adm708san Analog Devices, Inc., adm708san Datasheet - Page 11

• Manufacturer Part Number: adm708san
• Description: 3 V, Voltage Monitoring Microprocessor Supervisory Circuit
• Manufacturer: Analog Devices, Inc.

APPLICATIONS INFORMATION
A typical operating circuit is shown in Figure 20. The
unregulated dc input supply is monitored using the PFI input
via the resistive divider network. Resistor R1 and Resistor R2
should be selected so that when the supply voltage drops below
the desired level (for example, 5 V), the voltage on PFI drops
below the 1.25 V threshold, thereby generating an interrupt to
the microprocessor. Monitoring the preregulator input gives
additional time to execute an orderly shutdown procedure
before power is lost.
UNREGULATED
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, then WDO goes
low and a nonmaskable interrupt is generated. This interrupt
routine can be used to clear the problem.
If, in the event of inactivity on the WDI line, a system reset is
required, the WDO output should be connected to the input as
shown in Figure 21.
DC
MR
PFI
ADM706P/ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/ADM708T
ADM706R/
ADM706S/
ADM706T
MANUAL
GND
RESET
IN
Figure 20. Typical Application Circuit
ADM666A
GND
Figure 21. RESET from WDO
RESET
OUT
WDO
WDI
PFI
MR
ADM706R/
ADM706S/
ADM706T
GND
V
CC
3.3V
RESET
WDO
WDI
PFO
RESET
I/O LINE
MICROPROCESSOR
RESET
I/O LINE
NMI
INTERRUPT
MICROPROCESSOR
GND
V
CC
Rev. B | Page 11 of 16
MONITORING ADDITIONAL SUPPLY LEVELS
It is possible to use the power-fail comparator to monitor a
second supply as shown in Figure 22. The two sensing resistors,
R1 and R2, are selected such that the voltage on PFI drops
below 1.25 V at the minimum acceptable input supply. The PFO
output can 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 is generated.
MICROPROCESSORS WITH BIDIRECTIONAL RESET
To prevent contention for microprocessors with a bidirectional
reset line, a current limiting resistor should be inserted between
the ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/
ADM708T RESET output pin and the microprocessor reset pin.
This limits the current to a safe level if there are conflicting
output reset levels. A suitable resistor value is 4.7 kΩ. If the reset
output is required for other uses, it should be buffered as shown
in Figure 23.
R2
R1
V
ADM706R/ADM706S/
X
ADM706T/ADM708R/
ADM708S/ADM708T
Figure 22. Monitoring 3 V/3.3 V and an Additional Supply, V
GND
+3V/+3.3V
+3V/+3.3V
PFI
MR
Figure 23. Bidirectional Input/Output RESET
V
CC
ADM706R/
ADM706S/
ADM706T
GND
V
CC
RESET
RESET
WDI
PFO
RESET
MICROPROCESSOR
RESET
MICROPROCESSOR
BUFFERED
RESET
GND
X