Manual Reset Input Options
Unlike typical manual reset functions associated with
supervisors, each device in the MAX6443–MAX6452 fam-
ily includes at least one manual reset input, which must be
held logic-low for an extended setup period (tMR) before
the RESET output asserts. When valid manual reset input
conditions/setup periods are met, the RESET output is
one-shot pulse asserted low for a fixed reset timeout peri-
od (140ms min). Existing front-panel pushbutton switches
(i.e., power on/off, channel up/down, or mode select) can
be used to drive the manual reset inputs. The extended
manual reset setup period prevents nuisance system
resets during normal front-panel usage or resulting from
inadvertent short-term pushbutton closure.
The MAX6443/MAX6444, MAX6447/MAX6448, and
MAX6451/MAX6452 include a single manual reset
input with extended setup period (MR1). The MAX6445/
MAX6446 and MAX6449/MAX6450 include two manual
reset inputs (MR1 and MR2) with extended setup peri-
ods. For dual MR1, MR2 devices, both inputs must be
held low simultaneously for the extended setup period
(tMR) before the reset output is pulse asserted. The
dual extended setup provides greater protection from
nuisance resets. (For example, the user or service
technician is informed to simultaneously push both the
on/off button and the channel-select button for 6.72s
(L suffix) to reset the system.)
The MAX6443/MAX6452 RESET output is pulse asserted
once for the reset timeout period after each valid manual
reset input condition. At least one manual reset input
must be released (go high) and then be driven low for
the extended setup period before RESET asserts again.
Internal timing circuitry debounces low-to-high manual
reset logic transitions, so no external circuitry is required.
Figure 1 illustrates the single manual reset function of
the MAX6443/MAX6444 single-voltage monitors, and
Figure 2 represents the dual manual reset function of the
MAX6445/MAX6446 and MAX6449/MAX6450.
The MAX6447/MAX6448 and MAX6451/MAX6452
include both an extended setup period and immediate
setup period manual reset inputs. A low-to-high MR2
rising edge transition immediately pulse asserts the
RESET output for the reset timeout period (140ms min).
If the MAX6447/MAX6448 and MAX6451/MAX6452
MR2 input senses another rising edge before the end
of the 140ms timeout period (Figure 3), the internal
timer clears and begins counting again. If no rising
edges are detected within the 210ms timeout period,
RESET deasserts. The high-to-low transition on MR2
input is internally debounced for 210ms to ensure that
there are no false RESET assertions when MR2 is driven
from high to low (Figure 4). The MR2 input can be used
for system test purposes or smart-card-detect applica-
tions (see the Applications Information section).
Adjustable Input Voltage (RSTIN)
The MAX6449–MAX6452 monitor the voltage on RSTIN
using an adjustable reset threshold set with an external
resistor voltage-divider (Figure 5). Use the following
formula to calculate the externally monitored voltage
(VMON-TH):
VMON-TH = VTH-RSTIN × (R1 + R2)/R2
where VMON-TH is the desired reset threshold voltage and
VTH-RSTIN is the reset input threshold (0.63V). Resistors
R1 and R2 can have very high values to minimize current
consumption because of low leakage currents. Set R2 to
some conveniently high value (250kW, for example), and
calculate R1 based on the desired reset threshold volt-
age, using the following formula:
R1 = R2 × (VMON-TH/VTH-RSTIN - 1) W
Figure 2. MAX6445/MAX6446/MAX6449/MAX6450 Manual
Reset Timing Diagram
Figure 1. MAX6443/MAX6444 Manual Reset Timing Diagram
MR2
RESET
MR1
210ms
tMR
RESET TIMEOUT PERIOD
210ms
MR1 SETUP PERIOD
tMR
MR1
RESET
MAX6443–MAX6452 µP Reset Circuits with Long Manual Reset
Setup Period
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