ADM8698/ADM8699
CIRCUIT INFORMATION
Power Fail RESET
Watchdog Timer (ADM8699 Only)
On power-up, an internal monostable holds
RESET
low for
140 ms after V
CC
rises above the reset threshold. This allows the
power supply to stabilize on power-up and also prevents repeated
toggling of
RESET
even if the 5 V power drops out and recovers
with each power line cycle. In order to prevent mistriggering
due to transient voltage spikes, it is recommended that a 0.1
µF
capacitor be connected at the V
CC
pin.
The
RESET
output is guaranteed to remain low with V
CC,
as
low as 1 V. This holds the microprocessor in a stable shutdown
condition as the power supply comes up.
On the 16-lead SOIC package, an active high RESET output is
also provided. This is the complement of
RESET
and is in-
tended for microprocessors requiring an active high signal.
The WDI input is a three level input and will recognize a low-
to-high or high-to-low transition on its input. The watchdog
timer is reset by each WDI transition and then begins its timeout
period. If the WDI pin remains either high or low, reset pulses
will be issued every 1.6 seconds typically. If the watchdog timer
is not needed, the WDI input should be left floating.
The Watchdog Output (WDO) (SOIC package Only) provides
watchdog status information. It is driven low if WDI is not
toggled within the watchdog timeout period. It goes high at the
next WDI transition. It is also set high when V
CC
falls below the
reset threshold.
WDI
V
CC
V
2
V
1
V
2
V
1
WDO
RESET
t
1
t
1
RESET
t
2
t
2
V
1
= RESET VOLTAGE THRESHOLD
V
2
= RESET VOLTAGE THRESHOLD +
THRESHOLD HYSTERESIS
t
1
t
1
t
1
t
1
= RESET TIME
Figure 4. Watchdog Timeout Period vs. Temperature
t
1 = RESET TIME
t
2 = WATCHDOG TIME OUT PERIOD
Figure 5. Watchdog Timeout Period and Reset Active Time
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Pin Plastic DIP (N-8)
8-Pin SOIC (R-8)
0.1968 (5.00)
0.1890 (4.80)
16-Lead SOIC (R-16)
8
PIN 1
1
5
0.25 0.31
(6.35) (7.87)
4
8
5
16
9
0.2440 (6.20)
0.2284 (5.80)
1
4
0.1574 (4.00)
0.1497 (3.80)
0.299
(7.60)
0.419
(10.65)
0.430 (10.92)
MAX
0.035
(0.89)
PIN 1
0.102 (2.59)
0.094 (2.39)
1
8
0.18 (4.57)
0.018 (0.46)
0.1 (2.54)
BSC
0.125
(3.18)
MIN
SEATING
PLANE
0.033 (0.84)
0.18
(4.57)
MAX
0.011
(0.28)
0.3 (7.62)
0.0098 (0.25)
0.0040 (0.10)
0.0500 0.0192 (0.49)
SEATING (1.27) 0.0138 (0.35)
PLANE BSC
0.0196 (0.50)
x 45°
0.0099 (0.25)
0.413 (10.50)
0.012
(0.3)
0.104
(2.65)
0.05 (1.27)
REF
0.030
(0.75)
0.019 (0.49)
0.0098 (0.25)
0.0075 (0.19)
8°
0°
0.0500 (1.27)
0.0160 (0.41)
0.013
(0.32)
0.042
(1.07)
–4–
REV. 0
PRINTED IN U.S.A.
C2926–10–2/97
A precision voltage detector monitors V
CC
and generates a
RESET
output to hold the microprocessor’s Reset line low when
V
CC
falls below the reset threshold 4.65 V (see Figure 4). The
reset voltage threshold is set to accommodate a 5% variation on
V
CC
. The voltage detector has 40 mV hysteresis to ensure that
glitches on V
CC
do not activate the
RESET
output.
The watchdog timer input (WDI) monitors an I/O line from the
µP
system. The
µP
must toggle this input once every 1.6 sec-
onds to verify correct software execution. Failure to toggle the
line indicates that the
µP
system is not correctly executing its
program and may be tied up in an endless loop. If this happens,
a reset pulse is generated to initialize the processor.
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