RE: [SI-LIST] : Worst case VI curve for PCI spec a reality?

Andrew Ingraham (Andrew.Ingraham@digital.com)
Mon, 20 Apr 1998 12:22:46 -0400

> The worst case high state vi curve in the PCI spec shows Voh=2.4v for
> I=0 to 2mA (Figure 4-3, VI curve for 5V signaling). ...
> Granted, 3.3v devices are allowed to drive in a 5V environment,
> and specs for 3.3v parts do say that Voh_min=2.4v but isn't this a
> holdover from the TTL / Bipolar days?

Yes and no. It also allows N-channel pull-up devices; though maybe
2.4v is a bit low even for that.

Many 5v CMOS chips continue to use Voh(min)=2.4v, even though that
probably doesn't represent reality either. The PCI spec might have
taken the path of least resistance by conforming to the specs people
were used to designing and testing to.

Also, I wouldn't rule out bipolar devices, when PCI was new. The
notion of it being a "CMOS bus" might have come later.

> Does anyone know of PCI devices that actually put out 2.4v at I=0 amps
> (high temperature, low supply voltage) or is the spec just too worst
> case?

It can't be "too" worst case because it defines what is allowable.
But is it much worse than today's common practice? Maybe. OTOH,
someone might use a 3.0v part (or even 2.5v ... is that possible??).
You should design as if a device were close to the minimum curve.

> Note that I also question Vol=0.55v at I=0amps for the low state vi
> curve as well.

Maybe it was just for simplicity. It won't be Vol=0v, except in
theory (no ground pin + substrate IR drops).

Regards,
Andy Ingraham