Most of the microcontroller based designs are copied from reference design. While interfacing to external devices is one part, choosing the capacitors for a microcontroller is another part. Let us look at various scenarios and how to proceed with the capacitor selection:
- While we are using the capacitors as used in reference design, it is not good to use as is.
- The noise conditions (ultimately frequencies) are different in different designs.
- The capacitors used are meant to filter the noise and also meant to provide option to provide that small surge during the digital transitions
- First, we need to understand the kind of frequencies that are generated on the board and the danger they pose to the microcontrollers. Ultimately, the capacitor selection must be done.
- For a microcontroller, there are several power pins. They are not in sequence. As they are randomly distributed and noise could be distributed across the board, it is important that a capacitor is placed at each pin of microcontroller.
- The good starting value for the capacitor is 0.1uF which filters noise around 1.5MHz. While this doesn't guarantee that noise is filtered, a lower and higher capacitors than these covering large frequency band are used. This is all design dependent and designer has to decide based on the circuit demands
- As we said, normally 0.1uF is used at all pins and at pins where it is expected to have higher noise/operating conditions, choose an extra capacitor at those pins.
- The general design approach is to have capacitors following reference designs and design demands and have some do not stuff options around critical areas and place them later on need.
]]>
0 Comments