Jeff,
As is pointed out by others, the issue is the worry over NBTI.
Negative Bias Temperature Instability:
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Is an old issue, still around, where the pmos threshold voltage changes over time given different bias conditions (full ON, vs full OFF).
In V4, the DCM delay lines turned out to be the best ever NBTI shift characterization means ever designed. If you intentionally held the CLKIN high on one, vs. low on another, and didn't ever change it for perhaps three or four hundred hours, when hot, you would get vastly different pulse width out of the two lines. This led to concerns that if you didn't keep the DCM delay lines "busy" they might not be able to operate over all PVT, all frequencies, and all duty cycles.
Never proven in the lab (never have ever tested a single part that failed to meet specifications, no matter how stressed), but we decided to specify our way out of ever having to even deal with the issue, hence the strange specification.
As well, there is an "autocal" block that is inserted for every DCM, used or not, that keeps them all "busy" so that the NBTI effect occurs evenly, and no performance could be affected by the change in Vt (because the all happen symmetrically).
A bake at 150C for 48 hours brings all the Vt's back to normal (almost, not quite), so if you ever suspect something to not work from NBTI, a bake will then provide you with "proof" that it might have been NBTI.
Might have been HCI (hot carrier injection), too, so a bake is not conclusive. HCI should never be a problem with our parts, at least, the qualification reports show it isn't an issue...
Bottom line, I stopped worrying about NBTI three years ago, and in V5, all the NBTI stuff is completely hidden (and hardened) so there are no worries whatsoever. In V4, we have recommendations that we apply, but even those may be waived under specific circumstances.
I have written, and signed official variance letters, allowing customers to violate some of these overly severe NBTI precautions when we are informed of what the real stress can be, and have studied what the worst case NBTI can be (and found it to be no issue).
Please address these requests to me directly, as well as file a webcase at the same time (so everything gets tracked and officially recorded).
Austin