AN2860

PCB layout recommendations

Doc ID 15267 Rev 5

9/17

3

PCB layout recommendations

Figure 1 and Figure 2 defines the appropriate bill of materials but, for EMC performance,

special attention must be paid to the layout which defines ground plane, track width and

length, etc.

It is also important to pay special attention to the following tasks:

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The ground plane should be used as a shield below the device

●

The power supply planes must be designed around the device towards the other

components, as shown in Figure 3, Figure 4 and Figure 5. The purpose is to separate

the current flow supplying the rest of the application from the CPU power supply plane

●

Every supply pin should be decoupled by a ceramic capacitor. The capacitor should be

as possible.

STM8S and STM8AF devices require an additional external capacitor for a voltage regulator

●

The correct value of the capacitor is very critical (a min. 470 nF capacitance must be

kept in any condition)

●

ground plane.

Warning:

Special care must be taken on the device EMC robustness

when the application requires the use of an external (HSE)

oscillator (for instance, for permanent asynchronous

communication or time-dependent measuring) and the

internal clock source cannot be used.

A potential risk of EMC robustness degradation can be

minimized by applying the rules described below.

●

possible.

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whenever possible.

●

An additional shielding structure should surround the area of the crystal resonator and

of the associated components (see Figure 3, Figure 4 and Figure 5).