Watchdog Timer - National Instruments DAQ X Series User Manual

Chapter 6 Digital I/O
Figure 6-15 illustrates the difference between line and bus filtering.
Digital Input P0.A
Digital Input P0.B
Filter Clock
Filtered Input A
Filtered Input B
2A With line filtering, filtered input A would ignore the glitch on digital input P0.B and transition after two filter clocks.
3A Filtered input A goes high when sampled high for two consecutive filter clocks and transitions on the next filter
edge because digital input P0.B glitches.

Watchdog Timer

The watchdog timer is a software-configurable feature used to set critical
outputs to safe states in the event of a software failure, a system crash, or
any other loss of communication between the application and the X Series
device.
When the watchdog timer is enabled, if the X Series device does not receive
a watchdog reset software command within the time specified for the
watchdog timer, the outputs go to a user-defined safe state and remain in
that state until the watchdog timer is disarmed by the application and new
values are written, the device is reset, or the computer is restarted. The
expiration signal that indicates an expired watchdog will continue to assert
until the watchdog is disarmed. After the watchdog timer expires, the
device ignores any digital writes until the watchdog timer is disarmed.
When the watchdog timer is enabled and the computer enters a fault condition, ports
Note
that are set to tri-state remain tri-stated and do not go to user-defined safe states.
X Series User Manual
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1A 2A 3A
Figure 6-15. Line and Bus Filtering
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