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
VPP pin at VCC. When an external interrupt signal is asserted, the internal oscillato r circuitry is
enabled and turns on a weak internal pull-down. This weak pull-down causes the e xternal capac-
itor (C1) to begin discharging at a typical rate of 200 µA. When the VPP pin voltage drops below
the threshold voltage (about 2.5 V), the internal phase clocks are enabled and the device resume s
code execution.
At this time, the internal pull-up transistor turns on and quic kly pulls the pin bac k up to about
3.5 V. The pull-up becomes ineffective and the external resistor (R1) takes over and pulls the volt-
age up to VCC (see recovery time in Figure 14-4). The time constant foll ows an exponential charg-
ing curve. If R1 = 1 Mand C1 = 1 µF, the recovery time will be one second.
14.4.3.4 Selecting R1 and C1
The values of R1 and C1 are not critical. Select components that produce a sufficient discharge
time to permit the internal oscillator circuitry to stabilize. Because ma ny 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 M Typical
C
1
1µF Typical
VPP
A0279-01
VCC