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Does anyone know how many and what values of bypass capacitors I need for a PIC18F4455? The datasheet does not provide any information on this and I don't want to use more than what I strictly need, given that I don't have much real state available on my PCB. Also, if anyone has good guidelines for choosing number/values of bypass caps for MCUs in general, I'd also appreciate that.

Thank you, Chris.

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    \$\begingroup\$ Is the normal 100nF not working for you? \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 1 '15 at 14:27
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    \$\begingroup\$ picking caps for MCUs in general is a much more interesting question. Check out this answer for some good information electronics.stackexchange.com/questions/15135/… \$\endgroup\$ – Some Hardware Guy Jul 1 '15 at 14:30
  • \$\begingroup\$ @IgnacioVazquez-Abrams What do you mean by "normal 100nF"? Are you saying that what I need to use is only 1 cap of 100pF and that's all I need? If that's the case, what's the reasoning behind that choice? \$\endgroup\$ – Christopher Muñoz Cortés Jul 1 '15 at 14:45
  • \$\begingroup\$ @ChristopherMuñozCortés as my answer states, 0.1uF (a.k.a. 100nF) is the most common bypass capacitor size. Also, 100pF and 100nF are VERY different. \$\endgroup\$ – DerStrom8 Jul 1 '15 at 15:04
  • \$\begingroup\$ @derstrom8: Oops. Of course I know pico and nano are different. That was a typo. Thanks for your answer though. \$\endgroup\$ – Christopher Muñoz Cortés Jul 1 '15 at 15:09
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0.1uF is probably the most common, though I have seen 0.01uF used as well. Larger values (6.8uF, 10uF, etc) are used to filter out lower frequencies. It all depends on your exact circuit, where it will be used, what you are using for a power supply, and so on.

I generally just use a 0.1uF capacitor directly across the power pins of the microcontroller and don't have a problem. Just make sure you put the cap as close as you possibly can to the power pins when laying out your board.

Here is a tutorial on bypass capacitors:

http://www.electro-labs.com/bypass-capacitors-why-and-how-to-use-them/

There they discuss different types, why they're needed, how they're used, and how their values are calculated. Hopefully this helps =)

EDIT: Intersil also created a document to help determine which capacitors to use. I found this while reading through the previous link:

http://www.intersil.com/content/dam/Intersil/documents/an13/an1325.pdf

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I'd look at demo and evaluation boards for the processor or something similar to it. The first one I found had 0.1uF on all of regular VDD pins. There was a 6.8uF on the VDDCORE.

http://ww1.microchip.com/downloads/en/DeviceDoc/PIC18%20Starter%20Kit%20User%20Guide.pdf

If you are having problems with EMI though, you will have to tune those capacitors to your operating frequency and PCB layout.

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