ADM706: what happens if WDI Pin is left open or "High Impedance"

Document created by analog-archivist Employee on Feb 23, 2016
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our ADM706P/ADM706R/ADM706S/ADM706T supervisory chip used to offer a “Watchdog
Inhibit function”: If you left the WDI input unconnected or high impedance, an
internal voltage divider would pull the WDI input  to a third logic state, in
between 0 and 1, which disabled the Watchdog timer function.

 

This is useful in systems where the microprocessor needs a long time to boot
up, before it is capable of regularly triggering the Watchdog timer. In this
way you can inhibit the ADM706P/ADM706R/ADM706S/ADM706T watchdog function from
resetting the system, until the initialization phase is over and the system is
running.

Issue:
The Watchdog disable function mysteriously disappeared in RevC of the
datasheet, without any PCN being generated to explain it.
Maxim’s MAX706 datasheet  no longer featured this Watchdog disable function and
Maxim released a MAX706A, which guaranteed this function.

There is a 5V version, ADM706, where the Watchdog inhibit is robust.

Explanation:
The three-state input on the WDI input was not robust on the 3.3V Versions of
the ADM706. Leakage current could easily push the input to a 1 or a 0,
activating the Watchdog function  when it should be disabled. This affected
both the ADI and Maxim parts, but Maxim released a new part to address the
problem while ADI did not. Therefore we removed the function from the
ADM706P/R/S/T  datasheet. The three-state circuit is still on the chip, even
after the latest Fab transfer, but the feature is not guaranteed.

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