LM2984T National Semiconductor, LM2984T Datasheet - Page 13

• Manufacturer Part Number: LM2984T
• Description: IC,VOLT REGULATOR,FIXED,+5V,BIPOLAR,ZIP,11PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2984T

• Rohs Compliant: NO

Application Hints
This watchdog circuitry continuously monitors a pin on the
µP that generates a positive going pulse during normal op-
eration. The period of this pulse is typically on the order of
milliseconds and the pulse width is typically on the order of
10’s of microseconds. If this pulse ever disappears, the
watchdog circuitry will time out and a RESET low will be sent
to the µP. The time out period is determined by two external
components, R
The width of the RESET pulse is set by C
resistor according to the following:
A square wave signal can also be monitored for errors by fil-
tering the C
signal are detected. Figure 2 is a schematic diagram of a
typical circuit used to differentiate the input signal. Resistor
R
wave and create a short positive pulse suitable for the µP
tc
and capacitor C
mon
RESET
T
window
t
input such that only the positive edges of the
and C
tc
pw
pass only the rising edge of the square
= 0.82 R
mon
= 2000 C
, according to the formula:
t
(Continued)
C
mon
mon
(seconds)
(seconds)
mon
and an internal
13
monitor input. If the incoming signal continues in a high state
or in a low state for too long a period of time, a RESET low
will be generated.
The threshold voltage and input characteristics of this pin are
compatible with nearly all logic families.
There is a limit on the width of a pulse that can be reliably de-
tected by the watchdog circuit. This is due to the output re-
sistance of the transistor which discharges C
state is detected at the input. The minimum detectable pulse
width can be determined by the following formula:
FIGURE 2. Monitoring Square Wave µP Signals
PW
min
= 20 C
mon
(seconds)
mon
DS011252-14
when a high
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