ASM802LAEPA ALSC [Alliance Semiconductor Corporation], ASM802LAEPA Datasheet - Page 4

• Manufacturer Part Number: ASM802LAEPA
• Description: UP Power Supply Supervisor With Battery Backup Switch
• Manufacturer: ALSC [Alliance Semiconductor Corporation]

October 2003
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
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
power source connected at V
reset threshold voltage (V
V
2Ω PMOS switch but a brown-out or loss of V
switchover to V
Although both conditions (V
occur for the switchover to V
switched back to V
the voltage at V
device diode (D1 in Figure 3) will be forward biased if V
exceeds V
switched to V
V
rev 1.0
BATT
BATT
ASM690A/802A/805L Reset Threshold = 4.65V
ASM692A /ASM802M Reset Threshold = 4.4V
V
V
V
CC
CC
CC
. The V
be no greater than V
> Reset Threshold
< Reset Threshold
< Reset Threshold
V
V
Condition
CC
CC
WD
CC
OUT
> V
< V
OUT
+ t
by more than a diode drop when V
BATT
BATT
BATT
RS
BATT
pin is normally connected to V
. Because of this it is recommended that
CC
), a reset pulse will be asserted to return
. It should be noted that an internal
when V
by means of a 20Ω PMOS switch.
µP Power Supply Supervisor With Battery Backup Switch
CC
RT
CC
BATT
CC
+0.6V.
< V
< V
BATT
SW1/SW2
exceeds V
Notice: The information in this document is subject to change without notice
RT
closed
when V
RT
open
open
), and V
to occur, V
and V
CC
CC
RT
CC
to an alternate
CC
is less than the
CC
irrespective of
<V
SW3/SW4
CC
is less than
OUT
will cause a
closed
closed
BATT
open
through a
will be
) must
CC
BATT
is
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 V
and the current drawn by the device is typically 35µA. When
V
switchover comparator, the PFI comparator and WDI
comparator will shut off, reducing the quiescent current drawn
by the IC to less than 1µA.
CC
Figure 3: Internal device configuration of battery
RESET
V
V
PFO
WDI
Pin
PFI
BATT
OUT
drops more than 1.1V below V
CC
ASM690A
ASM692A
ASM802M
ASM802L
V
SW1 SW2
switch-over function
is still alive, the comparator circuits remain alive
BATT
Connected to V
switch
Connected to V
Disabled
Logic-LOW
Logic-LOW (except on ASM805 where it is
HIGH)
Watchdog timer disabled
BATT
OUT
Connection
ASM690A / 692A
ASM802L / 802M
D1
V
OUT
through internal PMOS
D3
D2
ASM805L
BATT
ASM805L
SW3 SW4
, the internal
V
CC
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