FreeLists / si-list / [SI-LIST] Re: I-V vs V-T endpoints In IBIS

["Peters, Stephen" <stephen.peters@xxxxxxxxx>]

Hi Tim:

  I would be very cautious about ignoring any warnings from the IBIS parser,
especially the DC endpoint warning.  This warning indicates that the I-V and
V-T tables do not correlate -- i.e. after the output transitions the final
(DC) value recorded in the V-T table does not match what the I-V table says
it should be (given the load resistor).  It's a red flag that something
isn't correct with the model. End point mismatches could be due to a number
of reasons:

1. The model creator did not properly document the R_fixture or V_fixture
(load resistance or voltage) used when creating the [Rising Waveform] or
[Falling Waveform] tables.

2. The model creator did not let the output make a full transition (i.e. the
V-T data was truncated before the output reached it's final DC level)

3. The I-V and V-T data were taken at different process/voltage/temperature
corners.

At any rate, don't ignore it.

(Also, I would caution against using the [Ramp] data rather than the V-T
tables.  IF the model creator included V-T tables there is a strong
likelihood that the model was not validated using the [Ramp] data.)

  Regards,
  Stephen Peters
  Intel Corp.

 

-----Original Message-----
From: Timothy Coyle [mailto:Timothy.Coyle@xxxxxxx]
Sent: Monday, June 03, 2002 9:57 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] I-V vs V-T endpoints In IBIS


Hello,
I am looking for some clarification on how the I-V and V-T data is used 
from an IBIS file.  When I make an IBIS file and run it through the 
parser, 
let's say I get a DC endpoint mismatch warning. I know that a load line is 
applied to the I-V data and the enpoints of this are matched to the 
endpoints
of the V-T data.  I'm not sure of the importance of why these endpoints 
should match.  My understanding is that the I-V data is used to model the 
endpoints 
of the device, where the V-T data is used to model the low-to-high and 
high-to-low transitions. I know that in HSPICE, when there is a mismatch, 
the V-T 
endpoint is modified to match the I-V endpoint. So how does this warning 
affect the accuracy of the model? I'm thinking the V-T data plays a bigger 
role than I think? 
I've correlated SPICE to IBIS models with good results even with these 
mismatches.  And you can use the Ramp data and get the same result without 
even using
the V-T data.  I have a customer who is asking about this, and I want to 
tell them that it's okay to have the warnings because of the above 
reasons, but I'm 
not entirely sure about that. 

Thanks for any responses,
Tim


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