Microprocessor Supervisory Circuits
= RESET TIMEOUT PERIOD
= NORMAL WATCHDOG TIMEOUT PERIOD
= WATCHDOG TIMEOUT PERIOD IMMEDIATELY AFTER RESET
Figure 1. Adding an external pull-down resistor ensures
RESET is valid with V
down to GND.
Figure 2. Watchdog Timeout Period and Reset Active Time
are asserted when V
the reset threshold (4.65V for the MAX691A/MAX800L,
4.4V for the MAX693A/MAX800M) and remain asserted
for 200ms typ after V
rises above the reset threshold
on power-up (Figure 5). The devices’ battery-
switchover comparator does not affect reset assertion.
However, both reset outputs are asserted in battery-
backup mode since V
must be below the reset
threshold to enter this mode.
The watchdog monitors µP activity via the Watchdog
Input (WDI). If the µP becomes inactive,
RESET are asserted. To use the watchdog function,
connect WDI to a bus line or µP I/O line. If WDI
remains high or low for longer than the watchdog time-
out period (1.6sec nominal),
are asserted (see
discussion on this page).
The Watchdog Output (WDO) remains high if there is a
transition or pulse at WDI during the watchdog timeout
period. The watchdog function is disabled and
a logic high when V
is below the reset threshold, bat-
tery-backup mode is enabled, or WDI is an open circuit.
In watchdog mode, if no transition occurs at WDI during
the watchdog timeout period, RESET and
asserted for the reset timeout period (200ms typical).
goes low and remains low until the next transition
at WDI (Figure 2). If WDI is held high or low indefinitely,
will generate 200ms pulses every
has a 2 x TTL output characteristic.
Selecting an Alternative
Watchdog and Reset Timeout Period
The OSC SEL and OSC IN inputs control the watchdog
and reset timeout periods. Floating OSC SEL and OSC
IN or tying them both to V
selects the nominal 1.6sec
watchdog timeout period and 200ms reset timeout peri-
od. Connecting OSC IN to GND and floating or connect-
ing OSC SEL to V
selects the 100ms normal
watchdog timeout delay and 1.6sec delay immediately
after reset. The reset timeout delay remains 200ms
(Figure 2). Select alternative timeout periods by con-
necting OSC SEL to GND and connecting a capacitor
between OSC IN and GND, or by externally driving OSC
IN (Table 1 and Figure 3). OSC IN is internally connect-
ed to a ±100nA (typ) current source that charges and
discharges the timing capacitor to create the oscillator
frequency, which sets the reset and watchdog timeout
Connecting a Timing Capacitor at OSC IN
A change of state (high to low, low to high, or a mini-
mum 100ns pulse) at the WDI during the watchdog
period resets the watchdog timer. The watchdog
default timeout is 1.6sec.
To disable the watchdog function, leave WDI floating.
An internal resistor network (100kΩ equivalent imped-
ance at WDI) biases WDI to approximately 1.6V.
Internal comparators detect this level and disable the
watchdog timer. When V
is below the reset thresh-
old, the watchdog function is disabled and WDI is dis-
connected from its internal resistor network, thus
becoming high impedance.