Re: [IBIS] VOL, VOH, Vmeas, VIL, VIH

> Is this right please? Then, could we know what are the thresholds really
> used by the receiver?

If you like doing things "by the seat of your pants" (i.e., barely
"getting by"), then ignore VIL, VIH, and everything in the data sheet.
 Simulate or build your circuit and see where it really switches. The
"real" switching threshold will be somewhere in there between VIL and
VIH. (You can find it if you want, using measurements or SPICE
simulations.) Your circuit will inevitably fail, eventually.

If you like guaranteeing your work, use VIL and VIH appropriately, and
forget where the real threshold is. It shouldn't matter to you.

VIL and VIH are never the actual switching thresholds. The receiver
does not compare the input voltage against VIL and VIH. Only we
humans do. And our timing tools. VIL and VIH are the levels below
and above which signals are defined as valid. They always straddle
the real switching threshold. They provide some margin to account for
variations, they give the input receiver a minimum amount of overdrive
(necessary for the receiver to switch appropriately fast), and they
provide some amount of noise immunity. Ignore VIL and VIH at your own
peril.

It is not necessarily bad if a signal crosses VIL or VIH more than
once ... but not for the reason you gave! (Well, yes there may be
some truth to it, but most good engineers would never use that
argument to support allowing multiple crossings ... unless the signal
barely crossed back below VIH and they had enough confidence in their
simulations to believe that the results were ultra conservative.)

If the signal is a clock or used to re-generate one, or otherwise
edge-sensitive, then keep it monotonic.

If it is a clocked data signal, then it can be non-monotonic, as long
as it reaches and stays valid (above VIH or below VIL) in time to be
clocked in. A lot of data bus signals have no need to be monotonic,
and it could be wasteful to engineer them to make them monotonic.

If you are doing a bleeding-edge design, your timing margins may be so
tight that you can't afford any non-monotonicity. But only you can
tell that, after evaluating your timing budget.

Regards,
Andy

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