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PIC18F2331/2431/4331/4431

DS39616D-page 28

2010 Microchip Technology Inc.

2.5

External Oscillator Pins

Many microcontrollers have options for at least two

oscillators: a high-frequency primary oscillator and a

low-frequency

secondary

oscillator

(refer

Section 3.0 “Oscillator Configurations”

for details).

The oscillator circuit should be placed on the same

side of the board as the device. Place the oscillator

circuit close to the respective oscillator pins with no

more than 0.5 inch (12 mm) between the circuit

components and the pins. The load capacitors should

be placed next to the oscillator itself, on the same side

of the board.

Use a grounded copper pour around the oscillator cir-

cuit to isolate it from surrounding circuits. The

grounded copper pour should be routed directly to the

MCU ground. Do not run any signal traces or power

traces inside the ground pour. Also, if using a two-sided

board, avoid any traces on the other side of the board

where the crystal is placed.

Layout suggestions are shown in Figure 2-4. In-line

packages may be handled with a single-sided layout

that completely encompasses the oscillator pins. With

fine-pitch packages, it is not always possible to com-

pletely surround the pins and components. A suitable

solution is to tie the broken guard sections to a mirrored

ground layer. In all cases, the guard trace(s) must be

returned to ground.

In planning the application’s routing and I/O assign-

ments, ensure that adjacent port pins and other signals

in close proximity to the oscillator are benign (i.e., free

of high frequencies, short rise and fall times, and other

similar noise).

For additional information and design guidance on

oscillator circuits, please refer to these Microchip

Application Notes, available at the corporate web site

(www.microchip.com):

AN826, “Crystal Oscillator Basics and Crystal

Selection for rfPIC and PICmicro Devices”

AN849, “Basic PICmicro Oscillator Design”

AN943, “Practical PICmicro Oscillator Analysis

and Design”

AN949, “Making Your Oscillator Work”

2.6

Unused I/Os

Unused I/O pins should be configured as outputs and

driven to a logic low state. Alternatively, connect a 1 k

to 10 k resistor to VSS on unused pins and drive the

output to logic low.

FIGURE 2-3:

SUGGESTED PLACEMENT

OF THE OSCILLATOR

CIRCUIT

GND

OSC1

OSC2

T1OSO

T1OS I

Copper Pour

Primary Oscillator

Crystal

Timer1 Oscillator

Crystal

DEVICE PINS

Primary

Oscillator

T1 Oscillator: C1

T1 Oscillator: C2

(tied to ground)

Single-Sided and In-Line Layouts:

Fine-Pitch (Dual-Sided) Layouts:

GND

OSCO

OSCI

Bottom Layer

Copper Pour

Oscillator

Crystal

Top Layer Copper Pour

DEVICE PINS

(tied to ground)

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