[IBIS] IBIS modellling for open drain cells

From: Vinayak Ghatawade <vinayak_at_.....>
Date: Mon Jan 09 2006 - 07:54:15 PST
All,

I had one more query for open-drain cells
for a buffer driving a huge capacitive load (bus capacitance)

For ex: a slow buffer with a large fall time

How do we model V-T data for such buffer. As we have only pulldown clamp
currents and RISING and FALLING WAVEFORM with R_Fixture tied to actual
pullup resistor(and not 50 ohms) connected to V_FIXTURE= VCC.

Should we have C_FIXTURE (bus capacitance) define for V-T data. What i 
feel is as the
buffer's fall time is so huge, there is no such Transmission line modelling
required for such a case.

Thanks and Regards
vinayak

Vinayak Ghatawade wrote:

> All,
>
> Thanks for the information and replies.
> The buffer is using a feedback which changes the drivestrengths.
>
> Currently we are going to chop the ringing portion
> of the IBIS V-T data which looks the safer option
> and go ahead.
>
> Thanks again
> vinayak
>
> Muranyi, Arpad wrote:
>
>>Andy,
>>
>>You are absolutely correct, there are different
>>types of feedbacks, and I should have defined
>>which one I was referring to in my posting...
>>
>>I was thinking of one that adjusts the drive
>>strength real time as the buffer is switching.
>>Legacy IBIS can't do that.  (Although one could
>>model these too using the *-AMS extensions of
>>IBIS).
>>
>>On the other hand, buffers which adjust their
>>own drive strength only once and a while to
>>compensate for temperature variations, for
>>example, can be modeled in IBIS as if they were
>>normal buffers, because they don't change skins
>>while driving signals onto the bus.  You may
>>need the [Model Selector] keyword to make a
>>few different driver models available for the
>>various strengths, but that's all.
>>
>>Arpad
>>------------------------------------------------
>>
>>-----Original Message-----
>>From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Andrew Ingraham
>>Sent: Wednesday, January 04, 2006 9:31 AM
>>To: ibis-users@eda.org; ibis@eda.org
>>Subject: [IBIS] Re: hspice vs IBIS model correlation at different capacitive loads
>>
>>  
>>
>>>I think he is saying that the oscillations happen
>>>because of the feedback in the buffer is trying to
>>>settle to a steady state condition.
>>>
>>>If so, legacy IBIS can not model your buffer, since
>>>there are no provisions for describing feedback.
>>>    
>>>
>>
>>Yes.
>>
>>Although it depends on the feedback.
>>
>>Linear negative feedback, such as used on op-amps to reduce output
>>impedance, should work OK, I think.  It's probably also possible that some
>>forms of feedback to reduce variations in drive strength (for example),
>>ought to be somewhat compatible with conventional IBIS.  But things like
>>positive feedback to give hysteresis, don't play well in IBIS.
>>
>>Regards,
>>Andy
>>
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Received on Mon Jan 9 07:54:40 2006

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