Phytec miniMODUL-537, Hardware Manual

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The External Watchdog-Timer and the Power-Fail-Option
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PHYTEC Meßtechnik GmbH 1993 L-006e_2
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8 The External Watchdog-Timer and the Power-Fail-
Option
The monitor chip U3 has an own watchdog timer and a comparator for
the creation of a interrupt signal /PFO.
8.1 The External Watchdog-Timer
The SAB80C537 has already an internal watchdog-timer, but in some
cases an external watchdog timer might be advantageous:
For applications where the time constant of the watchdog timer is too
small and for applications, where an external signal must be
monitored.
The controller watchdog reacts only to a program crash or error of the
clock. The additional watchdog timer in U3 allows the reaction to
external events and is activated by a cyclic application of a low level
to A62 (/WDI). If the signal fails for an adjustable period, the system
is reset. The external watchdog remains inactive as long as A62 is not
connected. The time constant is within the period of one to two
seconds.