IOPLL non-dedicated feedback option doesn't work correctly

[roeekalinsky]

In Stratix 10 and Agilex, the IOPLL supports an option for using a non-dedicated feedback path when using the normal compensation mode or source-synchronous compensation mode. However, the non-dedicated feedback option doesn't appear to work correctly.

With the non-dedicated feedback option selected, the IOPLL compensation anti-delay as reported in static timing analysis is insanely high, on the order of -30 ns, whereas it should be only a few ns for correct compensation. And consequently, this huge anti-delay causes huge clock skews of a similar magnitude, on the order of 30 ns, between the input clock and the clock generated by the IOPLL.

I'm providing a trivial design example that demonstrates this (see attached). It consists of little more than an IOPLL IP with normal mode and non-dedicated feedback option selected, and otherwise all defaults, plus a handful of registers at the top level to create timing paths to observe in STA. And note that the same design example produces reasonable compensation delays if the IOPLL is configured without the non-dedicated feedback option.

The design example provided is in the form of a Quartus Pro 24.1 project archive. But note that I've also tested older versions of Quartus Pro as far back as 21.3 -- same results.

Intel/Altera support folks, please have a look at the provided example and confirm that you can reproduce what I've reported.

And perhaps firstly can you even confirm, please: Is the non-dedicated feedback option known to work? Is it known to not work? Or am I missing something basic? Because it appears to be plainly broken.

[roeekalinsky]

Hello, Intel support, is somebody on this?  Can I get a response please?  Thank you.

-Roee

[AqidAyman_Intel]

Hi Roee,

Apologies for the delay reply.

I am looking into the resources and perhaps, this is to be expected when using non-dedicated path mode since it does not use a dedicated clock tree. Here’s the IP description:

Use Nondedicated Feedback Path: If this option is selected, the IOPLL will not use additional core clock resources in its feedback path.

Compensation modes that use nondedicated feedback conserve clock resources and have timing analysis benefits, but they create phase shift and frequency limitations. If compensation is not necessary, consider using direct mode.

Since when you turn this option off and the compensation timing look reasonable then that should rule out doing anything out of the ordinary with the PLL, such as cascading or using a non-dedicated clock pin to the PLL.

However, I will try to escalate this issue to the internal team for further investigation, but it may take some time if you are okay with that.

Regards,

Aqid

[roeekalinsky]

Hi Aqid,

Thank you for getting back to me.

Yes, please escalate it to the internal team for further investigation.

Per my understanding I don't believe that what I'm observing is expected, so I'll appreciate your support in getting to the bottom of it.

Thanks,

-Roee

[AqidAyman_Intel]

Hi Roee,

I have escalated this issue to get the attention from the internal team.

Currently, we are waiting for the response from them.

I will update you further once there is an update. I really appreciate your understanding.

Regards,

Aqid

[roeekalinsky]

Thanks for the update.  I'll look forward to hearing from you.

-Roee