user30761: 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?


user19409: 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:

MOS Transistor parasitic capacitances - OnMyPHD.com
MOSFET Parasitic Capacitance and Its Temperature Characteristic - ROHM


user30005: To make explicit the implicit parasitic capacitance on the pin.

30761: <p>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?</p>

<p><a href="https://i.stack.imgur.com/mIA54.png" rel="noreferrer"><img src="https://i.stack.imgur.com/mIA54.png" alt="i/o pin schematic"></a></p>
 19409: <p>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.</p>
<p>The input drives a MOSFET - and as we know, MOSFETs have some parasitic capacitance.</p>
<p>For more information on parasitic capacitance of MOSFETs see these pages:</p>
<ul>
<li><a href="http://www.onmyphd.com/?p=mosfet.parasitic" rel="nofollow noreferrer">MOS Transistor parasitic capacitances - OnMyPHD.com</a></li>
<li><a href="https://techweb.rohm.com/knowledge/si/s-si/03-s-si/4873" rel="nofollow noreferrer">MOSFET Parasitic Capacitance and Its Temperature Characteristic - ROHM</a></li>
</ul>
 30005: <p>To make explicit the implicit parasitic capacitance on the pin.</p>
