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
(minimum tWD + tRS), a reset pulse will be asserted to return
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/
692/802/805 by switching from the failed VCC to an
alternate power source connected at VBATT when VCC is less
than the reset threshold voltage (VCC < VRT), and VCC is less
than VBATT. The VOUT pin is normally connected to VCC
through a 2 W PMOS switch but a brown−out or loss of VCC
will cause a switchover to VBATT by means of a 20 W PMOS
switch. Although both conditions (VCC < VRT and VCC <
VBATT) must occur for the switchover to VBATT to occur,
VOUT will be switched back to VCC when VCC exceeds VRT
irrespective of the voltage at VBATT. It should be noted that
an internal device diode (D1 in Figure 5) will be forward
biased if VBATT exceeds VCC by more than a diode drop
when VCC is switched to VOUT. Because of this it is
recommended that VBATT be no greater than VRT + 0.6 V.
Table 4.
Condition SW1/SW2 SW3/SW4
VCC > Reset Threshold Open Closed
VCC < Reset Threshold
VCC > VBATT
Open Closed
VCC < Reset Threshold
VCC < VBATT
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
VOUT Connected to VBATT through internal
PMOS switch
VBATT Connected to VOUT
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.
While VCC is still alive, the comparator circuits remain alive
and the current drawn by the device is typically 35 mA.
When VCC drops more than 1.1 V below VBATT, the internal
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
D1 (see Figure 5) when VCC is less than VBATT and VCC >
VRT.
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
powered state as long as VCC remains above the reset
threshold voltage VRT. In the ASM devices, a floating
VBATT pin will not cause a power supply switchover as can
occur in some other supervisory circuits. If VBATT is not
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