regblock HWIF: field output value override for e.g. DFT-mode depending on specific set of selector signal/value sets

[galaviel]

Hi,

Just wondering if this is supported or can somehow be emulated/achieved using the existing SystemRDL/regblock functionality.

Sometimes a field controls some critical chip function.
When in test-mode (e.g. scan) and the value of the FF (field storage) may receive random values, you want to over-ride the output of that FF with an external mux (on the "Q" of the FF so to speak) with an override value.

something of the form:

assign hwif_out.i_regfile_0.my_split_reg.my_split_field.value = 
  ext_sel0 ? ext_val0 : 
  ext_sel1 ? ext_val1 : 
  field_storage.i_regfile_0.my_split_reg.my_split_field.value;

Make sense?
is there a way to achieve that?

I thought about using different reset signals, however that will reset the FF itself and not over-ride its output.

Any help appreciated!

[amykyta3]

DFT is a finicky subject that has wildly differing methodologies across the industry. In my opinion, having the regblock generator handle this feels like too much of a special-case to justify supporting. In this situation, I would recommend inserting a trivial test mode override mux externally. Having some minimal glue logic around a register file is often inevitable since it is not always practical/possible to describe all the behavior in RDL.

[galaviel]

Thanks allot for the reply! I can certainly agree that DFT support is an open topic.

Just wanted to make sure I'm not re-inventing the wheel here and I haven't accidentally missed some feature or property of the language/tool itself that I could have used to implement this.

BTW I still think that a general/generic purpose feature to override the output of the field's HWIF output might be useful, for other purposes as well. Perhaps there are other reasons, other than DFT that require this. It could be phrased as a general purpose override.

In any event it's easy enough to implement this using UDP's at the field level which I have already begun, following regblock's 'wr_swacc' as an example. So it's looking good.

Thank you for your support! Gal.