4.2 Connection for Power Supply

The basics and details of power supply design lie beyond the scope of these guidelines. See the application notes mentioned at the beginning of this section for more detailed information about this subject.

A decoupling capacitor must be placed close to the microcontroller for each supply pin pair (VDD 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 might form that will result in increased noise and increased radiated emission.

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

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

If the board is experiencing high-frequency noise (upward of tens of MHz), add a second ceramic type capacitor parallel to the decoupling capacitor described above. Place this second capacitor next to the primary decoupling capacitor.

On the board layout 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.

As mentioned at the beginning of this section, all values used in examples are typical values. The actual design may require other values.