MAX6822 MAXIM [Maxim Integrated Products], MAX6822 Datasheet - Page 7

• Manufacturer Part Number: MAX6822
• Description: Low-Voltage SOT23 ?P Supervisors with Manual Reset and Watchdog Timer
• Manufacturer: MAXIM [Maxim Integrated Products]

Figure 3. Watchdog Timing Relationship
Resets are issued to the µP during power-up, power-
down, and brownout conditions. The Typical Operating
Characteristics show a graph of the MAX6821–
MAX6825’s Maximum V
Threshold Overdrive, for which reset pulses are not
generated. The graph was produced using negative-
going V
voltage and ending below the reset threshold by the
magnitude indicated (reset threshold overdrive). The
graph shows the maximum pulse width that a negative-
going V
ing a reset pulse. As the amplitude of the transient
increases (i.e., goes farther below the reset threshold),
the maximum allowable pulse width decreases.
Typically, a V
reset threshold and lasts for 20µs or less will not trigger
a reset pulse.
One way to help the watchdog timer monitor software
execution more closely is to set and reset the watch-
dog input at different points in the program, rather than
pulsing the watchdog 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 watchdog from timing out.
Figure 5 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 beginning of
every subroutine or loop, then set high again when the
program returns to the beginning. If the program
should hang in any subroutine, the problem would
quickly be corrected, since the I/O is continually set
low and the watchdog timer is allowed to time out,
causing a reset or interrupt to be issued. As described
in the Watchdog Input Current section, this scheme
results in higher time average WDI input current than
does leaving WDI low for the majority of the timeout
period and periodically pulsing it low-high-low.
RESET*
*RESET IS THE INVERSE OF RESET.
V
WDI
CC
CC
CC
Watchdog Software Considerations
transient can typically have without trigger-
pulses, starting at the standard monitored
CC
t
RST
t
RP
transient that goes 100mV below the
_______________________________________________________________________________________
Low-Voltage SOT23 µP Supervisors with
CC
Transient Duration vs. Reset
Manual Reset and Watchdog Timer
t
WD
t
RP
Figure 4. Interfacing Open-Drain RESET to µPs with
Bidirectional Reset I/O
Figure 5. Watchdog Flow Diagram
MANUAL
RESET
GENERATOR
V
RESET
CC
MAX6822
Typical Operating Circuit
GND
V
CC
MR
RESET
SUBROUTINE OR
PROGRAM LOOP
MAX6823
SET WDI LOW
PROGRAM
SET WDI
RETURN
GND
START
V
CODE
HIGH
CC
RESET
WDI
RESET
RESET
I/O
GND
V
µ
CC
V
µP
P
CC
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