What is the purpose of the capacitor on the I/O pins on the ATmega328 (I imagine it's on other AVR microcontrollers as well)? Is it to reduce noise from other parts of the microcontroller?

i/o pin schematic

2 Answers 2


To make explicit the implicit parasitic capacitance on the pin.

  • 6
    So, it's not actually a capacitor, but just drawn in to the schematic to demonstrate that the pin has capacitance?
    – lemontwist
    Commented May 22, 2017 at 14:30
  • 6
    Correct. The input is actually the gate of a MOSFET. That gate acts like a capacitor. So they show it as one in the simplified block diagram.
    – Majenko
    Commented May 22, 2017 at 14:32

The pin does not have a capacitor added to the integrated circuit (IC). Rather this capacitor is drawn to show that the input has capacitance that you need to be aware of.

The input drives a MOSFET - and as we know, MOSFETs have some parasitic capacitance.

For more information on parasitic capacitance of MOSFETs see these pages:

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.