Datasheet

where V
MON_TH
is the desired reset threshold voltage
and V
TH
is the reset input threshold (1.235V). Resistors
R1 and R2 can have very high values to minimize
current consumption due to low leakage currents. Set R2
to some conveniently high value (500kΩ, for example)
and calculate R1 based on the desired reset threshold
voltage, using the following formula:
R1 = R2 x (V
MON_TH
/V
RESET IN
- 1) (Ω)
The MAX6748 and MAX6749 do not monitor V
CC
supply voltage; therefore, V
CC
must be greater than 1.5V
to guarantee RESET IN threshold accuracy and tim-
ing performance. The MAX6748 and MAX6749 can be
configured to monitor V
CC
voltage by connecting V
CC
to
V
MON_TH
.
Dual-Voltage Monitoring (MAX6750/MAX6751)
The MAX6750 and MAX6751 contain both factory-trimmed
threshold voltages and an adjustable reset threshold input,
allowing the monitoring of two voltages, V
CC
and V
MON_
TH
(see Figure 2). RESET is asserted when either of the
voltages fall below its respective threshold voltages.
Manual Reset (MAX6746/MAX6747)
Many μP-based products require manual-reset capability
to allow an operator or external logic circuitry to initiate a
reset. The manual-reset input (MR) can connect directly
to a switch without an external pullup resistor or debounc-
ing network. MR is internally pulled up to V
CC
and, there-
fore, can be left unconnected if unused.
MR is designed to reject fast, falling transients (typically
100ns pulses) and must be held low for a minimum of
1μs to assert the reset output. A 0.1μF capacitor from MR
to ground provides additional noise immunity. After MR
transitions from low to high, reset remains asserted for the
duration of the reset timeout period.
A manual-reset option can easily be implemented with
the MAX6748–MAX6751 by connecting a normally open
momentary switch in parallel with R2 (Figure 3). When
the switch is closed, the voltage on RESET IN goes to
zero, initiating a reset. Similar to the MAX6746/MAX6747
manual reset, reset remains asserted while the voltage at
RESET IN is zero and for the reset timeout period after
the switch is opened.
Figure 4a. Watchdog Timing Diagram, WDS = GND
Figure 3. Adding an External Manual Reset Function to the
MAX6748–MAX6751
V
CC
WDI
t
WD
t
RP
RESET
NORMAL MODE (WDS = GND)
V
CC
OV
OV
MAX6748
MAX6749
MAX6750
MAX6751
RESET IN
GND
V
CC
V
CC
V
MON_TH
R1
R2
MAX6746–MAX6753 μP Reset Circuits with Capacitor-Adjustable
Reset/Watchdog Timeout Delay
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