ADM705–ADM708
Manual Reset (ADM707/ADM708)
WATCHDOG
INPUT (WDI)
WATCHDOG
TRANSITION
DETECTOR
V
CC
250 A
MR
V
CC
4.65V*
POWER-FAIL
INPUT (PFI)
1.25V
RESET
GENERATOR
RESET
WATCHDOG
TIMER
WATCHDOG
OUTPUT (WDO)
RESET &
WATCHDOG
TIMEBASE
The manual reset input (MR) allows other reset sources, such as
a manual reset switch, to generate a processor reset. The input
is effectively debounced by the timeout period (200 ms typical).
The
MR
input is TTL/CMOS compatible, so it may also be
driven by any logic reset output.
V
CC
VRT
VRT
t
RS
t
RS
ADM705/
ADM706
RESET
POWER-FAIL
OUTPUT (PFO)
MR
MR
EXTERNALLY
DRIVEN LOW
*VOLTAGE
REFERENCE = 4.65V (ADM705), 4.40V (ADM706)
Figure 1. ADM705/ADM706 Functional Block Diagram
WDO
V
CC
250 A
MR
V
CC
4.65V*
POWER-FAIL
INPUT (PFI)
1.25V
RESET
GENERATOR
RESET
RESET
Figure 3.
RESET
,
MR
, and
WDO
Timing
Watchdog Timer (ADM705/ADM706)
ADM707/
ADM708
POWER-FAIL
OUTPUT (PFO)
*VOLTAGE
REFERENCE = 4.65V (ADM707), 4.40V (ADM708)
Figure 2. ADM707/ADM708 Functional Block Diagram
CIRCUIT INFORMATION
Power-Fail
RESET
Output
The watchdog timer circuit may be used to monitor the activity
of the microprocessor in order to check that it is not stalled in an
indefinite loop. An output line on the processor is used to toggle
the Watchdog Input (WDI) line. If this line is not toggled
within the timeout period (1.6 sec), the watchdog output
(WDO) goes low. The
WDO
output may be connected to a
nonmaskable interrupt (NMI) on the processor; therefore, if the
watchdog timer times out, an interrupt is generated. The inter-
rupt service routine should then be used to rectify the problem.
If a
RESET
signal is required when a timeout occurs, the
WDO
output should be connected to the manual reset input (MR).
The watchdog timer is cleared by either a high-to-low or by a
low-to-high transition on WDI. It is also cleared by
RESET
going low; therefore, the watchdog timeout period begins after
RESET
goes high.
When V
CC
falls below the reset threshold,
WDO
is forced low
whether or not the watchdog timer has timed out. Normally,
this would generate an interrupt, but it is overridden by
RESET
going low.
The watchdog monitor can be deactivated by floating the
Watchdog Input (WDI). The
WDO
output can now be used as
a low-line output since it will only go low when V
CC
falls below
the reset threshold.
t
WP
WDI
RESET
is an active low output that provides a RESET signal to
the Microprocessor whenever the V
CC
input is below the reset
threshold. An internal timer holds RESET low for 200 ms after
the voltage on V
CC
rises above the threshold. This is intended as
a power-on RESET signal for the microprocessor. It allows time
for both the power supply and the microprocessor to stabilize
after power-up. The
RESET
output is guaranteed to remain
valid (low) with V
CC
as low as 1 V. This ensures that the micro-
processor is held in a stable shutdown condition as the power
supply voltage ramps up.
In addition to
RESET,
an active high RESET output is also
available on the ADM707/ADM708. This is the complement of
RESET
and is useful for processors requiring an active high
RESET signal.
t
WD
t
WD
t
WD
WDO
RESET
RESET EXTERNALLY
TRIGGERED BY
MR
t
RS
Figure 4. Watchdog Timing
–4–
REV. B
AD [ ANALOG DEVICES ]
