3.5 External Oscillator Pins

Many microcontrollers have options for at least two oscillators: A high-frequency primary oscillator and a low-frequency secondary oscillator.

The oscillator circuit must 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 must 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 may 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 the following figure. 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 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.

Figure 3-3. Suggested Placement of the Oscillator Circuit

In planning the application’s routing and I/O assignments, 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, refer to these Microchip application notes, available at the corporate website (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”