Subject: Re: Simulating Pulldown and pullup curves.

From: Andy Ingraham <[email-masked]>
Date: Mon, 21 Jul 97 09:27:21 EDT

Eitan Medina wrote:

> I have an I/O pad with clamping-high and clamping-low circuits.
> When simulating the whole pad-circuit I get non-monotonic data (My
> checker wanrns about this). 
> Should I simulate the pullup-curve on just the pullup-circuitry (eliminating
> the clamping and other structures that exist in the pad-circuit) in order to 
> get monotonic curves ?

IBIS does not require monotonicity.

If your device is non-monotonic (if the total current is
non-monotonic), there is nothing you can do to the data to make it
seem monotonic.  Some simulators will have problems simulating with
your device, others will not.  You cannot help that, except by
redesigning your IC.

When separating the total device data into pullup/pulldown/clamp
curves for IBIS, even though the total current may be monotonic, some
of the resulting curves by themselves may become non-monotonic.  This
is unimportant, because the simulator will sum the currents and the
non-monotonicity should go away.  The IBIS checker complains because
it doesn't try to sum the currents back together.  It is just being
extra cautious.

I would not bother trying to make the separate curves monotonic just
to eliminate the warning message; especially if doing so results in
a less-than-correct representation of your device.  If you simulate
the currents separately, they may not add up to the total current
when you recombine them.

Regards,
Andy

---

From: "D. C. Sessions" <[email-masked]>
Date: Mon, 21 Jul 1997 08:54:15 -0700

> the currents separately, they may not add up to the total current

As a point of reference, some of our drivers have
non-monotonic I/V curves due to circuitry which
ensures that the outputs remain high-impedance when
the power is OFF; the weak devices used for this also
cause some loss of gate drive when the device drives
into a heavy load.

-- 
D. C. Sessions
[email-masked]