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AR# 13716

Virtex-II/-II Pro/-4/-5/-6 - Will the weak-keeper circuitry at the I/Os function with DCI-terminated standards?


The weak-keeper circuitry is designed to hold a bus at a known value in the event that all drivers on that bus are disabled. The circuitry essentially consists of a weak (~10K - 60K ohms) pull-up or pull-down resistor.

Will this feature work in the presence of DCI termination?


The weak-keeper circuitry will work on all I/Os (DCI or non-DCI) that are series terminated, such as LVDCI. However, any parallel termination resistors (for instance, those used with SSTL or HSTL standards) will prevent the weak-keeper from doing its job, because it is not nearly as strong the 50 ohm pull-ups/downs.

In addition, since the presence of the weak-keeper would upset the balance of such terminations, it is not activated for the DCI standards employing parallel termination (although the software might give you that impression by accepting the KEEPER attribute without complaint).

AR# 13716
Date Created 08/29/2007
Last Updated 12/15/2012
Status Active
Type General Article