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When you have development boards for microprocessors with every GPIO pinned out on headers around the edge of the board, is the board more susceptible to picking up noise than a board without all those unconnected lines?

For example say you were using the ADC on the board but everything else is disconnected (like in the image): enter image description here

Would the signal being read be noisier than if all those pink unused traces didn't exist if the board was placed near noisy things like SMPSs?

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Probably not by a lot. Proper grounding of the PCB and filtering of the input signal, the analog reference and PCB layout are more of an issue. That's built on a cheap 2-layer board so the layout cannot be all that great.

If you flip that board over, you'll probably find large areas of copper pour that are at AC ground potential (either tied to GND or to a power supply rail).

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It’s okay to leave I/O pins unconnected if you ensure that they’re programmed to have a pull-up or pull-down, or are programmed as outputs.

What you don’t want is a floating input. That can lead to unstable operation and excess power draw.

That said, if proximity to a noise source is a problem, you can bypass the (pull-up/down) pins with small capacitors.

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    \$\begingroup\$ It depends on the MCU. In some MCUs such as the STM32, the digital input stage is also disabled by default unless it is configured. So on these MCUs it does not matter if you leave it as a floating input, or configure it as digital input with pull-up or pull-down, or configure it as digital output. \$\endgroup\$ – Justme Aug 8 at 20:03
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The higher the impedance seen from outside, the more susceptible the pin to noise.

If the unused pin is configured as input (analog or digital) or open-drain output, the pin will be more susceptible to noise. The best practice would be configuring them as push-pull digital outputs as the impedance seen from outside would be much lower.

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