Subject: Drive-on-demand

From: "D.C. Sessions" <[email-masked]>
Date: Thu, 31 Aug 2000 11:26:49 -0700

Need to double-check this.  It seems that there are standard
logic components which, on output, sense the output voltage
and adjust drive current in response.

No, I don't mean bus hold.  These parts drive LOW hard when
the output voltage is above a certain threshold and cut the
drive when the output is below that threshold.  Rising-edge
drive is similar.

Yes, I know that this is of dubious stability.  Nobody asked
me if they should do it this way, but the manufacturers want
to make this a JEDEC standard and I'm trying to help them
put together an IBIS model.  Which, as far as I can tell, is
possible using

Submodel_type	Bus_hold

and having negative currents in the [Pulldown] and [Pullup]
tables.

1) Have I missed a better way to do this?
2) Will this be legal?
3) Will this break EDA tools?

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

---

From: Bob Ross <[email-masked]>
Date: Thu, 31 Aug 2000 18:23:39 -0700

D.C.:

Based on your description, I do not believe Bus_hold will work
directly.  I do not believe simulators will work with 
"negative" currents in the tables - meaning doing the
opposite of what is expected or creating a negative
impedance buffer that acts opposite of a conventional buffer.

Opposite polarity I-V tables will probably be flagged
as an error by the ibischk3 parser.  Therefore it would
not be legal.

I cannot think of any clever alternative with IBIS
Version 3.2.  

However, I believe the new macro language under discussion by
the IBIS Futures Working Group will be able to handle this
situation (this would be a good test case).

Bob Ross
Mentor Graphics