Even the best performing oscillator circuits and high-quality crystals
will not perform well if the layout and materials used during assembly are not carefully
considered. Ultra low-power 32 KHz oscillators typically dissipate significantly below 1
μW, and the current flowing in the circuit is, therefore, extremely small. In addition, the
crystal frequency is highly dependent on the capacitive load.
To increase the robustness of the oscillator, we recommend these
guidelines during PCB layout:
- Signal lines from XTAL1/TOSC1 and
XTAL2/TOSC2 to the crystal should be as short as possible to reduce parasitic
capacitance and increase noise and crosstalk immunity. Any kind of sockets should be
avoided.
- Shield the crystal and signal lines by surrounding it with a ground plane and guard
ring.
- Avoid routing digital lines,
especially clock lines, close to the crystal lines. For multilayer PCB boards, avoid
routing signals below the crystal lines.
- Use high-quality PCB and soldering materials.
- Dust and humidity will increase
parasitic capacitance and reduce signal isolation, so, protective coating is
recommended.