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SP813L

Low Power Microprocessor Supervisory Circuits

Exar Corporation 

tWP

tWD

tWD

+5V

WDI 0V

+5V

WDO 0V

+5V

RESET* 0V

+5V

RESET* 0V

tWD

tRS

* externally triggered LOW by MR,

RESET is for the SP813L only

Figure 14. SP705/706/813L Watchdog Timing Waveforms

Typically, WDO will be connected to the

non-maskable interrupt input (NMI) of a µP.

When VCC drops below the reset threshold,

WDO will go LOW whether or not the watch-

dog timer has timed out. Normally this would

trigger an NMI but RESET goes LOW simulta-

neously, and thus overrides the NMI.

If WDI is left unconnected, WDO can be used as

a low-line output. Since floating WDI disables

the internal timer, WDO goes LOW only when

VCC falls below the reset threshold, thus func-

tioning as a low-line output.

+5V

VRT

VRT

VCC

0V

+5V

WDO

0V

+5V

RESET

0V

tRS

tRS

MR*

+5V

0V

*externally driven LOW

tMD

tMR

Figure 15. SP705/706 Timing Diagrams with WDI Tri-stated. The SP707/708/813L RESET Output is the Inverse

of the RESET Waveform Shown.

June 2008 Rev C

SP705 Low Power Microprocessor Supervisory Circuits

10

© 2008 Exar Corporation

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