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 t
WD
+ t
RS
), 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 V
CC
to
an alternate power source connected at V
BATT
when V
CC
is less than the reset threshold voltage (V
CC
< V
RT
), and
V
CC
is less than V
BATT
. The V
OUT
pin is normally
connected to V
CC
through a 2Ω PMOS switch but a
brown-out or loss of V
CC
will cause a switchover to V
BATT
by means of a 20Ω PMOS switch. Although both
conditions (V
CC
< V
RT
and V
CC
<V
BATT
) must occur for
the switchover to V
BATT
to occur, V
OUT
will be switched
back to V
CC
when V
CC
exceeds V
RT
irrespective of the
voltage at V
BATT
. It should be noted that an internal
device diode (D1 in
Figure 3)
will be forward biased if
V
BATT
exceeds V
CC
by more than a diode drop when V
CC
is switched to V
OUT
. Because of this it is recommended
that V
BATT
be no greater than V
RT
+0.6V.
Condition
V
CC
> Reset Threshold
V
CC
< Reset Threshold
V
CC
> V
BATT
V
CC
< Reset Threshold
V
CC
< V
BATT
SW1/SW2
Open
Open
Closed
SW3/SW4
Closed
Closed
Open
Pin
V
OUT
V
BATT
PFI
PFO
RESET
WDI
ASM690A/692A
ASM802L/802M
ASM805L
Table 1. Pin Connections in Battery Backup Mode
Connection
Connected to V
BATT
through internal
PMOS switch
Connected to V
OUT
Disabled
Logic-LOW
Logic-LOW (except on ASM805 where
it is HIGH)
Watchdog timer disabled
During the backup power mode, the internal circuitry of
the supervisory circuit draws power from the battery
supply. While V
CC
is still alive, the comparator circuits
remain alive and the current drawn by the device is
typically 35µA. When V
CC
drops more than 1.1V below
V
BATT
, 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.
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 V
CC
is less than V
BATT
and
V
CC
>V
RT
.
ASM690A/802A/805L Reset Threshold = 4.65V
ASM692A /ASM802M Reset Threshold = 4.4V
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice.
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