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FILE IO TB

File I/O Based Testbench

Another way of getting the Stimulus is get the vectors from an external file. The external vector file is generally formatted so that each value in the file represents either a specific input pattern .Verilog HDL contains the $readmemb or $readmemh system tasks to do the file read if the file data is formatted in a specific way using either binary or hexadecimal data. TestBench is like just an interface between external vector source and DUT. Sometimes outputs are also to written to external files. For example, to verify a a dsp algorithm implemented as  DUT, get the input vectors from matlab tool and send the outputs to a file and then compare the outputs of the matlab for the same algorithm.

Fallowing example illustrates how to initialize a memory array from data stored as hexadecimal values in a data file, Simulate this file directly to see the results.
Note: The data file must reside in the same directory  as the .v file for the module in this example.

EXAMPLE: verilog file
module readmemh_demo;

   reg [31:0] Mem [0:11];

   initial $readmemh("data.txt",Mem);

   integer k;
   initial begin
      #10;
      $display("Contents of Mem after reading data file:");
      for (k=0; k<6; k=k+1) $display("%d:%h",k,Mem[k]);
   end

endmodule

EXAMPLE: data.txt file
   234ac
   23ca5
   b3c34
   23a4a
   234ca
   b3234

RESULT:

   0:000234ac
   1:00023ca5
   2:000b3c34
   3:00023a4a
   4:000234ca
   5:000b3234

Reading or writing to files during simulation is costly to performance, because the simulator must halt and wait while the OS completes each transaction with the file system. One way to improve performance is to replace ASCII vector files with a constant table in HDL itself. Do this using Perl script.

module readmemh_demo;

   reg [31:0] Mem [0:11];

   `include "data.v"

   integer k;
   initial begin
      #10;
      $display("Contents of Mem after reading data file:");
      for (k=0; k<6; k=k+1) $display("%d:%h",k,Mem[k]);
   end

endmodule

EXAMPLE: data.v file
initial
   begin
   Mem[0] = 32'h234ac;
   Mem[1] = 32'h23ca5;
   Mem[2] = 32'hb3c34;
   Mem[3] = 32'h23a4a;
   Mem[4] = 32'h234ca;
   Mem[5] = 32'hb3234;
   end

RESULT:

   0:000234ac
   1:00023ca5
   2:000b3c34
   3:00023a4a
   4:000234ca
   5:000b3234

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