ASM705/706/707/708 and ASM813L
Low Power
μP
Supervisor Circuits
General Description
The ASM705 / 706 / 707 / 708 and ASM813L are cost
effective CMOS supervisor circuits that monitor power-
supply and battery voltage level, and μP/μC operation.
The family offers several functional options. Each device
generates a reset signal during power-up, power-down and
during brownout conditions. A reset is generated when the
supply drops below 4.65V (ASM705/707/813L) or 4.40V
(ASM706/708). For 3V power supply applications, refer to
the ASM705P/R/S/T data sheet. In addition, the
ASM705/706/813L feature a 1.6 second watchdog timer.
The ASM707/708 have both active-HIGH and active-LOW
reset outputs but no watchdog function. The ASM813L has
the same pin-out and functions as the ASM705 but has an
active-HIGH reset output. A versatile power-fail circuit has
a 1.25V threshold, useful in low battery detection and for
monitoring non-5V supplies. All devices have a manual
reset (MR) input. The watchdog timer output will trigger a
reset if connected to MR.
All devices are available in 8-pin DIP, SO and MicroSO
packages.
Features
Precision power supply monitor
4.65V threshold (ASM705/707/813L)
4.40V threshold (ASM706/708)
Debounced manual reset input
Voltage monitor
1.25V threshold
Battery monitor / Auxiliary supply monitor
Watchdog timer (ASM705/706/813L)
200ms reset pulse width
Active HIGH reset output (ASM707/708/813L)
MicroSO package
Application
Computers and embedded controllers
Portable/Battery-operated systems
Intelligent instruments
Wireless communication systems
PDAs and hend-held equipment
Automative Systems
Safety Systems
Typical Operating Circuit
©2010 SCILLC. All rights reserved.
JANUARY 2010 – Rev. 2
Publication Order Number:
ASM705/D
ASM705/706/707/708 and ASM813L
Block Diagram
Pin Configuration
Rev. 2 | Page 2 of 14 | www.onsemi.com
ASM705/706/707/708 and ASM813L
Pin Description
Pin Number
ASM705/706
DIP/
SO
1
ASM707/708
DIP/
SO
1
ASM813L
DIP/
SO
1
Name
Function
MicroSO
MicroSO
MicroSO
Manual reset input. The active LOW input
triggers a reset pulse. A 250 μA pull-up
current allows the pin to be driven by
TTL/CMOS logic or shorted to ground with a
switch.
+5V power supply input.
Ground reference for all signals.
Power-fail input voltage monitor. With PFI
less than 1.25V, PFO goes LOW. Connect
PFI to Ground or V
CC
when not in use.
Power-fail output. The output is active LOW
and sinks current when PFI is less than
1.25V.
Watchdog input. WDI controls the internal
watchdog timer. A HIGH or LOW signal for
1.6sec at WDI allows the internal timer to
run-out, setting WDO LOW. The watchdog
function is disabled by floating WDI or by
connecting WDI to a high impedance three-
state buffer. The internal watchdog timer
clears when: RESET is asserted; WDI is
three-stated; or WDI sees a rising or falling
edge.
Not Connected.
Active LOW reset output. Pulses LOW for
200ms when triggered, and stays LOW
whenever V
CC
is below the reset threshold.
RESET remains LOW for 200ms after V
CC
rises above the reset threshold or MR goes
from LOW to HIGH. A watchdog timeout will
not trigger RESET unless WDO is
connected to MR.
Watchdog output. WDO goes LOW when
the 1.6 second internal watchdog timer
times-out and does not go HIGH until the
watchdog is cleared. In addition, when V
CC
falls below the reset threshold, WDO goes
LOW. Unlike RESET, WDO does not have a
minimum pulse width and as soon as V
CC
exceeds the reset threshold, WDO goes
HIGH with no delay.
Active HIGH reset output. The inverse of
RESET. The ASM813L only has a RESET
output.
3
3
3
MR
2
3
4
4
5
6
2
3
4
4
5
6
2
3
4
4
5
6
V
CC
GND
PFI
5
7
5
7
5
7
PFO
6
8
-
-
6
8
WDI
-
-
6
8
-
-
NC
7
1
7
1
-
-
RESET
8
2
-
-
8
2
WDO
-
-
8
2
7
1
RESET
Rev. 2 | Page 3 of 14 | www.onsemi.com
ASM705/706/707/708 and ASM813L
Detailed Description
A proper reset input enables a microprocessor /
microcontroller to start in a known state. ASM70X and
ASM813L assert reset to prevent code execution errors
during power-up, power-down and brown-out conditions.
RESET/RESET Timing
The RESET/RESET signals are designed to start a
μP/μC in a known state or return the system to a known
state.
The ASM707/708 have two reset outputs, one active-
HIGH RESET and one active-LOW RESET output. The
ASM813L has only an active-HIGH output. RESET is
simply the complement of RESET.
RESET is guaranteed to be LOW with V
CC
above 1.2V.
During a power-up sequence, RESET remains low until
the supply rises above the threshold level, either 4.65V or
4.40V. RESET goes high approximately 200ms after
crossing the threshold.
During power-down, RESET goes LOW as V
CC
falls
below the threshold level and is guaranteed to be under
0.4V with V
CC
above 1.2V.
In a brownout situation where V
CC
falls below the
threshold level, RESET pulses low. If a brown-out occurs
during an already initiated reset, the pulse will continue
for a minimum of 140ms.
Power Failure Detection With Auxiliary Comparator
All devices have an auxiliary comparator with 1.25V trip
point and uncommitted output (PFO) and noninverting
input (PFI). This comparator can be used as a supply
voltage monitor with an external resistor voltage divider.
The attenuated voltage at PFI should be set just below
the 1.25 threshold. As the supply level falls, PFI is
reduced causing the PFO output to transit LOW. Normally
PFO interrupts the processor so the system can be shut
down in a controlled manner.
Manual Reset (MR)
The active-LOW manual reset input is pulled high by a
250μA pull-up current and can be driven low by
CMOS/TTL logic or a mechanical switch to ground. An
external debounce circuit is unnecessary since the
140ms minimum reset time will debounce mechanical
pushbutton switches.
By connecting the watchdog output (WDO) and MR, a
watchdog timeout forces RESET to be generated. The
ASM813L should be used when an active-HIGH RESET
is required.
Watchdog Timer
The watchdog timer available on the ASM705/706/813L
monitors μP/μC activity. An output line on the processor
is used to toggle the WDI line. If this line is not toggled
within 1.6 seconds, the internal timer puts the watchdog
output, WDO, into a LOW state. WDO will remain LOW
until a toggle is detected at WDI.
If WDI is floated or connected to a three-stated circuit, the
watchdog function is disabled, meaning, it is cleared and
not counting. The watchdog timer is also disabled if
RESET is asserted. When RESET becomes inactive and
the WDI input sees a high or low transition as short as
50ns, the watchdog timer will begin a 1.6 second
countdown. Additional transitions at WDI will reset the
watchdog timer and initiate a new countdown sequence.
Rev. 2 | Page 4 of 14 | www.onsemi.com
ASM705/706/707/708 and ASM813L
WDO will also become LOW and remain so, whenever
the supply voltage, V
CC
, falls below the device threshold
level. WDO goes HIGH as soon as V
CC
transitions above
the threshold. There is no minimum pulse width for WDO
as there is for the RESET outputs. If WDI is floated, WDO
essentially acts as a low-power output indicator.
Monitoring Voltages Other Than VCC
The ASM705-708 can monitor voltages other than V
CC
using the Power Fail circuitry. If a resistive divider is
connected from the voltage to be monitored to the Power
Fail input (PFI), the PFO will go LOW if the voltage at PFI
goes below 1.25V reference. Should hysteresis be
desired, connect a resistor (equal to approximately 10
times the sum of the two resistors in the divider) between
the PFI and PFO pins. A capacitor between PFI and GND
will reduce circuit sensitivity to input high-frequency
noise. If it is desired to assert a RESET for voltages other
than V
CC
then the PFO output is to be connected to the
MR.
Application Information
Ensuring That RESET is Valid Down to V
CC
= 0V
When V
CC
falls below 1.1V, the ASM705-708 RESET
output no longer pulls down; it becomes indeterminate.
To avoid the possibility that stray charges build up and
force RESET to the wrong state, a pull-down resistor
should be connected to the RESET pin, thus draining
such charges to ground and holding RESET low. The
resistor value is not critical. A 100kΩ resistor will pull
RESET to ground without loading it.
Bi-directional Reset Pin Interfacing
The ASM705/6/7/8 can interface with μP/μC bi-directional
reset pins by connecting a 4.7kΩ resistor in series with
the RESET output and the μP/μC bi-directional RESET
pin.
Rev. 2 | Page 5 of 14 | www.onsemi.com
