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MAX6864UK29D3S

Nanopower µP Supervisory Circuits with Manual Reset and Watchdog Timer

Maxim Integrated 

Nanopower µP Supervisory Circuits with

Manual Reset and Watchdog Timer

Transient Immunity

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

power-down, and brownout conditions, the MAX6854/

MAX6855/MAX6856/MAX6858/MAX6860–MAX6869 are

relatively immune to short-duration supply transients, or

glitches. The Maximum VCC Transient Duration vs. Reset

Threshold Overdrive graph in the Typical Operating

Characteristics shows this relationship.

The area below the curve of the graph is the region in

which these devices typically do not generate a reset

pulse. This graph was generated using a falling pulse

applied to VCC, starting 100mV above the actual reset

threshold, VTH, and ending below this threshold (reset-

threshold overdrive). As the magnitude of the transient

increases, the maximum allowable pulse width

decreases. Typically, a 100mV VCC transient duration

of 40µs or less does not cause a reset.

Interfacing to Other Voltages for Logic

Compatibility

The open-drain RESET output can be used to interface

to a µP with other logic levels. As shown in Figure 4, the

open-drain output can be connected to voltages from 0

to 5.5V.

Generally, the pullup resistor connected to RESET con-

nects to the supply voltage that is being monitored at

the IC’s VCC input. However, some systems use the

open-drain output to level-shift from the monitored sup-

ply to reset circuitry powered by another supply voltage.

Keep in mind that as the supervisor’s VCC decreases,

so does the IC’s ability to sink current at RESET.

Ensuring a Valid RESET Down to VCC = 0V

(Push-Pull RESET)

When VCC falls below 1.1V, RESET’s current-sinking

capability declines drastically. The high-impedance

CMOS logic inputs connected to RESET can drift to

undetermined voltages. This presents no problems in

most applications, since most µPs and other circuitry

do not operate with VCC below 1.1V.

In those applications where RESET must be valid down

to 0, add a pulldown resistor between RESET and GND

for the MAX6854/MAX6858/MAX6861/MAX6864/

MAX6867 push-pull outputs. The resistor sinks any stray

leakage currents, holding RESET low (Figure 5). Choose

a pulldown resistor that accommodates leakages, such

that RESET is not significantly loaded and is capable of

pulling to GND. The external pulldown cannot be used

with the open-drain reset outputs.

Watchdog Software Considerations

One way to help the watchdog timer monitor software

execution more closely is to set and reset the watchdog

1.8V

VCC

MAX6856

MAX6860

MAX6863

MAX6866

MAX6869

RESET

N

GND

3.3V

100kΩ

VCC

RESET µP

GND

VCC

VCC

MAX6854

MAX6858

MAX6861

MAX6864

MAX6867

RESET

2MΩ

GND

Figure 4. Interfacing with Other Voltage Levels

Figure 5. Ensuring RESET Valid to VCC = Ground

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