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AR# 19826

Spartan-3 - Will the weak-keeper circuitry at the I/Os function with DCI-terminated standards?

Description

Keywords: Spartan-3, weak, keeper, I/O, IO, terminate, termination, bus, hold, DCI, XCITE, pullup, pull-up, pulldown, pull-down, resistor

The 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 pull-up or pull-down resistor. Will this feature work in the presence of DCI termination?

Solution

The keeper circuitry works on all I/Os (DCI or non-DCI) that are series terminated, such as LVTTL and LVCMOS. However, any parallel termination resistors (e.g., those used with SSTL or HSTL standards) prevent the keeper from performing its function because it is not as strong as the 50 Ohm pull-ups/downs.

In addition, since the keeper upsets the balance of such terminations, it is not activated for the DCI standards employing parallel termination (although the software might indicate this by accepting the KEEPER attribute without issuing an error message).
AR# 19826
Date Created 09/03/2007
Last Updated 09/12/2007
Status Active
Type General Article