Comparing results

Three options:

• Stare at waveforms by hand
  • useful in initial debugging
  • error prone and tedious for regression tests

• Use Verilog to compare results against an expected set
  • with a predictable progression can regenerate results in test harness
  • can import expected results from a preprepared file

• Dump a trace from the simulator and compare off-line
  • simple tests can alert user to anomalous conditions
  • data traces can be exported into files for analysis

Importing test results

Probably the easiest way to do this is to read a file of expected results into a memory residing in the test harness. Initialise a pointer to the start of the file and increment it every time a comparison is made.

reg  [7:0] results [0:1023];
reg [10:0] result_pointer;
reg  [7:0] comp_value;
 
initial $readmemh(<filename>, results);
 
initial
 begin
 result_pointer = 0;
 ...
 test_next(comp_value);   // Fetch value
 ...
 end  
task test_next;           // Use a task for convenience
 output [7:0] value;
 begin
 if (result_pointer > 1023)
   begin
   $display(“Out of data!”);
   $stop;                 // Oops! Error
   end
 else
   begin
   value = results[result_pointer];
   result_pointer = result_pointer + 1;
   end
 end
endtask

Exporting monitoring information

There are several ways to do this.

User prints

$display
$write

C-like print statements: (nearly) identical except $display(…) adds a LF to the string end.
If you want to stay on the same line $write(…) will do this.

Suggestions:

• Use to monitor progress through a stimulus file
• Use to pick up and diagnose errors

e.g.

$display(“Starting test”);
...
if (q != 32'h0000_0000)
 $display(“Unexpected value: q = %X”, q);

Sometimes it is convenient to watch particular signals rather than all signals through time. This can be done with the $monitor task.

initial
 $monitor(“this = %x that = %x at time %t”,
           this, that, $time);

This is similar to:

always @ (this, that)
 $display(“this = %x that = %x at time %t”,
           this, that, $time);

Writing to files

allow you to create your own output files. (“$fwrite” is the same as “$fdisplay” except for line-feeds.) Their use, including file handles, is similar that which should be familiar from C programming.

Up to simulation

Back to initialising simulations

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