ST STM8L - Recommendations; Printed Circuit Board; Component Position; Ground and Power Supply (VSS, VDD )

DocID16139 Rev 5 17/42
AN3029 Recommendations
41
6 Recommendations
6.1 Printed circuit board
For technical reasons, it is best to use a multi-layer PCB with a separate layer dedicated to
the V
SS
and another layer to the V
DD
supply. This results in a good decoupling, as well as a
good shielding effect. For many applications, economical requirements prohibit the use of
this type of board. In this case, the most important feature is to ensure a good structure for
the V
SS
and power supply.
6.2 Component position
A preliminary layout of the PCB must separate the different circuits according to their
electromagnetic interference (EMI) contribution. This reduces cross-coupling on the PCB,
for instance, noisy, high-current circuits, low voltage circuits, and digital components.
6.3 Ground and power supply (V
SS
, V
DD
)
The V
SS
should be distributed individually to every block (noisy, low level sensitive, and
digital) with a single point for gathering all ground returns. Loops must be avoided or have a
minimum surface. The power supply should be implemented close to the ground line to
minimize the surface of the supply loop. This is due to the fact that the supply loop acts as
an antenna, and is therefore the main emitter and receiver of EMI. All component-free
surfaces of the PCB must be filled with additional grounding to create a kind of shield
(especially when using single-layer PCBs).
6.4 Decoupling
The standard decoupler for the external power is a 1 µF pool capacitor. Supplementary 100
nF capacitors must be placed as close as possible to the V
SS
/V
DD
pins of the
microcontroller to reduce the area of the current loop.
As a general rule, decoupling all sensitive or noisy signals improves electromagnetic
compatibility (EMC) performances.
There are two types of decouplers:
• Capacitors close to components. Inductive characteristics, which apply to all capacitors
beyond a certain frequency, must be taken into account. If possible, parallel capacitors
with decreasing values (0.1, 0.01,... µF) should be used.
• Inductors. Although often ignored, ferrite beads, for example, are excellent inductors
due to their good dissipation of EMI energy and there is no loss of DC voltage (which is
not the case when simple resistors are used).