ASM690A/692A, ASM802L/802M, ASM805L

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6

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 second window

the microprocessor to the initial start−up state. Pulses as

short as 50 ns 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 ASM690/

through a 2

an internal device diode (D1 in Figure 5) will be forward

Table 4.

Condition

SW1/SW2

SW3/SW4

Open

Closed

Open

Closed

Closed

Open

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

ASM692A/ASM802M Reset Threshold = 4.4 V

Figure 5. Internal Device Configuration of Battery

Switch−over Function

Table 5. 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 supply.

and the current drawn by the device is typically 35

mA.

switchover comparator, the PFI comparator and WDI

comparator will shut off, reducing the quiescent current

drawn by the IC to less than 1

mA.

Backup Power Sources − Batteries

Battery voltage selection is important to insure that the

battery does not discharge through the parasitic device diode

Table 6. MAXIMUM BATTERY VOLTAGES

Part Number

Maximum Battery Voltage (V)

ASM690A

4.80

ASM802L

4.80

ASM805L

4.80

ASM692A

4.55

ASM802M

4.55

Although most batteries that meet the requirements of

Table 6 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

used, the pin should be grounded.

Backup Power Sources − SuperCapt

Capacitor storage, with very high values of capacitance,

can be used as a back−up power source instead of batteries.

SuperCap are capacitors with capacities in the fractional

farad range. A 0.1 farad SuperCap would provide a useful

backup power source. Like the battery supply, it is important