RE: [IBIS-Users] pulse width of IBIS model response

Yuancheng,

I assume by this "IBIS did rise faster than SPICE, but at the falling
edge, it nearly falls at the same time with SPICE model,"

you are saying not "faster" but "sooner" or "earlier".

Consider the following situation which holds true for at least

one popular commercial simulator with IBIS support.

The defaults of the stimulus threshold level are 0.8 and 0.2

volt for the rising and falling stimulus on the input of the

IBIS model. The assumed pulse levels are 0 and 1 volt for

low and high. This is a symmetric arrangement, and assuming

that the rising and falling edges of the stimulus waveform

have the same slope, the IBIS model will get triggered at the

same time with respect to when the rising and falling edges

start.

Now, consider that the stimulus levels are 0 and 5 volt instead.

In this case a rising edge stimulus will reach the 0.8 volt trigger

from the 0 V starting point much sooner than the falling edge

stimulus would reach 0.2 volt from the 5 V starting point.

This can cause significant duty cycle distortions.

Similarly, even if your stimulus levels are 0 and 1 volt, the

duty cycle will be distorted if the rising and falling edges

of your stimulus are different slopes.

I hope this answers your questions...

Arpad

====================================================================

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of ji xiao
Sent: Wednesday, April 20, 2011 11:12 AM
To: ibis-users
Subject: [IBIS-Users] pulse width of IBIS model response

Dear All,
Thanks for your replies for my previous question! This time I got a new
question about the pulse width of IBIS models.
Since IBIS model does not include the propagation delay of the driver,
it always has shorter time delay than SPICE model. And if we give a
pulse input, at the rising edge, IBIS did rise faster than SPICE, but
at the falling edge, it nearly falls at the same time with SPICE model,
i.e., the total pulse width of IBIS response is wider than the SPICE
response.
It is easy to understand the faster response at the first switching edge
(rise edge), but why it is not the same at the second switching edge
(fall edge)? Is it also related to the threshold voltage defined by
different simulation tool?
Thanks and regards,

Yuancheng

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