RE: [IBIS-Users] Open drain: Vmeas, Rref and Vref measurement

Shiv,

Thank you for your interest in IBIS!  Here are some general comments:

1) Rref can indeed be used for an open-drain buffer, if Vref is
non-zero.  In other words, Cref, Vref and Rref form a load definition
(Cref is connected to "ground" while Rref connects between the Vref node
and the output of the driver).  For this load, a time is measured when
the waveform of the buffer output pad connected to the load crosses
Vmeas.  

For your open-drain (I assume pulldown-only) driver, an Rref connected
to ground will certainly never cause the driver pad to cross a non-zero
Vmeas.  But an Rref connected to a non-zero Vref could indeed allow the
buffer to cross a non-zero Vmeas, depending on the Ron of the pulldown
and the Vmeas target.

1, 2 and 3) There are no "typical" values of Rref, nor are these values
intended to be calculated from other buffer information.  Per the IBIS
specification:

| The Cref and Rref subparameters correspond to the test load
| that the semiconductor vendor uses when specifying the
| propagation delay and/or output switching time of the model.
| The Vmeas subparameter is the reference voltage level that the
| semiconductor vendor uses for the model. Include Cref, Rref,
| Vref, and Vmeas information to facilitate board-level timing
| simulation. 

In other words, the buffer delay load should come from either industry
specifications (for a buffer to be used with an industry-defined
interface) or the buffer designers, to mimic the test environment
actually used to characterize the buffer.  The time provided by the
buffer delay load will often be used to adjust propagation delays into
flight times for use in a timing spreadsheet.

I hope this helps!

- Michael Mirmak
  Intel Corp.
  Chair, EIA IBIS Open Forum

-----Original Message-----
From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Shiv Harit MATHUR
Sent: Monday, February 26, 2007 20:50
To: ibis-users@server.eda.org
Cc: Shiv Harit MATHUR
Subject: [IBIS-Users] Open drain: Vmeas, Rref and Vref measurement

Hello,

We are working on open-drain buffer IBIS modeling v3.2. 

We have following queries for Vmeas, Rref and Vref parameters,
as per the cookbook v3.2 these parameters are required for timing
measurements :-

1> How do we calculate these parameters i.e. Vmeas, Rref and Vref for
open-drain buffer, 
   as the Rref cannot be put in such case. 

2> What are the typical values of Rref for any buffer,
   as in general we have only load values in specifications.

3> What could be the value of Vmeas if we have different high and low
values of the output 
   buffer in different PVT cases i.e. for 
   Nom Case High value = 3.3V and Low Value = 2.8V
   Min Case High value = 3.0V and Low Value = 2.5V
   Max Case High value = 3.6V and Low Value = 3.1V

Regards
Shiv Harit Mathur
STMicroelectronics




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