ADM705 Analog Devices, ADM705 Datasheet - Page 10

• Manufacturer Part Number: ADM705
• Description: Low Cost Microprocessor Supervisory Circuit
• Manufacturer: Analog Devices

Specifications of ADM705

• Batt-backup-flg: No
• Manual Reset Capability: Yes
• Package: DIP,SOIC
• Power Fail Indicator: Yes
• Reset Threshold Summary: 4.65V
• Reset Output-stage: Active-Low/Push-Pull
• Min Reset Timeout (ms): 160
• Typ Watchdog Timeout (ms): 1600
• Watchdog Timer: Yes

ADM705/ADM706/ADM707/ADM708
APPLICATIONS INFORMATION
A typical application circuit is shown in Figure 18. The
unregulated dc input supply is monitored using PFI via the
resistive divider network. Resistor R1 and Resistor R2 should
be selected so that when the supply voltage drops below the
desired level (such as 8 V), the voltage on PFI drops below
the 1.25 V threshold, thereby generating an interrupt to the
microprocessor. Monitoring the preregulator input provides
additional time to execute an orderly shutdown procedure
before power is lost.
Microprocessor activity is monitored using WDI. This is driven
using an output line from the processor. The software routines
should toggle this line at least once every 1.60 seconds. If a
problem occurs and this line is not toggled, 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, WDO should be connected to MR , as shown in
Figure 19.
INPUT POWER
7V TO 15V
R1
R2
MR
ADM705/
ADM706
Figure 18. Typical Application Circuit
ADP3367
GND
Figure 19. RESET From WDO
RESET
WDO
WDI
PFI
ADM705/ADM706/
ADM707/ADM708
5V
1.25V
V
CC
+
–
MICROPROCESSOR
I/O LINE
RESET
PFO
MICROPROCESSOR
RESET
Rev. G | Page 10 of 12
MONITORING ADDITIONAL SUPPLY LEVELS
It is possible to use the power-fail comparator to monitor a
second supply as shown in Figure 20. 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. PFO can be
connected to MR 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.
MICROPROCESSOR WITH BIDIRECTIONAL RESET
To prevent contention for microprocessors with a bidirectional
reset line, a current limiting resistor should be inserted between
the ADM70x 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 21.
R2
R1
Figure 20. Monitoring 5 V and an Additional Supply, V
V
X
ADM70x
Figure 21. Bidirectional Input/Output RESET
GND
V
5V
CC
RESET
PFI
MR
ADM705/
ADM706
GND
V
5V
CC
RESET
PFO
MICROPROCESSOR
RESET
MICROPROCESSOR
RESET
GND
BUFFERED
RESET
X