40.3.1 Power Supply Connections

The basics and details of power supply design are beyond the scope of these guidelines.

A decoupling capacitor C₁ must be placed close to the microcontroller for each supply pin pair (V_DD or other power supply pin and its corresponding GND pin). If the decoupling capacitor is placed too far from the microcontroller, a high-current loop can form, resulting in increased noise and radiated emissions.

Each supply pin pair (power input pin and ground pin) must have a separate decoupling capacitor.

It is recommended to place the decoupling capacitor on the same side of the PCB as the microcontroller. If space does not allow this, the decoupling capacitor may be placed on the other side through a via, but be sure to keep the distance to the supply pin as short as possible.

If the board is experiencing high-frequency noise (upwards of tens of MHz), add a second ceramic-type capacitor, C2, in parallel with the decoupling capacitor, C₁. Place this second capacitor right next to the primary decoupling capacitor.

In the board layout, starting from the power supply circuit, run the power and return traces to the decoupling capacitors first, and then to the device pins, ensuring that the decoupling capacitors are first in the power chain. Equally important is to keep the trace length between the capacitor and the power pins to a minimum, thereby reducing PCB trace inductance.

All values used in the examples are typical values. The actual design may require different values.