MAX6323/MAX6324
µP Supervisory Circuits with Windowed
(Min/Max) Watchdog and Manual Reset
_______________________________________________________________________________________ 7
RESET
WDPO
WDI
FAST FAULT
t
WDI
< t
WD1
(min)
Figure 2. Fast Fault Timing
RESET
WDPO
WDI
SLOW FAULT
t
WDI
< t
WD2
(max)
Figure 3. Slow Fault Timing
RESET
WDPO
WDI
NORMAL OPERATION (NO PULSING, OUTPUT STAYS HIGH)
t
WD1
(max)
< t
WDI
< t
WD2
(min)
H
L
Figure 4. Normal Operation, WDPO Not Asserted
MAX6323/MAX6324
falls below the preset threshold or when the manual
reset input (MR) is asserted. The RESET output remains
asserted for at least 100ms after V
CC
has risen above
the reset threshold and MR is deasserted (Figure 5).
For noisy environments, bybass V
CC
with a 500pF (min)
capacitor to ensure correct operation.
The MAX6323 has a push-pull output stage, and the
MAX6324 utilizes an open-drain output. Connect a pull-
up resistor on the RESET output of the MAX6324 to any
supply up to +6V. Select a resistor value large enough
to register a logic low (see
Electrical Characteristics
)
and small enough to register a logic high while supply-
ing all input leakage currents and leakage paths con-
nected to the RESET line. A 10k pullup is sufficient in
most applications.
Manual Reset Input
Many µP-based products require manual reset capabil-
ity to allow an operator or external logic circuitry to initi-
ate a reset. The manual reset input (MR) can connect
directly to a switch without an external pullup resistor or
debouncing network. MR is internally pulled up to V
CC
and, therefore, can be left unconnected if unused. MR
is designed to reject fast, negative-going transients
(typically 100ns pulses), and it must be held low for a
minimum of 1µs to assert the reset output (Figure 5). A
0.1µF capacitor from MR to ground provides additional
noise immunity. After MR transitions from low to high,
reset will remain asserted for the duration of the reset
timeout period, at least 100ms.
Applications Information
Negative-Going V
CC
Transients
The MAX6323/MAX6324 are relatively immune to short-
duration negative-going V
CC
transients (glitches),
which usually do not require the entire system to shut
down. Typically, 200ns large-amplitude pulses (from
ground to V
CC
) on the supply will not cause a reset.
Lower amplitude pulses result in greater immunity.
Typically, a V
CC
transient that falls 100mV below the
reset threshold and lasts less than 20µs will not trigger
a reset (see
Typical Operating Characteristics
). An
optional 0.1µF bypass capacitor mounted close to V
CC
provides additional transient immunity.
Ensuring a Valid Reset Output
Down to V
CC
= 0V
When V
CC
falls below +1.2V, the MAX6323 RESET out-
put no longer sinks current; it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs con-
nected to RESET can drift to undetermined voltages.
This does not present a problem in most applications,
since most µPs and other circuitry are inoperative with
V
CC
below +1.2V. However, in applications where
RESET must be valid down to 0, adding a pulldown
resistor to RESET causes any stray leakage currents to
flow to ground, holding RESET low (Figure 6). R1’s
value is not critical; 100k is large enough not to load
RESET and small enough to pull RESET to ground. This
scheme does not work with the open-drain output of the
MAX6324.
µP Supervisory Circuits with Windowed
(Min/Max) Watchdog and Manual Reset
8 _______________________________________________________________________________________
MR
RESET
V
CC
V
TH
100ms
(min)
120ns
(typ) 20µs
(typ)
1µs
(min)
100ms
(min)
Figure 5. RESET Timing Relationship
Interfacing to µPs with
Bidirectional Reset Pins
Since the RESET output on the MAX6324 is open-drain,
this device easily interfaces with µPs that have bidirec-
tional reset pins, such as the Motorola 68HC11.
Connecting the µP supervisor’s RESET output directly
to the microcontroller’s (µC’s) RESET pin with a single
pullup resistor allows either device to assert reset
(Figure 7).
MAX6324 Open-Drain
RESET
Output
Allows Use with Multiple Supplies
Generally, the pullup resistor connected to the
MAX6324 will connect to the supply voltage that is
being monitored at the IC’s V
CC
pin. However, some
systems may use the open-drain output to level-shift
from the monitored supply to reset circuitry powered by
some other supply (Figure 8). Keep in mind that as the
MAX6324’s V
CC
decreases below +1.2V, so does the
IC’s ability to sink current at RESET. Also, with any pull-
up resistor, RESET will be pulled high as V
CC
decays
toward 0. The voltage where this occurs depends on
the pullup resistor value and the voltage to which it is
connected.
Watchdog Software Considerations
To help the watchdog timer monitor software execution
more closely, set and reset the watchdog input at dif-
ferent 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
time would continue to be reset within the loop, keeping
the watchdog from timing out.
Figure 9 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 be quickly
corrected, since the I/O is continually set low and the
watchdog time is allowed to time out, causing a reset or
interrupt to be issued.
MAX6323/MAX6324
µP Supervisory Circuits with Windowed
(Min/Max) Watchdog and Manual Reset
_______________________________________________________________________________________ 9
MAX6323
GND
V
CC
V
CC
RESET
R1
100k
Figure 6. RESET Valid to V
CC
= Ground Circuit
MAX6324
GND GND
V
CC
V
CC
V
CC
µP
RESET
RESET
INPUT
Figure 7. Interfacing to µPs with Bidirectional Reset Pins
MAX6324
GND GND
V
CC
+3.3V +5.0V
V
CC
5V SYSTEM
RESET
RESET
INPUT
R
PULLUP
Figure 8. MAX6324 Open-Drain RESET Output Allows Use with
Multiple Supplies

MAX6324AUT31+T

Mfr. #:
Manufacturer:
Maxim Integrated
Description:
Supervisory Circuits uPower Supervisor
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union