3-Pin, Ultra-Low-Voltage, Low-Power
µP Reset Circuits
MAX6332/MAX6333/MAX6334
______________Applications Information
Interfacing to µPs with Bidirectional
Reset Pins
Since the
RESET
output on the MAX6334 is open-drain,
this device interfaces easily 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
pull-up resistor allows either device to assert reset
(Figure 1).
V
CC
V
CC
V
CC
µP
MOTOROLA
68HCXX
RESET
INPUT
GND
MAX6334
RESET
GND
Negative-Going V
CC
Transients
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, these devices are
relatively immune to short-duration, negative-going V
CC
transients (glitches). The
Typical Operating Characteristics
show the Maximum Transient Duration vs. Reset
Comparator Overdrive graph. The graph shows the maxi-
mum pulse width that a negative-going V
CC
transient may
typically have without issuing a reset signal. As the ampli-
tude of the transient increases, the maximum allowable
pulse width decreases.
Ensuring a Valid Reset Output
Down to V
CC
= 0
When V
CC
falls below 1V and approaches the minimum
operating voltage of 0.7V, push/pull-structured reset
sinking (or sourcing) capabilities decrease drastically.
High-impedance CMOS-logic inputs connected to the
RESET
pin can drift to indeterminate voltages. This
does not present a problem in most cases, since most
µPs and circuitry do not operate at V
CC
below 1V. For
the MAX6333, where
RESET
must be valid down to 0,
adding a pull-down resistor between
RESET
and GND
removes stray leakage currents, holding
RESET
low
(Figure 2a). The pull-down resistor value is not critical;
100kΩ is large enough not to load
RESET
and small
enough to pull it low. For the MAX6332, where RESET
must be valid to V
CC
= 0, a 100kΩ pull-up resistor
between RESET and V
CC
will hold RESET high when
V
CC
falls below 0.7V (Figure 2b).
Since the MAX6334 has an open-drain, active-low out-
put, it typically uses a pull-up resistor. With this device,
RESET will most likely not maintain an active condition,
but will drift to a non-active level due to the pull-up
resistor and the reduced sinking capability of the open-
drain device. Therefore, this device is not recommend-
ed for applications where the
RESET
pin is required to
be valid down to V
CC
= 0.
Figure 1. Interfacing to µPs with Bidirectional Reset Pins
V
CC
V
CC
100k*
MAX6333
RESET
MAX6332
RESET
GND
100k
(b)
GND
(a)
*ASSUMES HIGH-Z RESET INPUT TO THE
µP
Figure 2. Ensuring Reset Valid Down to V
CC
= 0
Table 1. Factory-Trimmed Reset
Thresholds*
RESET-
THRESHOLD
SUFFIX
MAX633_UR25D_
MAX633_UR24D_
MAX633_UR23D_
MAX633_UR22D_
MAX633_UR21D_
MAX633_UR20D_
MAX633_UR19D_
MAX633_UR18D_
MAX633_UR17D_
MAX633_UR16D_
T
A
= +25°C
MIN TYP MAX
2.46 2.50
2.36 2.40
2.26 2.30
2.16 2.20
2.06 2.10
1.96 2.00
1.87 1.90
2.55
2.44
2.34
2.24
2.14
2.04
1.93
T
A
= -40°C to +85°C
MIN
2.43
2.33
2.23
2.13
2.04
1.94
1.84
1.75
1.65
1.55
MAX
2.58
2.47
2.37
2.27
2.16
2.06
1.96
1.85
1.75
1.65
1.77 1.80 1.83
1.67 1.70 1.73
1.57 1.60 1.63
*
Factory-trimmed reset thresholds are available in approximately
100mV increments, with a ±1.8% room-temperature variance.
4
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