Choice of bypass capacitors for PIC18F4455

Question

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.

picking caps for MCUs in general is a much more interesting question. Check out this answer for some good information http://electronics.stackexchange.com/questions/15135/decoupling-caps-pcb-layout/15143#15143 — Some Hardware Guy, Jul 01 '15 at 14:30
@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? — Christopher Muñoz Cortés, Jul 01 '15 at 14:45
@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. — DerStrom8, Jul 01 '15 at 15:04
@derstrom8: Oops. Of course I know pico and nano are different. That was a typo. Thanks for your answer though. — Christopher Muñoz Cortés, Jul 01 '15 at 15:09
I had a feeling it was a typo but had to check. Not everyone knows the difference =P ;) — DerStrom8, Jul 01 '15 at 15:10
I doubted that there is no recommendation in the datasheet, but that's indeed correct. Just about your chip: If you want to use USB and the internal 3.3V USB voltage regulator, you need 220nF between Vusb (pin 18 or 37) and GND, but this is written in the datasheet. — sweber, Jul 01 '15 at 20:12

score 1 · Answer 1 · answered Jul 01 '15 at 14:50

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.

DerStrom8 · Accepted Answer · 2015-07-01T15:03:00.427

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

Choice of bypass capacitors for PIC18F4455

2 Answers2