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

Hello, 

I am not able to understand the significance of Vmeas and Vref.

1. I simulated my model by changing different values for Rref, but I dont see any change in the waveform timings. Can anybody explain how the timing information is impacted by Rref.

2. There could be many possible values of both, i.e. if Vref (Vref > 0) is changed to different values we would have different values of Vmeas.
  can I write Vref = 3.0V (i.e. supply voltage) and calculate the drop across the Rref and then dump the value of Vmeas.

3. Also does Vref and Vmeas have any effect on Rising and Falling waveforms of the buffer.
  as the cookbook refers these parameters in relation to timing values. 
  Although I personally think this is not the case, but still I am not able to figure out the importance these parameters.


Regards
Shiv 



Mirmak, Michael wrote:

>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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