78M6612 Hardware Design Guidelines
AN_6612_007
18
Rev 2
6.2 PCB Layout Recommendations
6.2.1 Power Supply Noise and Electromagnetic Noise
Supply noise and electromagnetic noise are common causes of crystal oscillator failures. The oscillator
gain, and the slow rise and fall times (the signal is near sinusoidal), are typical characteristics of a
low-power and low-frequency oscillator. Because of these characteristics, the 32-kHz oscillator is
sensitive to power supply noise and to electromagnetic coupling.
Do not locate power magnetic components near the crystal oscillator components. Select a PCB layout
topology that places the crystal components on the opposite PCB side from the power magnetic
components and resulting magnetic fields.
6.2.2 Component Placement
To minimize noise sensitivity to spurious coupling or parasitic antenna phenomena on the PCB, the
connections of the crystal to oscillator input and output and to other components must be as short as
possible. The best practice is to place the crystal and phasing capacitors as close as possible to the
78M6612. This helps to minimize the length of the connections.
The currents flowing through the two load capacitors (C1 and C2) are in opposition. The best practice is
to connect the two capacitors before connecting to the ground reference. At that time, the current back to
the ground is significantly reduced. The connections must be as short as possible and of identical lengths.
Avoid long connections from these capacitors that make a large loop on the PCB, which behaves like an
antenna and can collect surrounding high-frequency radiation.
6.2.3 Layout
The reference ground of the oscillator must be as quiet as possible; otherwise, high-frequency noise is
transmitted directly to the oscillator input and output, resulting in degradation of the oscillator
performance.
To prevent cross-coupling to fast signals with high-level harmonic content, do not route signal traces
through the crystal area. Both oscillator pin connections are critical.
Vias in the oscillator circuit should only be used for connections to the ground plane. Do not share ground
connections; instead, make a separate connection to ground for each component that requires grounding.
If possible, place multiple vias in parallel for each connection to the ground plane.
The use of high-quality components in the oscillator circuit is equally important to achieve correct and
reliable operation. Capacitors should be high-quality capacitors with very low ESR, designed for use in
high-frequency applications (i.e., NP0 and COG).
Figure 20, Figure 21, and Figure 22 show layout examples for the crystal oscillator. Figure 20 and Figure
21 show the placement and layout of the crystal oscillator components on the opposite side of the PCB
from the 78M6612. The 32.768 kHz crystal and its two 27 pF capacitors are placed on the GND layer.
This allows the crystal and the two capacitors to be surrounded with a ground shield. Place the XIN and
XOUT vias as close as possible to the 78M6612 pins. Shield the XIN and XOUT signal vias with a
Ground plane flood sectioned out of the V3P3 layer.
