Date: 11/29/04

SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over

© Copyright 2004 Sipex Corporation

10

The SP805L/M active-high RESET output is

the inverse of the SP690A/SP692A/SP802 RE-

SET output, and is valid with V

Some

µP's, such as Intel's 80C51, require an

active-high reset pulse.

Watchdog Input

The watchdog circuit monitors the

µP's activity.

If the

µP does not toggle the watchdog input

(WDI) within 1.6sec, a reset pulse is triggered.

The internal 1.6sec timer is cleared by either a

reset pulse or by floating the WDI input. As long

as RESET is asserted or the WDI input is

floating, the timer remains cleared and does not

count. As soon as RESET is released and WDI

is driven high or low, the timer starts counting.

It can detect pulses as short as 50ns.

Power-Fail Comparator

The Power-Fail Comparator can be used as an

under-voltage detector to signal the failing of a

power supply (it is completely separate from the

rest of the circuitry and does not need to be

dedicated to this function). The PFI input is

compared to an internal 1.25V reference. If PFI

is less than 1.25V, PFO goes low. The external

voltage divider drives PFI to sense the

unregulated DC input to the +5V regulator. The

voltage-divider ratio can be chosen such that the

voltage at PFI falls below 1.25V just before the

+5V regulator drops out. PFO then triggers an

interrupt which signals the

µP to prepare for

power-down.

When V

comparator is turned off and PFO is forced low

to conserve backup-battery power.

Backup-Battery Switchover

In the event of a brownout or power failure, it

may be necessary to preserve the contents of

RAM. With a backup battery installed at V

the RAM is assured to have power if V

As long as V

V

power switch. Once V

threshold, V

switches to V

V

through a 5

Ω switch only when V

reset threshold and V

When V

connected to V

applied to V

the diode (D1) between V

conduct current from V

more than .6V above V

When V

activated and the internal circuitry will be pow-

ered from the battery Figure 14. When V

just below V

current drawn from V

30

µA. When V

V

off and the supply current falls to less than 0.6

µA.

Reset Threshold = 4.65V in SP690A/802L/805L

Reset Threshold = 4.40V in SP692A/802M/805M

Figure 13. BACKUP-BATTERY Switchover Block Diagram

SW1

D1

D2

D3

SW2

GND

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I

T

I

D

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C1

W

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