Re: [IBIS-Users] Rref_diff keyword

Hi Cynthia:

My answers are in your text.

Best Regards,
Bob

Cynthia Low wrote:
> Dear IBIS experts
>  
> I am currently looking at the Rref_diff subparameters under the [model] 
> keyword and the usage of this subparameters. This subparameters is only 
> available in IBIS specification of v4.1 but not in older version. May I 
> know how this subparameters keyword is being used? the detail of the 
> question is as below.
>  
> IBIS Spec:
> Rref_diff is used for true differential buffer timing test loads. where 
> the buffer hv to be specified only when the [Diff Pin]
> keyword connects two models, and each buffer references the same model.  
> The Rref_diff and Cref_diff subparameters can be used with the Rref, 
> Cref, and Vref subparameters for a combined differential and 
> signal-ended timing test load.
> 1. My questions will be, can this Rref_diff sub-parameters be used ONLY 
> when we engaged using the true differential [External Model] with either 
> spice or vhdl-ams? As shown in the IBIS spec.

Yes, Rref_diff can be the only timing test load parameter for true
differential buffers, as illustrated in Section 6b.

>  
> 2. Can this Rref_diff keyword be used in the context of a normal true 
> differential buffer under the [Model] keyword, and at the same time the 
> model is define under [Diff Pin] section to describe the timing test 
> loads just like how the rest of the Cref, Vref keyword are used to 
> facilitate for board-level timing simulation, with the setup as 
> attached. Can the Rref_diff keyword used under the example below?

Rref_diff and Cref_diff can also be used for [Model]s when
they are connected by the [Diff Pin] keyword without the [External
Model] extension.  However, to prevent parser warning messages,
it is strongly encouraged to still add the Cref and/or Vref and Rref
subparameters.  Cref can be 0 F and/or Rref can be very large.
The parser check was added to be sure that the model maker intentially
meant to have a negligible single-ended timing test load.

So the example below may produce parser Warning messages.  If that is
the case, I would add

Cref = 0F

and/or possibly

Vref=1.163
Rref=1E9

as needed to remove any Warning messages.

>  
>  
> [Model] 25_lvds
> Model_type Output
> Polarity Non-Inverting
> Enable Active-Low
> Rref_diff = 100
> Vmeas = 1.163
> C_comp  1.8pF   1.8pF  1.8pF  
>  
> Please kindly comment. Thanks.
>  
> Regards
> Cynthia
> 
> 
> -- 


-- 
Bob Ross
Teraspeed Consulting Group LLC     Teraspeed Labs
121 North River Drive              13610 SW Harness Lane
Narragansett, RI 02882             Beaverton, OR 97008
401-284-1827                       503-430-1065
http://www.teraspeed.com           503-246-8048 Direct
bob@teraspeed.com

Teraspeed is a registered service mark of Teraspeed Consulting Group LLC


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