Don't understand pin configuration

could someone of the experts explain the following screenshots pin connections to me.
I currently try do create some kind of a prototype of a little mainboard and got stuck at some point 'cause i am not sure if the schematic is wright. So you see the pins which are all connected to vdd and to capacitors as well but there are some caps missing to connect to every pin. Does that mean the missing ones should just be connected to vdd or did someone forget to insert them and which cap belongs to which pin? Which pins don’t have caps?

Thanks for your help

This part of the schematic shows all the pins on the left N9…R11 are connected to Vdd. As for being correct, you need to understand the function of each pin and how it is being used in the design.

You should always not assume that we are clairvoyants.
“to create a prototype” - so you have a schematic and try to desing the PCB.
“missing to connect to every pin” - ok - I see 8 capacitors and 9 pins - one C missing.
" someone forgat to insert them" - insert at schematic or insert at board?
Do you have only schematic or schematic and board with not all parts assembled?

I do have this schematic from the manufacturer. This pic is just from the SoC schematic there is no board and nothing i just have to assembly. I asked if someone has forgotten the 3rd cap in the schematic and last thing i don’t have the board.

I will speculate that this is some kind of a microcontroller. Which one? Can you post a copy of the Data Sheet? (No, I am NOT inclined to wade through a manufacturer’s web pages, loaded with advertising and pop-ups, to find the Date Sheet for a part in YOUR design project.)

Did you notice that those capacitors have different values? There are three, 100 nF (0.1 uF); one, 10 nF (0.01 uF); two, 10 uF; and two, 22 uF. Now, 0.1 uF is probably the “standard” value for power supply decoupling (and is currently experiencing a supply shortage). Manufacturers typically advise us to locate these “as close as possible” to supply pins.

0.01 uF (10 nF) is much less common. That value might be used on a supply pin dedicated to some special function - perhaps the A/D converter’s supply pin, or an analog signal conditioning block, or a high-speed line transceiver, etc. Something that is either especially sensitive to power supply noise, or something that is inclined to corrupt a supply rail with noise.

Manufacturers will sometimes call for bulk filtering capacitors, in the range of 10 uF to 100 uF or so, on a one-per-board basis. Usually they are specified as “good-quality tantalum or aluminum electrolytic” components, rather than ceramic capacitors. The 10 uF and 22 uF parts seem to fit this bill, though I don’t understand why there are two of each value.

I’ll also mention that the note on your schematic snippet, “Note: Place close to CPU power pin.” is a little ambiguous. Does it only apply to the three capacitors immediately adjacent to it, or to all eight? The white space between C808 and C810 on the drawing suggests that the Note applies to only C806, C807, and C808.

I suggest you spend some time carefully reading the Data Sheet. You may find the information you are looking for in a section called “Power Supply”, or “Layout Considerations”. It may be buried in a footnote with tiny print. Or, if the manufacturer has published “Application Notes” or “Reference Designs” for this part, they may contain more complete discussions of the power supply decoupling requirements.



Now I understand.
As you wrote “some caps missing” I understood not one but more. So may be you see some empty places at board. It happenes that board is designed full and produced with lower costs.
About missing C809.
If it would be my schematic (I am not designing mainboards) it would be probably the following story:

  1. Makeing schematic I have put the number of capacitors I suppose is adequate.
  2. Designing PCB I find that whatever I am trying to do two the same value capacitors I have to put together - so I delete one of them.

Missing C numbers (C809 here) is no problem.

When you carefully read WIZNET W5200 Data Sheet

In pin by pin description (page 11) you see about VDD pin: “Connect a capacitor of 10.1uF to the ground.”
If not schematic added here I would start to look for distributor of 10.1uF capacitors :slight_smile:
1 Like

Yeah, sometimes blunders slip into Data Sheets despite our best efforts. “10.1 uF” isn’t even a value in the E96 series, so you’ll probably have to pay somebody to custom fabricate the things. Even so, I go through the Data Sheet before looking in other places for answers about a part.

And that particular Data Sheet actually shows (on pages 11 and 12) some good examples of non-trivial power supply bypassing, like I was referring to in my previous post.


I have read many times to not connect parallelly hi Q (low ESR) capacitors of different values as their ESL resonates giving you some frequency (betwean serial resonances of each of them) at which blocking really disapeares. If any harmonic of IC signals will be close to that resonance you are in trouble.
So I think connecting ceramic 100n and 1u here is not a good idea.

1 Like

That advice is repeated in quite a few places, which may be why the larger capacitor is typically specified as an aluminum electrolytic, or tantalum, type.


On the other hand, It is very common to use use parallel combinations of MLCC’s of different sizes and capacitances to get effective decoupling over a wide frequency range.
Someting like 1uF 1206, 100n 0603, 10n 0402, 1n 0201.
If you need effective decoupling @500MHz and beyond, just slapping on some 100nF caps ain’t going to cut it anymore.

There are also manufacturers who make MLCC’s with a controlled ESR, with the goal of dampening the resonance peaks when used with other MLCC’s.

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.