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Basic Electronics » TQFP ATmega 168 capacitors

July 30, 2013
by dvdsnyd
dvdsnyd's Avatar

Hi all,

I want to use a TQFP packaged ATmega168. It is essentially the same chip provided in the kit, however it has 32 pins, as opposed to 28. A few extra ADC pins, as well as an extra VCC and GND pin. My question is, do I need to put a 0.1 uf capacitor between VCC and GND for both sets or only one?

Thanks

Dave

July 31, 2013
by Ralphxyz
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Hi Dave, essentially one capacitor would filter out your power supply but:

I do not believe two would hurt. I believe (hopefully some who actually knows will correct me) the 0.1 uf capacitor at the mcu should be supplemented with three capacitors immediately off your power supply I think 100uf,1uf.0.1uf. Darn now that I think about it they might want to be immediately at the mcu so you could use six capacitors plus a 100 uf at the power supply.

This is of course to filter noise from the power supply so you really can play with them to see what you actually need. Of course a scope would help but you might see performance difference in sensitive circuits.

My 2 cents.

Ralph

July 31, 2013
by pcbolt
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Dave -

My guess would be that if you want to use both Vcc pins, they should be tied together (same with the 2 Gnd pins). Therefore you would only need one capacitor between them and ground (actually you don't "need" it but it's a good thing to have). I thought maybe the added power pins are there to give you more options when routing your PCB traces but that wouldn't explain why they are next to each other.

July 31, 2013
by esoderberg
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Dave,

Here's a link to an ATMEL schematic checklist. It's for the Xmega, but it's generic enough that much of it should apply to the Atmega too.Schematic_checklist

Eric

July 31, 2013
by dvdsnyd
dvdsnyd's Avatar

Thanks all,

From the document supplied by Eric, it looks like each supply pin pair should have decoupling caps. This is kind of what I was thinking, but have seen schematics from Sparkfun that only use one.

Thanks again!

Dave

August 01, 2013
by scootergarrett
scootergarrett's Avatar

I use the TQFP ATmega328 on a PCB and its so packed and small that the only place I have caps are before and after the voltage regulator. I have also used it with out any caps and it worked ok (A/D was not the best). Also the way I have the PCB set up the power traces go under the chip so if there is a cap close to pins on the left side there actually close to the pins on the right. Here is a screen cap of the PCB design I use, on the left is where the chip is with wide power traces under it the voltage reg and caps are on the other side.

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