Re[3]: Interpretation of I/O data

I wasn't the one who asked the questions, but I'll throw in my two bits,
if for no other reason than to give another opinion.

> Shouldn't the Bus-hold circuit be modeled as an output rather than an input?

Bus-hold circuits are typically used on bi-directs, but because they
come into play when the output is disabled, one tends to think of them
as being part of the input circuit rather than the output. It seems to
make more sense to model them as input currents, since they are always
there whether the output drives or not.

Since IBIS is predicated on modeling static I/V characteristics, and
since the I/V curves of bus-hold circuits depend on past history (a
bi-stable state), there is a problem of how to model them. Anyway,
they may be problematic because of their positive feedback.

> Inputs usually clamp below and above the supply rail. If the failing power
> supply shorts to GND, the input will clamp below -0.6 volts and above +0.6
> volts.

Not necessarily. Many IC inputs do not clamp in the positive direction,
or if they do, it is not at Vcc +0.6 volts.

CMOS inputs need not clamp at all, in principle. Generally speaking,
clamps are added for their protection.

Some IC inputs are designed not to conduct when power is removed;
important for hot-swapping boards or running independent units on a
common bus. I think this is the question John Fitzpatrick is getting
at. Will the clamp characteristics be the same (relative to Vcc) with
the power removed as when it is present? I can't say, though I think
there is a good likelihood that the gross behavior would be similar.
It is probably best to contact the IC manufacturer or IBIS model writer
to see under what conditions those parameters apply.

Regards,
Andy
Received on Mon Jan 22 19:46:13 1996