Generics and force

[Anselmo95]

Hello,

I was trying out NVC and I was wondering if it is possible to
•change the value of a generic for an entity that is not the top one
•force a signal to a certain value even if it has a driver inside the code

I tried looking at the manual and at the tests inside the repo but I can't figure out if it is possible or not. The vhpi seemed promising for forcing signal values but I can't get it to work so I figured it was better to ask :)

[nickg]

change the value of a generic for an entity that is not the top one

You mean during elaboration, with the -g option, right? It's not possible at the moment but I suppose it could be added. Is this a capability that already exists in other tools like Questa/RiveraPRO?

force a signal to a certain value even if it has a driver inside the code

This should work fine, modulo bugs. See test/regress/force1.vhd for an example. You can raise an issue if you run into any problems.

[Anselmo95]

You mean during elaboration, with the -g option, right?

Yes exactly and I tried the option but I saw in the help that it's limited to the top entity.

Is this a capability that already exists in other tools like Questa/RiveraPRO?

I don't know about RiveraPRO never used it, but I can confirm that it is possible to do it in Questa/Modelsim and VCS

Thanks for pointing out an example for the force, but what I was looking for is more similar to the force TCL command. I don't know if you are familiar with it, I usually use VCS and there I can pass a script tcl with commands similar to this one
force -freeze /top/module_a/signal_b 16#cafe01
Is there a similar feature to avoid using the force keywords inside vhdl ?

Thanks for the quick reply!

[nickg]

Is there a similar feature to avoid using the force keywords inside vhdl ?

I added force and noforce commands to the TCL shell in fdc75c2 so something like the following works:

nvc -e example --do script.tcl

force /x '1'
force /z "101"
run 1 ns
noforce /z
run

It's slightly different to Modelsim in that the force command schedules an update on the next delta cycle rather than changing the value immediately (it's like the force statement in VHDL-2008 or the VHPI vhpiForcePropagate mode). Is this enough for your use case?

Note the TCL interface is something of a work-in-progress at the moment. You need to enable it with configure --enable-tcl. There's an interactive version you can access with -i instead of --do.

[Anselmo95]

Wow, thanks ! I didn't expect you to add the feature over the weekend. I tried it out and that's exactly what I was looking for. I also tried the vhpiForcePropagate via the VHPI api but I must say that it's a bit cumbersome compared with tcl.

I would like to give it a go to extend the -goption to any generic in the design, but if you don't mind could you give me a little help/direction to start ?

[nickg]

It requires some refactoring of the existing code so probably easier if I do it. See 4dd2021.

[Anselmo95]

Thanks a lot for the feature! I'll test it tomorrow and also look at the commit it can't hurt to know a bit how nvc works.

[avelure]

Modelsim has signal_spy, signal_force, signal_release and Cadence has nc_mirror, nc_force, nc_deposit, nc_release in custom libraries. These are built in functions in the respective simulators and have a bit more flexibility on what can be forced. The deposit functionality is quite handy, where the value driving value is changed instead of overridden. These also support force/release across mixed-language borders.

[nickg]

Thanks, I wasn't aware of those. Sounds quite useful. Presumably they specify the signal as a path string rather than taking an actual signal argument / external name?

[avelure]

Yes it is taken as a string. There is some description in the manual starting from page 486 https://www.microsemi.com/document-portal/doc_view/131619-modelsim-user