Oscillator

AN2353

6/18

1.3

PCB layout for ST10F27x oscillator

The following

Figure 2 shows the proposed layout for the ST10F27x oscillator.

Figure 2.

Example of layout for external crystal

1.4

Oscillator and EMC

The ST10F27x oscillator has an integrated gain control to minimize EMC and power

consumption. However, this does not prevent users from observing / respecting the following

rules / recommendations:

●

Avoid other high frequency signals near the oscillator circuitry. These can influence the

oscillator.

●

Lay out/configure the ground supply on the basis of low impedance.

●

Shield the crystal with an additional ground plane underneath the crystal.

●

Do not lay out sensitive signals near the oscillator. Analyze cross-talk between different

layers.

●

●

Capacitors are placed at both ends of the crystal, directly connected to the ground

plane while keeping the overall loop as small as possible.

●

The crystal package, when metallic, is directly connected to the ground.

1.5

32 kHz oscillator

The same recommendations are valid for the 32 kHz oscillator. In any case, when the

32 kHz oscillator amplifier is not used, to avoid spurious consumption, XTAL3 must be tied

to ground while XTAL4 is left open. Moreover, bit OFF32 in the RTCCON register should be

set. The 32 kHz oscillator can only be driven by an external crystal and not by a different

clock source.

ST10F27x

Decoupling capacitor

C2

C1

Vias to GND

Vias to GND

Crystal

Ground plate

CD

Vias to VDD

XTAL1

XTAL2

Crystal package

soldered to GND