this article is second part of creating your first design with FPGA and verilogHDL, in particular WireFrame FGPA Board Xilinx XC3S250E board. we are going to use modelSim software From altera which is limited free software available to download from Altera website
|ModelSim Simulation Software From Altera|
First part of this project have created and implemented a basic counter module so that we can blink few leds. now we will verify that counter module with the help of a test-bench module and Altera ModelSim Software, there are many software which you can you to simulate your design none of the component we use are platform dependent.
so lets start , first of all we are going to need a testbench module , testbench module is nothing but a simple verilog module which feeds the inputs to our counter module , monitor the output and ultimately validates that the under test module is working as it should be, to achieve this testbench modules have few extra features or you can call them have extra powers , like you can specify initialization block and specify time when a instruction will execute due to testbench module never ever get synthesize , they just run in the simulation software that is all.
lets start writing a test bench ,it will be added to modelsim project as counter_testbench.v
- you need a module ,
- few reg and wire declaration for ports of the module under test,
- an instance of the module inter test,
- and a initial block to do one time events,
- and finally a always block to generate clock for the module under test.
|`timescale 1ns/1ps // simmulation happen in picosecond time base ,|
// we have up scaled it it to ns
// when we say #5 it will mean 5 ns , and when we say #1000 it will 1000ns, 1us
reg clock_tb,reset; //mapped with ports of the module under test
counter t0(reset,clock_tb,out_tb); //create an instance of the counter module which is under test
$monitor("%g,out_tb=%d,reset=%b",$time,out_tb,reset); // these values will be visible in the transcript window at the bottom
reset=1; //enable the counter module ,reset is active low input
#100 // right here we should have (200/5)/2 = 20 in the cout output value
reset=0; //reset the timer module for a while
$stop(); // right here we will have (1000/5)2=100 in the count output
always #5 clock_tb=~clock_tb; //generate clock by toggling Clock line ever 5 unit of time
now you we need to create a new project in ModelSim and add
First ModelSim Project
ModelSim starter edition is limited free software you can download it from Altera Website , after installation , launch modelSim, now got to file menu-> New -> Project
Specify the project name and project location ,
Create New file or add Existing file , you will need only two file for this project , one is testbech another is the actual module under test. (module under test we have already developed in last post)
add existing file to project
compile all the file , see transcript for any error or serious warnings , counter.v is same file as be developed in last post, with Xilinx ISE. counter_testbench.v is the file which we have just written
Select the testbech file only!!!
Run all , till it stuck $stop() , see line number 20 in the test bench source
simulation finish now , you need to go to wave view, though it open automatically , but if it doesn't then go to view menu and select wave and also sect object window ,
now you need to drag and drop signals from object window to wave window.
Zoom in or out with the buttons in the tool bar so that you can see the whole view
you can view signal value as tool tip and on the signal it self ,
as you can see we expected 10 value after 200ns , as we are clocking every 10ns (5ns+5ns of always block ) . and soon as reset goes low , the module set to zero.
now after running simulations for 1000 ns we got 100 in the counter output.
result is as be expected, every thing is fine , you can end simulations here or play with test bench for different tests.
Source Code and Design file
all the source code is available in the github repo WireFrame-FPGA/VerilogHDL-Modules/counter_wireframe,
Test in the WireFrame FPGA Board