Re: Differential pins in IBIS

Weston Beal wrote:
>
> Stephen,
>
> Although this receiver is implemented as a differential receiver, from the
> outside it looks more like a single ended receiver because the system really
> only feeds in one data signal. Vref is held constant at 1.25V. You can
> simply use the parameters Vinl and Vinh to specify the thresholds Vref -
> 350mV (0.9V) and Vref + 350mv (1.6V).
>
> Now, since Vref should follow Vcc, it changes for min and max cases. For
> this we use Vinl and Vinh under the [Model_Spec] keyword. They work the
> same but now have typ, min, and max values.

Not good enough. Vref and Vtt are related to Vddq, but there are both
tolerances and noise terms involved which don't track the simplistic
typ/min/max axis. In particular, for the devices Mr. Nolan is discussing,
the common-mode rejection of the receivers is an issue. Differential is
a must.

For instance, right now there is a JC-42.3 task group dealing with the
impact of SSI noise on Vref. Crosstalk analysis where is can make or
break standard technology.

> The IBIS committee is still working on the AC thresholds vs. DC thresholds
> problem.

One of the reasons that Mr. Nolan now represents TI on the IBIS committee.

-- 
D. C. Sessions
dc.sessions@vlsi.com