MAX6301–MAX6304
In those applications where RESET must be valid down
to 0V, adding a pulldown resistor between RESET and
ground sinks any stray leakage currents, holding
RESET low (Figure 9). The value of the pulldown resistor
is not critical; 100kΩ is large enough not to load RESET
and small enough to pull RESET to ground. For applica-
tions using the MAX6304, a 100kΩ pullup resistor
between RESET and V
CC
will hold RESET high when
V
CC
falls below 1V (Figure 10).
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
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 within the loop, keeping
the watchdog from timing out.
Figure 11 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. When using extended mode,
as described in the
Watchdog Input Current
section,
this scheme does result in higher average WDI input
current than does the method of leaving WDI low for the
majority of the timeout period and periodically pulsing it
low-high-low.
Layout Considerations
SRT and SWT are precision current sources. When
developing the layout for the application, be careful to
minimize board capacitance and leakage currents
around these pins. Traces connected to these pins
should be kept as short as possible. Traces carrying
high-speed digital signals and traces with large voltage
potentials should be routed as far from these pins as
possible. Leakage currents and stray capacitance
(e.g., a scope probe) at these pins could cause errors
in the reset and/or watchdog timeout period. When
evaluating these parts, use clean prototype boards to
ensure accurate reset and watchdog timeout periods.
RESET IN is a high-impedance input that is typically
driven by a high-impedance resistor-divider network
(e.g., 1MΩ to 10MΩ). Minimize coupling to transient sig-
nals by keeping the connections to this input short. Any
DC leakage current at RESET IN (e.g., a scope probe)
causes errors in the programmed reset threshold. Note
that sensitive pins are located on the GND side of the
device, away from the digital I/O, to simplify board layout.
+5V, Low-Power µP Supervisory Circuits
with Adjustable Reset/Watchdog
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