
ADM705–ADM708
REV. B
–7–
If, in the event of inactivity on the WDI line, a system reset is
required, then the
WDO
output should be connected to the
MR
input as shown in Figure 16.
ADM705/
ADM706
RESET
GND
I/O LINE
P
MR
WDI
WDO
RESET
Figure 16.
RESET
from
WDO
Monitoring Additional Supply Levels
It is possible to use the power-fail comparator to monitor a
second supply as shown in Figure 17. The two sensing resistors,
R1 and R2, are selected so that the voltage on PFI drops below
1.25 V at the minimum acceptable input supply. The
PFO
output may be connected to the
MR
input so that a RESET is
generated when the supply drops out of tolerance. In this case, if
either supply drops out of tolerance, a
RESET
will be generated.
ADM705/
ADM706
PFI
RESET
GND
P
MR
PFO
RESET
V
CC
R1
R2
V
X
5V
Figure 17. Monitoring 5 V and an Additional Supply, V
X
Ps With Bidirectional
RESET
In order to prevent contention for microprocessors with a bidi-
rectional reset line, a current limiting resistor should be inserted
between the ADM70x
RESET
output pin and the
μ
P reset pin.
This will limit the current to a safe level if there are conflicting
output reset levels. A suitable resistor value is 4.7 k
. If the
reset output is required for other uses, it should be buffered as
shown in Figure 18.
ADM70x
RESET
GND
P
RESET
GND
BUFFERED
RESET
5V
V
CC
Figure 18. Bidirectional I-O
RESET
APPLICATIONS
A Typical Operating Circuit is shown in Figure 15. The unregu-
lated dc input supply is monitored using the PFI input via the
resistive divider network. Resistors R1 and R2 should be selected
so that when the supply voltage drops below the desired level
(e.g., 8 V), the voltage on PFI drops below the 1.25 V threshold
thereby generating an interrupt to the
μ
P. Monitoring the pre-
regulator input gives additional time to execute an orderly
shutdown procedure before power is lost.
ADM705/
ADM706
PFI
RESET
I/O LINE
GND
P
MR
WDI
PFO
RESET
V
CC
5V
WDO
V
CC
UNREGULATED
DC
INTERRUPT
NMI
MANUAL
RESET
GND
IN
OUT
ADM666
R1
R2
Figure 15. Typical Application Circuit
Microprocessor activity is monitored using the WDI input. This
is driven using an output line from the processor. The software
routines should toggle this line at least once every 1.6 seconds.
If a problem occurs and this line is not toggled,
WDO
goes low
and a nonmaskable interrupt is generated. This interrupt rou-
tine may be used to clear the problem.
4V
V
CC
5V
2 s/DIV
0V
5V
RESET
T
A
= 25 C
Figure 14. ADM705/ADM707
RESET
Response Time