April 2008
rev 1.7
Detailed Description
It is important to initialize a microprocessor to a known
state in response to specific events that could create
code execution errors and “lock-up”. The reset output of
these supervisory circuits send a reset pulse to the
microprocessor
in
response
to
power-up,
power-
down/power-loss or a watchdog time-out.
RESET/RESET Timing
Power-up reset occurs when a rising V
CC
reaches the
reset threshold, V
RT
, forcing a reset condition in which
the reset output is asserted in the appropriate logic state
for the duration of t
RS
. The reset pulse width, t
RS
, is
typically around 200ms and is LOW for the ASM690A,
ASM692A, ASM802 and HIGH for the ASM805L.
Figure 1
shows the reset pin timing.
Power-loss or “brown-out” reset occurs when V
CC
dips
below the reset threshold resulting in a reset assertion for
the duration of t
RS
. The reset signal remains asserted as
long as V
CC
is between V
RT
and 1.1V, the lowest V
CC
for
which thesedevices can provide a guaranteed logic-low
output. To ensure logic inputs connected to the ASM690A
/ ASM692A/ASM802 RESET pin are in a known state
when V
CC
is under 1.1V, a 100kΩ pull-down resistor at
RESET is needed: the logic-high ASM805L will need a
pull-up resistor to V
CC
.
Watchdog Timer
A Watchdog time-out reset occurs when a logic “1” or
logic “0” is continuously applied to the WDI pin for more
than 1.6 seconds. After the duration of the reset interval,
the watchdog timer starts a new 1.6 second timing
interval; the microprocessor must service the watchdog
input by changing states or by floating the WDI pin before
this interval is finished. If the WDI pin is held either HIGH
or LOW, a reset pulse will be triggered every 1.8 seconds
(the 1.6 second timing interval plus the reset pulse width
tRS).
ASM690A/692A
ASM802L/802M
ASM805L
Application Information
Microprocessor Interface
The ASM690 has logic-LOW RESET output while the
ASM805 has an inverted logic-HIGH RESET output.
Microprocessors with bidirectional reset pins can pose a
problem
when
the
supervisory
circuit
and
the
microprocessor output pins attempt to go to opposite
logic states. The problem can be resolved by placing a
4.7kΩ resistor between the RESET output and the
microprocessor reset pin. This is shown in
Figure 2.
Since the series resistor limits drive capabilities, the reset
signal to other devices should be buffered.
µP Power Supply Supervisor With Battery Backup Switch
Notice: The information in this document is subject to change without notice.
3 of 15
PULSECORE [ PulseCore Semiconductor ]
