This mini-tutorial uses gtkwave which can be installed with
sudo apt install gtkwave on ubuntu.
In chisel this is the only viable way to do whitebox testing (i.e peer inside the circuit) other than printf, thus it can be a very useful tool if you learn it.
In order for a test to generate a .vcd file the test must specify this in the driver argument.
Open the DelaySpec test found in
/user/home/path/to/exercise/Examples/stateful.scala
Now, replace the line that says
chisel3.iotesters.Driver(() => new SimpleDelay) { c =>
with
chisel3.iotesters.Driver.execute(Array("--generate-vcd-output", "on", "--backend-name", "treadle"), () => new SimpleDelay) { c =>
and run the associated test from sbt with
sbt: testOnly Examples.DelaySpec
You should now be able to find a .vcd file in your project root, located somewhere near
/user/home/path/to/exercise/test_run_dir/Example.DelaySpec$Number/SomethingSomething.vcd
which you can open with gtkwave.
When you open your file in gtkwave you will be met with something similar to this:
First, click the SimpleDelay file pointed at by the red arrow.
This will open the top level signal list where the red circle is.
Next, select some of the signals and drag them into the box to the right that says “signals”
so your view will look like this:
This view is a little hard to read, so you can add colors and change the type of encoding.
After playing around a little I get something that looks like this:
Once you’ve set up a nice waveform window you can reuse it by reloading the waveform (ctrl shift r).
Try changing the code in stateful by adding a constant to delayReg and observe how, after running the
test (which will now fail) you will get new values in gtkwave after reloading.
In order to save your layout use write save file as (ctrl shift S) and upon opening everything will be restored as you left it.
This is especially relevant for the later exercises when you will build a RISC-V 5-stage pipeline.
In order to give special names to values, right click the signal and select
Data Format -> Translate Filter File -> Enable and Select
You can use the supplied filterTest which is located at the project root, which will change 0 to “zero”,
1 to “One” and 2 to “Two”.
The syntax of a filter file is simple: Key <Tab> Value
