2.8 External Oscillator Layout Guidance
Use best practices during PCB layout to ensure robust start-up and operation. The
oscillator circuit should be placed on the same side of the board as the device. Also,
place the oscillator circuit close to the respective oscillator pins, not exceeding
one-half inch (12 mm) distance between them. 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 circuit 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. If using a two-sided board, avoid any traces on the
other side of the board where the crystal is placed. Suggested layouts are shown in
Figure 2-5. With fine-pitch packages, it is not always
possible to completely 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. For additional information and design guidance on oscillator
circuits, please refer to these Microchip Application Notes, available at the Microchip
website (www.microchip.com):
- AN943, “Practical PICmicro® Oscillator Analysis and Design"
- AN949, "Making Your Oscillator Work"