6

IMP690A

, 802L, 802M, 805L

Application Information

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

a reset pulse will be 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 IMP690/692/802/805 by

Figure 4. Internal device configuration of battery

switch-over function

IMP690A / IMP802L / IMP805L Reset Threshold = 4.65V

IMP692A / IMP802M Reset Threshold = 4.4V

Closed

Open

Open

Closed

Closed

Open

CONDITION

SW1/SW2

SW3/SW4

690A_06.eps

SW4

SW3

D2

D1

D3

IMP690A

IMP692A

IMP802L

IMP802M

IMP805L

SW1

SW2

Table 1. Pin Connections in Battery Backup Mode

During the backup power mode, the internal circuitry of the

supervisory circuit draws power from the battery supply. While

rent drawn by the device is typically 35

PFI comparator and WDI comparator will shut off, reducing the

quiescent current drawn by the IC to less than 1

µA.

Backup Power Sources - Batteries

Battery voltage selection is important to insure that the battery

does not discharge through the parasitic device diode D1 (see

Table 2. Maximum Battery Voltages

Although most batteries that meet the requirements of Table 2 are

acceptable, lithium batteries are very effective backup source due

to their high-energy density and very low self-discharge rates.

Battery Replacement while Powered

Batteries can be replaced even when the device is in a powered

supply switchover as can occur in some other supervisory

Part No.

MAXIMUM Battery Voltage

IMP690A

4.80

IMP802L

4.80

IMP805L

4.80

IMP692A

4.55

IMP802M

4.55

Pin

Connection

PFI

Disabled



PFO

Logic-LOW





RESET

Logic LOW (except on IMP805 where it is HIGH)

WDI

Watchdog timer disabled