MAX694CSLASHD datasheet(7/18 Pages) MAXIM

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Watchdog Timer

The microprocessor drives the WATCHDOG INPUT

(WDI) with an I/O line. When OSC IN and OSC SEL are

unconnected, the microprocessor must toggle the WDI

pin once every 1.6s to verify proper software execution. If

a hardware or software failure occurs such that WDI not

toggled the MAX691/MAX693 will issue a 50ms* RESET

pulse after 1.6s. This typically restarts the microproces-

sor’s power-up routine. A new RESET pulse is issued

every 1.6s until the WDI is again strobed.

The WATCHDOG OUTPUT (WDO) goes low if the

watchdog timer is not serviced within its timeout period.

Once WDO goes low it remains low until a transition

occurs at WDI. The watchdog timer feature can be dis-

abled by leaving WDI unconnected. OSC IN and OSC

SEL also allow other watchdog timing options, as shown

in Table 1 and Figure 8.

MAX690, MAX692, and MAX694

The 8 pin MAX690, MAX692, and MAX694 have most

of the features of the MAX691, MAX693, and MAX695.

Figure 2 shows the MAX690/MAX692/MAX694 in a typical

application. Operation is much the same as with the

MAX691/MAX693/MAX695 (Figure 1), but in this case, the

power- fail input (PFI) monitors the unregulated input to the

7805 regulator. The MAX690/MAX694 RESET output goes

low when VCC falls below 4.65V. The RESET output of the

MAX692 goes low when VCC drops below 4.4V.

The current consumption of the battery-backed-up power

bus must be less than 50mA. The MAX690/MAX692/

MAX694 does not have a BATT ON output to drive an

external transistor. The MAX690/MAX692/MAX694 also

does not include chip enable gating circuitry that is avail-

able on the MAX690/MAX692/MAX694. In many systems

though, CE gating is not needed since a low input to

the microprocessor RESET line prevents the processor

from writing to RAM during power-up and power-down

transients.

The MAX690/MAX692/MAX694 watchdog timer has a

fixed 1.6s timeout period. If WDI remains either low or

high for more than 1.6s, a RESET pulse is sent to the

microprocessor. The watchdog timer is disabled if WDI is

left unconnected.

*200ms for MAX695

Figure 2. MAX690/692/694 Typical Application

7805

3-TERMINAL

REGULATOR

+8V

+5V

0.1µF

MAX690

MAX692

MAX694

GND

MICROPROCESSOR

MICRO-

PROCESSOR

POWER

CMOS

RAM

VCC

VOUT

VBATT

RESET

PFI

RESET

PFO

NMI

WDI

I/O LINE

MAX690–MAX695

Microprocessor Supervisory Circuits

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