MAX6747KA46-T Datasheet, PDF (8/14 Pages) Maxim Integrated Products – muP Reset Circuits with Capacitor-Adjustable Reset/Watchdog Timeout Delay

µP Reset Circuits with Capacitor-Adjustable
Reset/Watchdog Timeout Delay
V_{MON_TH}
R1
RESET IN
V_CC
V_CC
R2
MAX6748
MAX6749
MAX6750
GND _MAX6751
Figure 3. Adding an External Manual Reset Function to the
MAX6748–MAX6751
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 cur-
rent 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 sup-
ply voltage, therefore, V_CC must be greater than 1.5V to
guarantee RESET IN threshold accuracy and timing
performance. The MAX6748 and MAX6749 can be con-
figured 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 falls below it respective
threshold voltages.
Manual Reset (MAX6746/MAX6747)
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, falling transients (typically
100ns pulses) and it 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 time-
out period after the switch is opened.
VCC
WDI
tWD
tRP
OV
VCC
RESET
OV
NORMAL MODE (WDS = GND)
Figure 4a. Watchdog Timing Diagram, WDS = GND
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