Chapter 6
Digital I/O
X Series User Manual
6-28
ni.com
Fig
u
re 6-15 ill
u
strates the difference between line and b
u
s filtering.
Figure 6-15.
Line and Bus Filtering
Watchdog Timer
The watchdog timer is a software-config
u
rable feat
u
re
u
sed to set critical
o
u
tp
u
ts to safe states in the event of a software fail
u
re, a system crash, or
any other loss of comm
u
nication 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 o
u
tp
u
ts go to a
u
ser-defined safe state and remain in
that state
u
ntil the watchdog timer is disarmed by the application and new
val
u
es are written, the device is reset, or the comp
u
ter is restarted. The
expiration signal that indicates an expired watchdog will contin
u
e to assert
u
ntil the watchdog is disarmed. After the watchdog timer expires, the
device ignores any digital writes
u
ntil the watchdog timer is disarmed.
Note
When the watchdog timer is enabled and the comp
u
ter enters a fa
u
lt condition, ports
that are set to tri-state remain tri-stated and do not go to
u
ser-defined safe states.
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.
Digit
a
l Inp
u
t P0.A
Digit
a
l Inp
u
t P0.B
Filter Clock
Filtered Inp
u
t A
Filtered Inp
u
t B
1A
2A
3
A
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