8XC196MC, MD, MH USER’S MANUAL

8XC196

Device

VPP

VCC

R1 1 MΩ Typical

C1 1μF Typical

A0279-01

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 oscillator circuitry is enabled and turns on a weak internal pull-down. This weak pull-down causes the external 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 resumes code execution.

At this time, the internal pull-up transistor turns on and quickly pulls the pin back up to about

3.5V. 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 follows an exponential charg- ing curve. If R1 = 1 MΩ and C1 = 1 μF, the recovery time will be one second.

14.4.3.4Selecting 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 many factors can influence the discharge time requirement, you should always fully characterize your design under worst-case conditions to verify proper operation.

14-8

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Intel 8XC196MD, 8XC196MH, 8XC196MC manual External RC Circuit, Selecting R1 and C1