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PICDEM™ PIC18 Explorer Demonstration Board User’s Guide

DS51721B-page 14

2.3.2

Varying the Device Voltage (5V/3.3V)

By default, the PICDEM PIC18 Explorer Demonstration Board’s VDD supply is 5V. The

VDD can be varied, for PIM-mounted microcontrollers, from 5 to 3.3V to accommodate

devices running at 5 or 3.3V. This VDD is named VAR.

The PICDEM PIC18 Explorer Demonstration Board enables the voltage change with

PIM connection headers and a variable voltage regulator. PIMs mounted with 3.3V

devices implement the voltage change through two resistors with values that produce

the desired voltage. (See “Calculating Other VDD Values” on page 15.)

The voltage varying hardware includes:

• An adjustable voltage regulator, the LM317 – Located on the board, left of the PIM

connectors and marked as U2 (recognizable by the TO-220 package commonly

used for transistors)

• Header J2 – Located above the PIM connectors

• Resistors R25 and R26 – Located below jumper J13

• Resistors R101 and R102 – Located on the PIM board

In setting the board’s voltage:

• For the default, 5V voltage –

- For board-mounted PIC18F8722 device:

• Board resistor R25 = 1 k

• Board resistor R26 = 330

- For a PIM-mounted, 5V microcontroller:

• Board resistors R25 and R26 – Same values of 1 k

Ω and 330Ω,

respectively

• PIM-mounted resistors R101 and R102 – Unpopulated

• For 3.3V VDD (achieved only with a PIM with a mounted 3.3V device, such as the

PIC18F87J11) –

- Header J2 goes into the PIM board where resistors R101 and R102 are

inserted in parallel to the board resistors R25 and R26

- PIM board resistor R101 can be unpopulated

- PIM board resistor R102 can be 1.18 k

Ω.

Note:

For precise adjustment of VDD, 1% resistors are recommended.

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