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  ../images/main/bullet_green_ball.gif Test Benches

Ok, we have code written according to the design document, now what?

   

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Well we need to test it to see if it works according to specs. Most of the time, it's the same we use to do in digital labs in college days: drive the inputs, match the outputs with expected values. Let's look at the arbiter testbench.

   

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  1 module arbiter (
  2 clock, 
  3 reset, 
  4 req_0,
  5 req_1, 
  6 gnt_0,
  7 gnt_1
  8 );
  9 
 10 input clock, reset, req_0, req_1;
 11 output gnt_0, gnt_1;
 12 
 13 reg gnt_0, gnt_1;
 14 
 15 always @ (posedge clock or posedge reset)
 16 if (reset) begin
 17  gnt_0 <= 0;
 18  gnt_1 <= 0;
 19 end else if (req_0) begin
 20   gnt_0 <= 1;
 21   gnt_1 <= 0;
 22 end else if (req_1) begin
 23   gnt_0 <= 0;
 24   gnt_1 <= 1;
 25 end
 26 
 27 endmodule
 28 // Testbench Code Goes here
 29 module arbiter_tb;
 30 
 31 reg clock, reset, req0,req1;
 32 wire gnt0,gnt1;
 33 
 34 initial begin
 35   $monitor ("req0=%b,req1=%b,gnt0=%b,gnt1=%b", req0,req1,gnt0,gnt1);
 36   clock = 0;
 37   reset = 0;
 38   req0 = 0;
 39   req1 = 0;
 40    #5  reset = 1;
 41    #15  reset = 0;
 42    #10  req0 = 1;
 43    #10  req0 = 0;
 44    #10  req1 = 1;
 45    #10  req1 = 0;
 46    #10  {req0,req1} = 2'b11;
 47    #10  {req0,req1} = 2'b00;
 48    #10  $finish;
 49 end
 50 
 51 always begin
 52   #5  clock =  ! clock;
 53 end
 54 
 55 arbiter U0 (
 56 .clock (clock),
 57 .reset (reset),
 58 .req_0 (req0),
 59 .req_1 (req1),
 60 .gnt_0 (gnt0),
 61 .gnt_1 (gnt1)
 62 );
 63 
 64 endmodule
You could download file arbiter.v here
   

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It looks like we have declared all the arbiter inputs as reg and outputs as wire; well, that's true. We are doing this as test bench needs to drive inputs and needs to monitor outputs.

   

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After we have declared all needed variables, we initialize all the inputs to known state: we do that in the initial block. After initialization, we assert/de-assert reset, req0, req1 in the sequence we want to test the arbiter. Clock is generated with an always block.

   

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After we are done with the testing, we need to stop the simulator. Well, we use $finish to terminate simulation. $monitor is used to monitor the changes in the signal list and print them in the format we want.

   

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 req0=0,req1=0,gnt0=x,gnt1=x
 req0=0,req1=0,gnt0=0,gnt1=0
 req0=1,req1=0,gnt0=0,gnt1=0
 req0=1,req1=0,gnt0=1,gnt1=0
 req0=0,req1=0,gnt0=1,gnt1=0
 req0=0,req1=1,gnt0=1,gnt1=0
 req0=0,req1=1,gnt0=0,gnt1=1
 req0=0,req1=0,gnt0=0,gnt1=1
 req0=1,req1=1,gnt0=0,gnt1=1
 req0=1,req1=1,gnt0=1,gnt1=0
 req0=0,req1=0,gnt0=1,gnt1=0
   

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I have used Icarus Verilog simulator to generate the above output.

   

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Copyright 1998-2014

Deepak Kumar Tala - All rights reserved

Do you have any Comment? mail me at:deepak@asic-world.com