ADM690–ADM695
P O WER FAIL RESET O UTP UT
Watchdog Tim er RESET
RESET is an active low output which provides a RESET signal
to the Microprocessor whenever VCC is at an invalid level. When
VCC falls below the reset threshold, the RESET output is forced
low. T he nominal reset voltage threshold is 4.65 V (ADM690/
ADM691/ADM694/ADM695) or 4.4 V (ADM692/ADM693).
T he watchdog timer circuit monitors the activity of the micro-
processor 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
selected timeout period, a RESET pulse is generated. T he
nominal watchdog timeout period is preset at 1.6 seconds on the
ADM690/ADM692/ADM694. The ADM691/ADM693/ADM695
may be configured for either a fixed “short” 100 ms or a “long”
1.6 second timeout period or for an adjustable timeout period.
If the “short” period is selected, some systems may be unable to
service the watchdog timer immediately after a reset, so the
ADM691/ADM693/ADM695 automatically selects the “long”
timeout period directly after a reset is issued. T he watchdog
timer is restarted at the end of reset, whether the reset was
caused by lack of activity on WDI or by VCC falling below the
reset threshold.
V
V2
V2
CC
V1
V1
t1
t
1
RESET
LOW LINE
t1 = RESET TIME.
T he normal (short) timeout period becomes effective following
the first transition of WDI after RESET has gone inactive. T he
watchdog timeout period restarts with each transition on the
WDI pin. T o ensure that the watchdog timer does not time out,
either a high-to-low or low-to-high transition on the WDI pin
must occur at or less than the minimum timeout period. If WDI
remains permanently either high or low, reset pulses will be
issued after each “long” timeout period (1.6 s). T he watchdog
monitor can be deactivated by floating the Watchdog Input
(WDI) or by connecting it to midsupply.
V1 = RESET VOLTAGE THRESHOLD LOW
V2 = RESET VOLTAGE THRESHOLD HIGH
HYSTERESIS = V2–V1
Figure 2. Power Fail Reset Tim ing
On power-up RESET will remain low for 50 ms (200 ms for
ADM694 and ADM695) after VCC rises above the appropriate
reset threshold. T his allows time for the power supply and mi-
croprocessor to stabilize. On power-down, the RESET output
remains low with VCC as low as 1 V. T his ensures that the
microprocessor is held in a stable shutdown condition.
WDI
T his RESET active time is adjustable on the ADM691/ADM693/
ADM695 by using an external oscillator or by connecting an
external capacitor to the OSC IN pin. Refer to T able I and
Figure 4.
WDO
T he guaranteed minimum and maximum thresholds of the
ADM690/ADM691/ADM694/ADM695 are 4.5 V and 4.73 V,
while the guaranteed thresholds of the ADM692/ADM693 are
4.25 V and 4.48 V. The ADM690/ADM691/ADM694/ADM695
is, therefore, compatible with 5 V supplies with a +10%, –5%
tolerance while the ADM692/ADM693 is compatible with 5 V
± 10% supplies. T he reset threshold comparator has approxi-
mately 50 mV of hysteresis. T he response time of the reset volt-
age comparator is less than 1 µs. If glitches are present on the
VCC line which could cause spurious reset pulses, then VCC
should be decoupled close to the device.
t2
t3
RESET
t1
t1
t1
t
t
t
1 = RESET TIME.
2 = NORMAL (SHORT) WATCHDOG TIMEOUT PERIOD.
3 = WATCHDOG TIMEOUT PERIOD IMMEDIATELY FOLLOWING A RESET.
In addition to RESET the ADM691/ADM693/ADM695 con-
tain an active high RESET output. T his is the complement of
RESET and is intended for processors requiring an active high
RESET signal.
Figure 3. Watchdog Tim eout Period and Reset Active
Tim e
–6–
REV. A