Reaction Timer | Reflex Tester in Verilog and FPGA

This post will be about programming a reaction timer in verilog and make it ready for FPGA implementation which in our case will be on the BASYS2 (Spartan 3E) board but can be programmed for any other board by simply changing the .ucf file. The reaction timer or reflex tester will check and time how fast you can respond after seeing a visual stimulus or in other words it will test your hand eye co-ordination.

The code for this is a bit more busy than any of my other projects, but it has been heavily commented so just by reading the code you can easily understand what is going on. The code will be followed by a video demonstration in Isim along with it working on the FPGA board.

Here we will be using three inputs; reset, start, stop and one output led along with displaying the time on the seven segment display.

• When the reset button is pressed it will obviously reset all registers and counters and will make the system ready for the next reflex test. Also when in this state it will display a "Hi" message on the seven segment display to welcome the user and to show that it is ready for the next input.
• When the start button is pressed the output led will turn on after a random interval between 0 seconds and 10 seconds. The random time is achieved using the random number generator which I have explained in detail in my previous post. Randomness here is very important, if the test did not have a random delay we could mentally memorize the delay between reset and the stimulus thus ruining the test.
• After the random delay is achieved the led will turn on along with the timer.
• The user is now to press the stop button. This will stop the timer and display your reaction time. The normal reaction time should be between 0.15s and 0.30s.

Something to note here is that since I will be using the seven segment to display the "Hi" as well as the time so in order to make it work I would have to use a multiplexing circuit. The multiplexing circuit will use a decision based code to decide what to display on the 7-segment and at what time. It was instantiated into the main module. So I think it would be best to explain the multiplexing circuit first.

module muxer(
    input clock,
    input reset,
    input [3:0] fourth,
    input [3:0] third,
    input [3:0] second,
    input [3:0] first,
    output a_m,
    output b_m,
    output c_m,
    output d_m,
    output e_m,
    output f_m,
    output g_m,
    output dp_m,
    output [3:0] an_m
    );

//The Circuit for 7 Segment Multiplexing - 

localparam N = 18; 

reg [N-1:0]count; //the 18 bit counter which allows us to multiplex at 1000Hz

always @ (posedge clock or posedge reset)
 begin
  if (reset)
   count <= 0;
  else
   count <= count + 1;
 end

reg [3:0]sseg; //the 4 bit register to hold the data that is to be output
reg [3:0]an_temp; //register for the 4 bit enable
reg reg_dp;
always @ (*)
 begin
  case(count[N-1:N-2]) //MSB and MSB-1 for multiplexing
   
   2'b00 : 
    begin
     sseg = first;
     an_temp = 4'b1110;
     reg_dp = 1'b1;
    end
   
   2'b01:
    begin
     sseg = second;
     an_temp = 4'b1101;
     reg_dp = 1'b0;
    end
   
   2'b10:
    begin
     sseg = third;
     an_temp = 4'b1011;
     reg_dp = 1'b0;
    end
    
   2'b11:
    begin
     sseg = fourth;
     an_temp = 4'b0111;
     reg_dp = 1'b1;
    end
  endcase
 end
assign an_m = an_temp;

reg [6:0] sseg_temp; 
always @ (*)
 begin
  case(sseg)
   4'd0 : sseg_temp = 7'b1000000; //display 0
   4'd1 : sseg_temp = 7'b1111001; //display 1
   4'd2 : sseg_temp = 7'b0100100; //display 2
   4'd3 : sseg_temp = 7'b0110000; //display 3
   4'd4 : sseg_temp = 7'b0011001; //display 4
   4'd5 : sseg_temp = 7'b0010010; //display 5
   4'd6 : sseg_temp = 7'b0000010; //display 6
   4'd7 : sseg_temp = 7'b1111000; //display 7
   4'd8 : sseg_temp = 7'b0000000; //display 8
   4'd9 : sseg_temp = 7'b0010000; //display 9
   4'd10 : sseg_temp = 7'b0001001; //to display H
   4'd11 : sseg_temp = 7'b1001111; //to display I
   default : sseg_temp = 7'b0111111; //dash
  endcase
 end
assign {g_m, f_m, e_m, d_m, c_m, b_m, a_m} = sseg_temp; 
assign dp_m = reg_dp;

endmodule

The multiplexer module has 4 inputs which will have data passed to them from the main module. The outputs are connected to the 7 segment display. For detailed explanation on the above code visit the post: 7 Segment LED multiplexing in Verilog.

And now for the code  of the reaction / reflex tester:

module reaction_main(
  input clock, reset, start, stop,
  output led,
  output [3:0] an,
  output a, b, c, d, e, f, g, dp
  );
reg [3:0] regd3, regd2, regd1, regd0; //the main output registers

wire db_start, db_stop;
reg dffstr1, dffstr2, dffstp1, dffstp2;
 
always @ (posedge clock) dffstr1 <= start;
always @ (posedge clock) dffstr2 <= dffstr1;
 
assign db_start = ~dffstr1 & dffstr2; //monostable multivibrator to detect only one pulse of the button
 
always @ (posedge clock) dffstp1 <= stop;
always @ (posedge clock) dffstp2 <= dffstp1;
 
assign db_stop = ~dffstp1 & dffstp2; //monostable multivibrator to detect only one pulse of the button


// Instantiate the 7 segment multiplexing module
muxer display (
    .clock(clock), 
    .reset(reset), 
    .fourth(regd3), 
    .third(regd2), 
    .second(regd1), 
    .first(regd0), 
    .a_m(a), 
    .b_m(b), 
    .c_m(c), 
    .d_m(d), 
    .e_m(e), 
    .f_m(f), 
    .g_m(g), 
    .dp_m(dp), 
    .an_m(an)
    );

//Block for LFSR random number generator  

reg [28:0] random, random_next, random_done; //**29 bit register to keep track upto 10 seconds. 


reg [4:0] count_r, count_next_r; //to keep track of the shifts. 5 bit register to count up to 30

wire feedback = random[28] ^ random[26]; 

always @ (posedge clock or posedge reset)
begin
 if (reset)
 begin
  random <= 29'hF; //An LFSR cannot have an all 0 state, thus reset to FF. 
  count_r <= 0;
 end
 
 else
 begin
  random <= random_next;
  count_r <= count_next_r;
 end
end

always @ (*)
begin
 random_next = random; //default state stays the same
 count_next_r = count_r;
  
  random_next = {random[27:0], feedback}; //shift left the xor'd every posedge clock
           

 if (count_r == 29) 
 begin
  count_next_r = 0;
  random_done = random; //assign the random number to output after 30 shifts
 end
 else
 begin
  count_next_r = count_r + 1;
  random_done = random; //keep previous value of random
 end
 
end
//random number block ends


reg [3:0] reg_d0, reg_d1, reg_d2, reg_d3; //registers that will hold the individual counts
(* KEEP = "TRUE" *)reg [1:0] sel, sel_next; //for KEEP attribute see note below
localparam [1:0]
      idle = 2'b00,
      starting = 2'b01,
      time_it = 2'b10,
      done = 2'b11;
      
reg [1:0] state_reg, state_next;
reg [28:0] count_reg, count_next; 

always @ (posedge clock or posedge reset)
begin
 if(reset)
  begin 
   state_reg <= idle;
   count_reg <= 0;
   sel <=0;
  end
 else
  begin
   state_reg <= state_next;
   count_reg <= count_next;
   sel <= sel_next;
  end
end

reg go_start;
always @ (*)
begin
 state_next = state_reg; //default state stays the same
 count_next = count_reg;
 sel_next = sel;
 case(state_reg)
  idle:
   begin
    //DISPLAY HI HERE
    sel_next = 2'b00;
    if(db_start)
    begin
     count_next = random_done; //get the random number from LFSR module
     state_next = starting; //go to next state
    end
   end
  starting:
   begin
    if(count_next == 500000000) // **500M equals a delay of 10 seconds. and starting from 'rand' ensures a random delay
    begin  
     state_next = time_it; //go to next state
    end
    
    else
    begin
     count_next = count_reg + 1; 
    end
   end  
  time_it:
   begin
     sel_next = 2'b01; //start the timer
     state_next = done;     
   end
    
  done:
   begin
    if(db_stop)
     begin
      sel_next = 2'b10; //stop the timer
     end
    
   end
   
  endcase
  
 case(sel_next) //this case statement that will control what is sent to the 7 segment based on the sel signal
  2'b00: //hi
  begin
   go_start = 0; //make sure timer module is off
   regd0 = 4'd12; 
   regd1 = 4'd11;
   regd2 = 4'd10;
   regd3 = 4'd12;
  end
  
  2'b01: //timer
  begin
   
   go_start = 1'b1; //enable start signal to start timer
   regd0 = reg_d0;
   regd1 = reg_d1;
   regd2 = reg_d2;
   regd3 = reg_d3;
  end
  
  2'b10: //stop timer
  begin
   go_start = 1'b0;
   regd0 = reg_d0;
   regd1 = reg_d1;
   regd2 = reg_d2;
   regd3 = reg_d3;
  end
  
  2'b11: //Although this condition is of no use to us it is placed here for the sake of completion, case statements left uncompleted will create a latch in implementation
  begin
   regd0 = 4'd12; //4'd12 to siplay '-'
   regd1 = 4'd12;
   regd2 = 4'd12;
   regd3 = 4'd12;
   go_start = 1'b0;
  end
  
  default:
  begin
   regd0 = 4'd12;
   regd1 = 4'd12;
   regd2 = 4'd12;
   regd3 = 4'd12;
   go_start = 1'b0;
  end
 endcase   
end


//the stopwatch block


reg [18:0] ticker; //19 bits needed to count up to 500K bits
wire click;

//the mod 500K clock to generate a tick ever 0.01 second

always @ (posedge clock or posedge reset)
begin
 if(reset)

  ticker <= 0;

 else if(ticker == 500000) //if it reaches the desired max value of 500K reset it
  ticker <= 0;
 else if(go_start) //only start if the input is set high
  ticker <= ticker + 1;
end

assign click = ((ticker == 500000)?1'b1:1'b0); //click to be assigned high every 0.01 second

always @ (posedge clock or posedge reset)
begin
 if (reset)
  begin
   reg_d0 <= 0;
   reg_d1 <= 0;
   reg_d2 <= 0;
   reg_d3 <= 0;
  end
  
 else if (click) //increment at every click
  begin
   if(reg_d0 == 9) //xxx9 - the 0.001 second digit
   begin  //if_1
    reg_d0 <= 0;
    
    if (reg_d1 == 9) //xx99 
    begin  // if_2
     reg_d1 <= 0;
     if (reg_d2 == 5) //x599 - the two digit seconds digits
     begin //if_3
      reg_d2 <= 0;
      if(reg_d3 == 9) //9599 - The minute digit
       reg_d3 <= 0;
      else
       reg_d3 <= reg_d3 + 1;
     end
     else //else_3
      reg_d2 <= reg_d2 + 1;
    end
    
    else //else_2
     reg_d1 <= reg_d1 + 1;
   end 
   
   else //else_1
    reg_d0 <= reg_d0 + 1;
  end
end

//If count_reg == 500M - check if 'stop' key is pressed, if yes disable led, otherwise enable it. If count_reg ~= 500M keep led off.
assign led = ((count_reg == 500000000)?((db_stop == 1)?1'b0:1'b1):1'b0);

endmodule

If you go through the above code you will note that its pretty self explanatory as all steps have been commented. The only thing worth mentioning here is the

(* KEEP = "TRUE" *)

attribute. This attribute has nothing to do with the working of the code, it is needed for implementation only. If you are going to simulate this code you can remove this attribute as it wont have any effect on its working. The reason I felt to add it was because during implementation and synthesis Xilinx for some reason decided to remove the sel signal entirely after giving this warning:

Xst:1710 - FF/Latch <sel_3> (without init value) has a constant value of 0 in block <reaction_main>. This FF/Latch will be trimmed during the optimization process.

I checked it up with Xilinx documentation and it was a known problem with the version of Xilinx I am using. For some reason Xilinx thought it would optimize the design by removing one of my most vital signals :) So in order to force Xilinx to keep the signal the KEEP attribute must be used.

I tried to write the code in a very easy way and commented it in detail so its easy to read and understand. Below is the video demonstration of this code working.


And here is the simulation of the above code: