Forum Discussion
Occasional ContributorQuestasim INTEl EDITION counter simulation
@GOMEZ_IT, I think you were right, this looks like a simulator problem.
I was able to reproduce your original observation, and it appears to be a bug in older versions of Questa, up to the version shipped with Quartus Pro 23.3.
The version of Questa shipped with Quartus Pro 23.4 doesn't appear to exhibit this symptom. Or at least not with this test case. But I don't know if this was a known bug that got fixed, or if the symptom just went away with other changes in Questa. That's something you should confirm with Intel and/or Mentor/Siemens.
For ease of testing, I modified your test case slightly to be able to run in a more minimalist command line mode, with console messages in lieu of waveforms. See attached zip.
Running this test case with all the simulators I currently have available to me I got the following results:
- Questa from Quartus Pro 22.2: bad
- Questa from Quartus Pro 22.4: bad
- Questa from Quartus Pro 23.1: bad
- Questa from Quartus Pro 23.2: bad
- Questa from Quartus Pro 23.3: bad
- Questa from Quartus Pro 23.4: good
- Modelsim DE 2022.4: good
- ghdl 4.0.0-dev: good
See attached log.txt for the above results.
-Roee
Super ContributorIt does not make sense why your changed code works. As mentioned, COUNT gets reset every time the CLK changes so you can only count to 1 with this design, as you see in your original simulation.
For a counter, you should have a reset mechanism (synchronous or asynchronous control signal) and not initialize the variable in the variable declaration.
Occasional Contributor
@sstrell wrote:It does not make sense why your changed code works. As mentioned, COUNT gets reset every time the CLK changes so you can only count to 1 with this design, as you see in your original simulation.
For a counter, you should have a reset mechanism (synchronous or asynchronous control signal) and not initialize the variable in the variable declaration.
Looks like I was mistaken. An initial value at the variable declaration in the process takes effect at time zero, not at every entry into the process. Learn something every day.
