] hundreds, // hundreds value of the input nnumber
    input [3:0] thousands ,// thousands value of the input number
    output reg [6:0] SEG,           // 7 Segments of Displays
    output reg [3:0] AN             // 4 Anodes Display
    );
  
  

  
    // Parameters for segment patterns
    parameter ZERO  = 7'b000_0001;  // 0
    parameter ONE   = 7'b100_1111;  // 1
    parameter TWO   = 7'b001_0010;  // 2
    parameter THREE = 7'b000_0110;  // 3
    parameter FOUR  = 7'b100_1100;  // 4
    parameter FIVE  = 7'b010_0100;  // 5
    parameter SIX   = 7'b010_0000;  // 6
    parameter SEVEN = 7'b000_1111;  // 7
    parameter EIGHT = 7'b000_0000;  // 8
    parameter NINE  = 7'b000_0100;  // 9


    // To select each digit in turn
    reg [1:0] anode_select;       
    reg [16:0] anode_timer;           
    // Logic for controlling digit select and digit timer
    always @(posedge clk_100MHz) begin  // 1ms x 4 displays = 4ms refresh period
        if(anode_timer == 99_999) begin         // The period of 100MHz clock is 10ns (1/100,000,000 seconds)
            anode_timer <= 0;                   // 10ns x 100,000 = 1ms
            anode_select <=  anode_select + 1;
        end
        else
            anode_timer <=  anode_timer + 1;
    end
  
    // Logic for driving the 4 bit anode output based on digit select
    always @(anode_select) begin
        case(anode_select)
            2'b00 : AN = 4'b1110;   // Turn on ones digit
            2'b01 : AN = 4'b1101;   // Turn on tens digit
            2'b10 : AN = 4'b1011;   // Turn on hundreds digit
            2'b11 : AN = 4'b0111;   // Turn on thousands digit
        endcase
    end
  
    always @*
        case(anode_select)
            2'b00 : begin               
                                case(ones)
                            4'b0000 : SEG = ZERO;
                            4'b0001 : SEG = ONE;
                            4'b0010 : SEG = TWO;
                            4'b0011 : SEG = THREE;
                            4'b0100 : SEG = FOUR;
                            4'b0101 : SEG = FIVE;
                            4'b0110 : SEG = SIX;
                            4'b0111 : SEG = SEVEN;
                            4'b1000 : SEG = EIGHT;
                            4'b1001 : SEG = NINE;
                        endcase
                    end
                      
            2'b01 : begin
                                case(tens)
                            4'b0000 : SEG = ZERO;
                            4'b0001 : SEG = ONE;
                            4'b0010 : SEG = TWO;
                            4'b0011 : SEG = THREE;
                            4'b0100 : SEG = FOUR;
                            4'b0101 : SEG = FIVE;
                            4'b0110 : SEG = SIX;
                            4'b0111 : SEG = SEVEN;
                            4'b1000 : SEG = EIGHT;
                            4'b1001 : SEG = NINE;
                        endcase
                    end
              
                  
            2'b10 : begin     
                        case(hundreds)
                            4'b0000 : SEG = ZERO;
                            4'b0001 : SEG = ONE;
                            4'b0010 : SEG = TWO;
                            4'b0011 : SEG = THREE;
                            4'b0100 : SEG = FOUR;
                            4'b0101 : SEG = FIVE;
                            4'b0110 : SEG = SIX;
                            4'b0111 : SEG = SEVEN;
                            4'b1000 : SEG = EIGHT;
                            4'b1001 : SEG = NINE;
                        endcase
                    end
                  
            2'b11 : begin     
                        case(thousands)
                            4'b0000 : SEG = ZERO;
                            4'b0001 : SEG = ONE;
                            4'b0010 : SEG = TWO;
                            4'b0011 : SEG = THREE;
                            4'b0100 : SEG = FOUR;
                            4'b0101 : SEG = FIVE;
                            4'b0110 : SEG = SIX;
                            4'b0111 : SEG = SEVEN;
                            4'b1000 : SEG = EIGHT;
                            4'b1001 : SEG = NINE;
                        endcase
                    end
        endcase
 
endmodule

Can someone help me in writing the test bench for the code or give me some hints as to how to write a test bench for this code.

[moderator action: added CODE tags]

 

[std_match]

You don't have start (reset) values for anode_select and anode_timer.
Since the assignments to them depends on the previous value, you get stuck.
Add a reset signal, and check in simulation that anode_select and anode_timer have the expected values.