ASM690A/692A

ASM802L/802M

April 2008

ASM805L

rev 1.7

µP Power Supply Supervisor With Battery Backup Switch

4 of 15

Notice: The information in this document is subject to change without notice.

Watchdog Input

As discussed in the Reset section, the Watchdog input is

used to monitor microprocessor activity. It can be used to

insure that the microprocessor is in a continually

responsive state by requiring that the WDI pin be toggled

every second. If the WDI pin is not toggled within the 1.6

asserted to return the microprocessor to the initial

start-up state. Pulses as short as 50ns can be applied to

the WDI pin. If this feature is not used, the WDI pin

should be open circuited or the logic placed into a high-

impedance state to allow the pin to float.

Backup-Battery Switchover

A power loss can be made less severe if the system RAM

contents are preserved. This is achieved in the

Ω PMOS switch but a

by means of a 20

Ω PMOS switch. Although both

device diode (D1 in Figure 3) will be forward biased if

Condition

SW1/SW2

SW3/SW4

Open

Closed

Open

Closed

Closed

Open

ASM690A/802A/805L Reset Threshold = 4.65V

ASM692A /ASM802M Reset Threshold = 4.4V

Table 1. Pin Connections in Battery Backup Mode

Pin

Connection

PMOS switch

PFI

Disabled

PFO

Logic-LOW

RESET

Logic-LOW (except on ASM805 where

it is HIGH)

WDI

Watchdog timer disabled

During the backup power mode, the internal circuitry of

the supervisory circuit draws power from the battery

remain alive and the current drawn by the device is

comparator and WDI comparator will shut off, reducing

the quiescent current drawn by the IC to less than 1µA.

Backup Power Sources - Batteries

Battery voltage selection is important to insure that the

battery does not discharge through the parasitic device