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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

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To make explicit the implicit parasitic capacitance on the pin.

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    So, it's not actually a capacitor, but just drawn in to the schematic to demonstrate that the pin has capacitance? – lemontwist May 22 '17 at 14:30
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    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 May 22 '17 at 14:32
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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:

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