Decoupler Caps - AtTiny24/44

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I am currently soldering a few AtTiny based boards, when I realised I didn't know something.

The required size of the decoupling caps and wether those should be ceramic or electrolytes. Now, since they are both dirt cheap and I've got a whole bunch of the boards lying around, I've tried building them with both. Both work.

O have a 0.1 Uf Cap on the reset (according to the Atmel note that was correct) and a 10 Uf Cap on the VCC.

My questions are; are those values sensible? And should I use ceramics or electrolytes?

By the way, all this board does is register a button and flash leds. Still hobby'ing around.

Code, justify, code - Pitr Dubovich

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You need ceramic caps for decoupling and close to the chip.

You probably don't need electrolytics near your AVR at all. You have them at the 7805. (and ceramic)

You won't do any harm by having electrolytic as well as ceramic.

David.

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Thanks, so Electrolytes at the regulator, I get that.

And I should use ceramics for decoupling the reset switch. Is there a reason to do that? Why use electrolytes at one point and ceramics at another?

Code, justify, code - Pitr Dubovich

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CMOS chips only really draw current when a signal switches state. A ceramic capacitor has low self-inductance, and can supply these short pulses of current.

An electrolytic capacitor has a large capacitance 1uF - 47000uF. But is likely to have a high self-inductance and internal resistance. So it is very good for a 'reservoir' capacitor in 50Hz or 60Hz power circuits. It will be lousy at giving a smooth VCC for your digital chips switching at 20MHz.

A typical AVR project drawing 0.1uA to 100mA @ 5V, will hardly tax a power supply electrolytic. If you are using serious currents, there are several things to watch for in electrolytics. They can explode very spectacularly !

You don't need a capacitor at the RESET switch, but if you do, a 10nF .. 100nF will protect the pin from noisy environment. (and stop debugWire working)
I doubt that the construction of this capacitor is too critical. OTOH, a ceramic will be better.

David.

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Ceramics can handle spikes.
Electrolyes have a larger capacitance.

That's why the ATtiny needs a ceramic 100nF at Vcc and GND for the high frequency digital noise.
Also the 7805 needs two 100nF, otherwise its regulator feedback inside can go nuts.

Those 100nF are very small, a glitch in the power supply will reset the ATtiny. So a bigger (electrolyte) capacitance of roughly 22uF ... 470uF will make sure it keeps working fine. Most people (I do) place two electrolyte capacitors at both sides of the 7805 next to the 100nF's.

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A clear explanation, thank you very much for that.

I think I understand now. Ceramics for spikes, electrolytes for 'reservoir'. Now, I have a battery powered system, running at the internal clock at 1Mhz. Also, I run at 3 volts.

Let me see if I got this part straight then;

I have one cap on the VCC, one on the Reset, both 0.1 uF (or 100nF)those caps should both be ceramic. Altough in my simple system, I will most likely not notice if I do use electrolytes.

I am not using a regulator, but if I did, I should use electrolytes on that.

One final question, is it bad to place a cap at 10000nF where a cap of 100nF would do?

Code, justify, code - Pitr Dubovich

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Unless your device is switching some heavy loads that the battery struggles with, you probably don't need electrolytic in a battery system.

Regarding 10000nF and 100nF. Yes the 100nF is preferable for decoupling. It will be physically smaller, with shorter track length to the AVR pins.

Just look at other people's designs. Copy them. Follow manufacturer's advice.

David.

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It is advisable to use an electrolytic also in a battery system. It compensates the (growing) battery internal resistance.

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Quote:
Regarding 10000nF and 100nF. Yes the 100nF is preferable for decoupling. It will be physically smaller, with shorter track length to the AVR pins.

Ceramic 10uF caps in a 0603 package rated for 10V are available... but likely loads more expensive then a 100nF one.