+3V Voltage Monitoring, Low-Cost µP

Supervisory Circuits

_______________________________________________________________________________________

7

RESET and

RESET Outputs

A microprocessor’s (µP’s) reset input starts in a known

state. When the µP is in an unknown state, it should be

held in reset. The MAX706P/R/S/T and the MAX706AP/

code execution errors during power-up, power-down,

or brownout conditions.

teed to be logic-low and RESET is guaranteed to be

internal timer causes RESET and RESET to be

deasserted after a time equal to the reset pulse width,

which is typically 200ms (Figure 3).

drops below the reset threshold), RESET and RESET

threshold, the internal timer is continually reset, causing

the RESET and RESET outputs to remain asserted.

Thus, a brownout condition that interrupts a previously

initiated reset pulse causes an additional 200ms delay

from the time the latest interruption occurred. On

RESET and RESET are guaranteed to be asserted for

The MAX706P/MAX706AP provide a RESET signal, and

the MAX706R/S/T and MAX706AR/AS/AT provide a

RESET signal. The MAX708R/S/T provide both RESET

and RESET.

Watchdog Timer

The MAX706P/R/S/T and the MAX706AP/AR/AS/AT

watchdog circuit monitor the µP’s activity. If the µP

does not toggle the watchdog input (WDI) within 1.6s,

the watchdog output (WDO) goes low (Figure 4). If the

reset signal is asserted, the watchdog timer will be

reset to zero and disabled. As soon as reset is

released, the timer starts counting. WDI can detect puls-

es as narrow as 100ns with a 2.7V supply and 50ns with a

4.5V supply. The watchdog timer for the MAX706P/R/S/T

cannot be disabled. The watchdog timer for the

MAX706AP/AR/AS/AT operates similarly to the

MAX706P/R/S/T. However, the watchdog timer for the

MAX706AP/AR/AS/AT disables when the WDI input is

left open or connected to a tri-state output in its high-

impedance state and with a leakage current of less

than 600nA. The watchdog timer can be disabled any-

time, provided WDO is not asserted.

RESET

GENERATOR

70µA

8

7

RESET

RESET

5

PFO

GND

3

1.25V

2.63V MAX708R

2.93V MAX708S

3.08V MAX708T

1

MR

2

4

PFI

MAX708R/S/T

Figure 1. MAX706_ Functional Diagram

Figure 2. MAX708_ Functional Diagram

WATCHDOG

TRANSITION

DETECTOR

WATCHDOG

TIMER

TIMEBASE FOR

RESET AND

WATCHDOG

RESET

GENERATOR

70µA

8

WDO

7

RESET

(RESET)

5

PFO

GND

3

1.25V

2.63V MAX706P/R

2.93V MAX706S

3.08V MAX706T

( ) ARE FOR MAX706P/AP.

6

WDI

1

MR

2

4

PFI

MAX706P/R/S/T

MAX706AP/AR/AS/AT