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MAX6369 Datasheet(PDF) 8 Page - Maxim Integrated Products

Part No. MAX6369
Description  Pin-Selectable Watchdog Timers
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX6369 Datasheet(HTML) 8 Page - Maxim Integrated Products

 
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Pin-Selectable Watchdog Timers
8
_______________________________________________________________________________________
Selecting 011 (SET2 = 0, SET1 = 1, SET0 = 1) disables
the watchdog timer function on all devices in the family.
Operation can be reenabled without powering down by
changing the set inputs to the new desired setting. The
device assumes the new selected timing characteris-
tics and enter the startup phase after the 300µs setup
time elapses (Figure 3). WDO is high when the watch-
dog timer is disabled.
The MAX6373/MAX6374 offer a first-edge feature. In
first-edge mode (settings 101 or 110, Table 1), the
internal timer does not control the startup delay period.
Instead, startup terminates when WDI sees a transition.
If changing to first-edge mode while the device is oper-
ating, disable mode must be entered first. It is then safe
to select first-edge mode. Entering disable mode first
ensures the output is unasserted when selecting first-
edge mode and removes the danger of WDI being
masked out.
Output
The MAX6369/MAX6371/MAX6373 have an active-low,
open-drain output that provides a watchdog output
pulse of 100ms. This output structure sinks current
when WDO is asserted. Connect a pullup resistor from
WDO to any supply voltage up to +5.5V.
Select a resistor value large enough to register a logic
low (see
Electrical Characteristics), and small enough
to register a logic high while supplying all input current
and leakage paths connected to the WDO line. A 10kΩ
pullup is sufficient in most applications. The MAX6370/
MAX6372/MAX6374 have push-pull outputs that pro-
vide an active-low watchdog output pulse of 1ms.
When WDO deasserts, timing begins again at the
beginning of the watchdog timeout period (Figure 2).
Usage in Noisy Environments
If using the watchdog timer in an electrically noisy envi-
ronment, a bypass capacitor of 0.1µF should be con-
nected between VCC and GND as close to the device
as possible, and no further away than 0.2 inches.
________________Watchdog Software
Considerations
To help the watchdog timer monitor software execution
more closely, set and reset the watchdog input at differ-
ent points in the program, rather than pulsing the watch-
dog input high-low-high or low-high-low. This technique
avoids a stuck loop, in which the watchdog timer would
continue to be reset inside the loop, keeping the watch-
dog from timing out. Figure 4 shows an example of a
flow diagram where the I/O driving the watchdog input is
set high at the beginning of the program, set low at the
end of every subroutine or loop, then set high again
when the program returns to the beginning. If the pro-
gram should hang in any subroutine, the problem would
be quickly corrected, since the I/O is continually set low
and the watchdog timer is allowed to time out, causing
WDO to pulse.
START
SET WDI
HIGH
PROGRAM
CODE
SUBROUTINE OR
PROGRAM LOOP
SET WDI LOW
RETURN
POSSIBLE
INFINITE LOOP PATH
Figure 4. Watchdog Flow Diagram


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