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
bidirectional 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 pullup
resistor allows either device to assert reset (Figure 1).
Negative-Going VCC 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 VCC transients (glitches). The Typical Operating
Characteristics show the Maximum Transient Duration vs.
Reset Comparator Overdrive graph. The graph shows the
maximum pulse width that a negative-going VCC transient
may typically have without issuing a reset signal. As the
amplitude of the transient increases, the maximum allowable
pulse width decreases.
Ensuring a Valid Reset Output Down to
VCC = 0
When VCC 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 VCC 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 VCC =
0, a 100kΩ pull-up resistor between RESET and VCC will
hold RESET high when VCC falls below 0.7V (Figure 2b).
Since the MAX6334 has an open-drain, active-low
output, 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 recommended for applications
where the RESET pin is required to be valid down to VCC = 0.
Figure 1. Interfacing to μPs with Bidirectional Reset Pins
*Factory-trimmed reset thresholds are available in approximately
100mV increments, with a ±1.8% room-temperature variance.
Figure 2. Ensuring Reset Valid Down to VCC = 0
Table 1. Factory-Trimmed Reset
Thresholds*
VCC
VCC
GND
MAX6334
RESET
VCC
GND
RESET
INPUT
MOTOROLA
68HCXX
µP
VCC
GND
MAX6333
RESET
100kΩ
(a)
VCC
GND
MAX6332
RESET
100kΩ*
(b)
*ASSUMES HIGH-Z RESET INPUT TO THE µP
RESET-
THRESHOLD
SUFFIX
TA = +25°C TA = -40°C to
+125°C BIT 4
MIN TYP MAX MIN MAX
MAX633_UR25D_ 2.46 2.50 2.55 2.43 2.58
MAX633_UR24D_ 2.36 2.40 2.44 2.33 2.47
MAX633_UR23D_ 2.26 2.30 2.34 2.23 2.37
MAX633_UR22D_ 2.16 2.20 2.24 2.13 2.27
MAX633_UR21D_ 2.06 2.10 2.14 2.04 2.16
MAX633_UR20D_ 1.96 2.00 2.04 1.94 2.06
MAX633_UR19D_ 1.87 1.90 1.93 1.84 1.96
MAX633_UR18D_ 1.77 1.80 1.83 1.75 1.85
MAX633_UR17D_ 1.67 1.70 1.73 1.65 1.75
MAX633_UR16D_ 1.57 1.60 1.63 1.55 1.65
MAX6332/MAX6333/
MAX6334
3-Pin, Ultra-Low-Voltage, Low-Power
μP Reset Circuits
www.maximintegrated.com
Maxim Integrated
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