Re: the 4 V-t curves of IBIS 2.1

On Wed, 27 Sep 2000, Betty Luk wrote:
> Why do we need four V-t curves to describe the rising and falling
edges of a CMOS buffer (as described in the IBIS cookbook)?

I assume you mean 2 per edge.

One per edge describes what it does with a particular load. What it
does into any other load is a guess.

By supplying more than one waveform per edge, with different loads, it
is possible to represent changes in the output resistance in the
driver, in addition to the voltage.

Usually, if you can provide 2 tables per edge, the best loads are to
use the same resistance in both cases, but connect one to ground and
the other to the power supply. To see why, consider that the typical
driver consists of a pull-up device and a pull-down device. A
resistor from the terminal to ground will tend to swamp out the
pull-down device, and characterize the pull-up device fairly well. A
resistor to power will do the opposite. Usually, the best resistance
to use is as close as possible to the load it will actually drive.

 
Received on Thu Sep 28 19:27:21 2000