MAX6369 データシートの表示（PDF） - Maxim Integrated

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MAX6369

Pin-Selectable Watchdog Timers

Maxim Integrated 

Pin-Selectable Watchdog Timers

START

SET WDI

HIGH

PROGRAM

CODE

SUBROUTINE OR

PROGRAM LOOP

SET WDI LOW

RETURN

POSSIBLE

INFINITE LOOP PATH

Figure 4. Watchdog Flow Diagram

Selecting 011 (SET2 = 0, SET1 = 1, SET0 = 1) disables

the watchdog timer function on all devices in the family.

Operation can be reenabled without powering down by

changing the set inputs to the new desired setting. The

device assumes the new selected timing characteris-

tics and enter the startup phase after the 300µs setup

time elapses (Figure 3).

The MAX6373/MAX6374 offer a first-edge feature. In

first-edge mode (settings 101 or 110, Table 1), the

internal timer does not control the startup delay period.

Instead, startup terminates when WDI sees a transition.

If changing to first-edge mode while the device is oper-

ating, disable mode must be entered first. It is then safe

to select first-edge mode. Entering disable mode first

ensures the output is unasserted when selecting first-

edge mode and removes the danger of WDI being

masked out.

Output

The MAX6369/MAX6371/MAX6373 have an active-low,

open-drain output that provides a watchdog output

pulse of 100ms. This output structure sinks current

when WDO is asserted. Connect a pullup resistor from

WDO to any supply voltage up to +5.5V.

Select a resistor value large enough to register a logic

low (see Electrical Characteristics), and small enough

to register a logic high while supplying all input current

and leakage paths connected to the WDO line. A 10kΩ

pullup is sufficient in most applications. The MAX6370/

MAX6372/MAX6374 have push-pull outputs that pro-

vide an active-low watchdog output pulse of 1ms.

When WDO deasserts, timing begins again at the

beginning of the watchdog timeout period (Figure 2).

Usage in Noisy Environments

If using the watchdog timer in an electrically noisy envi-

ronment, a bypass capacitor of 0.1µF should be con-

nected between VCC and GND as close to the device

as possible, and no further away than 0.2 inches.

________________Watchdog Software

Considerations

To help the watchdog timer monitor software execution

more closely, set and reset the watchdog input at differ-

ent points in the program, rather than pulsing the watch-

dog input high-low-high or low-high-low. This technique

avoids a stuck loop, in which the watchdog timer would

continue to be reset inside the loop, keeping the watch-

dog from timing out. Figure 4 shows an example of a

flow diagram where the I/O driving the watchdog input is

set high at the beginning of the program, set low at the

end of every subroutine or loop, then set high again

when the program returns to the beginning. If the pro-

gram should hang in any subroutine, the problem would

be quickly corrected, since the I/O is continually set low

and the watchdog timer is allowed to time out, causing

WDO to pulse.

Pin Configuration

TOP VIEW

WDI 1

GND 2

N.C. 3

SET0 4

MAX6369

MAX6370

MAX6371

MAX6372

MAX6373

MAX6374

8 VCC

7 WDO

6 SET2

5 SET1

SOT23-8

Chip Information

TRANSISTOR COUNT: 1500

PROCESS: BiCMOS

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2003 Maxim Integrated Products

Printed USA

is a registered trademark of Maxim Integrated Products.

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