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8XC196MC, MD, MH USER’S MANUAL
14-8
Figure 14-3. External RC Circuit
During normal operation (before entering powerdown mode), an internal pull-up holds the
V
PP
pin at V
CC
. When an external interrupt signal is asserted, the internal oscillator circuitry is
enabled and turns on a weak internal pull-down. This weak pull-down causes the external capac-
itor (C
1
) to begin discharging at a typical rate of 200 µA. When the V
PP
pin voltage drops below
the threshold voltage (about 2.5 V), the internal phase clocks are enabled and the device resumes
code execution.
At this time, the internal pull-up transistor turns on and quickly pulls the pin back up to about
3.5 V. The pull-up becomes ineffective and the external resistor (R
1
) takes over and pulls the volt-
age up to V
CC
(see recovery time in Figure 14-4). The time constant follows an exponential charg-
ing curve. If R
1
= 1 MΩ and C
1
= 1 µF, the recovery time will be one second.
14.4.3.4 Selecting R
1
and C
1
The values of R
1
and C
1
are not critical. Select components that produce a sufficient discharge
time to permit the internal oscillator circuitry to stabilize. Because many factors can influence the
discharge time requirement, you should always fully characterize your design under worst-case
conditions to verify proper operation.
8XC196
Device
R
1
1 MΩ Typical
C
1
1µF Typical
V
PP
A0279-01
V
CC