PIC18F87J90 Datasheet, PDF (34/450 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt Technology

PIC18F87J90 FAMILY
2.6 External Oscillator Pins
Many microcontrollers have options for at least two
oscillators: a high-frequency primary oscillator and a
low-frequency secondary oscillator (refer to
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”
2.7 Unused I/Os
Unused I/O pins should be configured as outputs and
driven to a logic low state. Alternatively, connect a 1 kΩ
to 10 kΩ resistor to VSS on unused pins and drive the
output to logic low.
FIGURE 2-4:
SUGGESTED PLACEMENT
OF THE OSCILLATOR
CIRCUIT
Single-Sided and In-Line Layouts:
Copper Pour Primary Oscillator
(tied to ground)
Crystal
DEVICE PINS
Primary
Oscillator
C1
`
C2
`
Timer1 Oscillator
Crystal
`
T1 Oscillator: C1
T1 Oscillator: C2
OSC1
OSC2
GND
T1OSO
T1OS I
Fine-Pitch (Dual-Sided) Layouts:
Top Layer Copper Pour
(tied to ground)
Bottom Layer
Copper Pour
(tied to ground)
OSCO
GND
OSCI
C2
Oscillator
Crystal
C1
DEVICE PINS
DS39933D-page 34
 2010 Microchip Technology Inc.