ASM690A/692A
ASM802L/802M
ASM805L
April 2008
rev 1.7
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 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
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 2Ω PMOS switch but a
Table 1. Pin Connections in Battery Backup Mode
Pin
Connection
Connected to VBATT through internal
PMOS switch
VOUT
VBATT
Connected to VOUT
Disabled
PFI
brown-out or loss of VCC will cause a switchover to VBATT
Logic-LOW
by means of a 20Ω PMOS switch. Although both
PFO
RESET
WDI
Logic-LOW (except on ASM805 where
it is HIGH)
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 3) 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 +0.6V.
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 35µA. When VCC drops more than 1.1V 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 1µA.
Condition
SW1/SW2
Open
SW3/SW4
Closed
Closed
Open
VCC > Reset Threshold
VCC < Reset Threshold
VCC > VBATT
Open
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 3) when VCC is less than VBATT and
VCC >VRT.
VCC < Reset Threshold
VCC < VBATT
Closed
ASM690A/802A/805L Reset Threshold = 4.65V
ASM692A /ASM802M Reset Threshold = 4.4V
µP Power Supply Supervisor With Battery Backup Switch
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Notice: The information in this document is subject to change without notice.
PULSECORE [ PulseCore Semiconductor ]
