PICkit
TM
2 User’s Guide
DS51553B-page 20 © 2006 Microchip Technology Inc.
3.2 ISOLATE VPP/MCLR
/PORT PIN
When VPP voltage is applied, the application circuit needs to take into consideration
that the typical VPP voltage is +12V. This may be an issue in the following situations:
3.2.1 If the VPP pin is used as a MCLR
pin.
The application circuit typically is connected to a pull-up resistor/capacitor circuit, as
recommended in the device data sheet. Care must be taken so that the VPP voltage
slew rate is not slowed down and exceeds the rise time in the programming
specification (typically 1 µs).
If a supervisory circuit or a push button is interfaced to the MCLR
pin, it is
recommended that they be isolated from the VPP voltage by using a Schottky-type
diode or limiting resistor as shown in Figure 3-1. For more information about using
supervisory circuits with ICSP, see Application Note, AN820 “System Supervisors in
ICSP™ Architectures” (DS00820).
3.2.2 If the VPP pin is used as an I/O port pin.
The application circuit that connects to the I/O pin may not be able to handle the +12V
voltage. It is recommended to use a Schottky-type diode or limiting resistor as shown
in Figure 3-1 to isolate the circuitry.
3.3 ISOLATE ICSPCLK OR PGC AND ICSPDAT OR PGD PINS
The ICSPCLK or PGC and ICSPDAT or PGD pins need to be isolated from the
application circuit to prevent the programming signals from being affected by the
application circuitry. ICSPCLK or PGC is a unidirection synchronous serial
programming clock line from the programmer to the target. ICSPDAT or PGD is a
bidirectional synchronous serial programming data line.
If the design permits, dedicate these pins for ICSP. However, if the application circuit
requires that these pins be used in the application circuit, design the circuitry in a
manner that does not alter the signal level and slew rates. Isolation circuitry will vary
according to the application. Figure 3-1 shows one possibility by using series resistors
to isolate the ICSP signals from the application circuit.
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