Datasheet

Detailed Description
Reset Output
A microprocessor’s (µP’s) reset input starts the µP in a
known state. The MAX6841–MAX6845 assert a reset dur-
ing power-up, power-down, and brownout conditions.
When the V
CC
supply voltage falls below a preset thresh-
old (MAX6841/MAX6842) or RESET IN falls below the
adjustable threshold (MAX6843/MAX6844/MAX6845),
reset asserts and stays asserted for a fixed timeout delay
(Table 2) after V
CC
or RESET IN has risen above the
reset threshold. The MAX6841–MAX6845 are available
with five typical timeout options for each part: 150µs
(voltage detector), 1.5ms, 30ms, 210ms, and 1.68s.
The MAX6841/MAX6842 have two reset outputs
(RESET and RESET); one is the inverse of the other.
The MAX6841 has one active-high push-pull output and
one active-low push-pull output. The MAX6842 has one
active-high push-pull output and one active-low open-
drain output. The MAX6843 features an active-low
push-pull output. The MAX6844 features an active-
high push-pull output, and the MAX6845 has an active-
low open-drain output stage. Connect a pullup resistor
from the open-drain RESET output to any voltage
between 0 and 6V.
Manual Reset Input
Many µP-based systems require a manual reset capa-
bility, allowing the operator, a test technician, or exter-
nal logic circuitry to initiate a reset. Reset remains
asserted while MR is low, and for a fixed timeout delay
after MR returns high. This input has an internal 20kΩ
pullup resistor, so it can be left open if it is not used.
MR can be driven with CMOS logic level or with open-
drain/collector outputs. To create a manual reset func-
tion, connect a normally open momentary switch from
MR to ground; external debounce circuitry is not
required. If MR is driven from long cables or the device
is used in a noisy environment, connect a 0.1µF capac-
itor from MR to ground to provide additional noise
immunity.
RESET IN
The MAX6843/MAX6844/MAX6845 feature a RESET IN
input, which allows users to adjust the reset threshold
voltage. The internal reference voltage is 187.4mV with
respect to V
CC
. An external resistive-divider network
can be used to set voltage monitoring thresholds, as
shown in Figure 1. The resistive-divider network must
be referenced to V
CC
. As V
CC
falls, the voltage at
MAX6841–MAX6845
Ultra-Low-Voltage µP Reset Circuits and
Voltage Detectors
_______________________________________________________________________________________ 5
V
CC
RESET
OUTPUT
RESET
RESET
TIMEOUT
PERIOD
V
REF
MANUAL
RESET
DETECT
GND
MR
MR
PULLUP
RESET
MAX6841
MAX6842
Functional Diagrams
V
CC
RESET
OUTPUT
RESET
TIMEOUT
PERIOD
V
REF
= 187.4mV
MANUAL
RESET
DETECT
GND
MR
MR
PULLUP
RESET
RESET
RESET IN
MAX6843
MAX6844
MAX6845
MAX6843
MAX6844
MAX6845
V
CC
GND
RESET IN
MR
RESET
(RESET)
( ) ARE FOR
MAX6844
R1 =
R2
R2
R1
V
CC-TH
V
REF
- 1
WHERE V
REF
= 187.4mV (WITH RESPECT TO V
CC
) AND
V
CC-TH
IS THE V
CC
THRESHOLD.
V
CC
Figure 1. Setting the Adjustable Threshold Externally