3-Pin, Ultra-Low-Voltage, Low-Power

µP Reset Circuits

4

_______________________________________________________________________________________

______________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).

In addition to issuing a reset to the µP during power-up,

power-down, and brownout conditions, these devices are

transients (glitches). The

Typical Operating Characteristics

show the Maximum Transient Duration vs. Reset

Comparator Overdrive graph. The graph shows the maxi-

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

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

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;

100k

Ω is large enough not to load RESET and small

enough to pull it low. For the MAX6332, where RESET

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

GND

MAX6334

RESET

GND

RESET

INPUT

MOTOROLA

68HCXX

µP

*

Factory-trimmed reset thresholds are available in approximately

100mV increments, with a ±1.8% room-temperature variance.

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

RESET-

THRESHOLD

SUFFIX

Table 1. Factory-Trimmed Reset

Thresholds*

Figure 1. Interfacing to µPs with Bidirectional Reset Pins

GND

MAX6333

RESET

100k

(a)

GND

MAX6332

RESET

100k*

(b)

*ASSUMES HIGH-Z RESET INPUT TO THE

µP