Fuse bits for selecting EXTERNAL CRYSTAL OSCILLATOR for ATMEGA168A?

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Can any one please explain the ways in which atmega chip can be clocked for example difference between ceramic resonator and normal crystal oscillator we found on arudino(16Mhz), external clock etc. PLEASE SEE THE ATTACHMENTS WHERE I INCLUDED SEVERAL TABLES THAT CONFUSES ME FROM AVR DATASHEET..

It's really confusing and even on internet no correct and straight forward simple explanation.

Consider I want to use 20Mhz crystal oscillator (normal one as like arudino's 16Mhz) to clock my avr. What fuse bit should I select. I attached a screenshot of a table from datasheet, please any explain what all those low, high medium power means plus do they really mean using normal CRYSTAL OSCILLATOR?

My doubts topic are listed below:

1.What is really this ceramic resonator means?? Difference between it and a normal crystal oscillator we use??

2. Low power crystal oscillator and Full swing crystal oscillator? Do I really choose Full swing crystal oscillator since I need a clock of 20Mhz??

3. Also explain Ceramic resonator_fast rising power, Ceramic resonator_slowly rising power all this thins means. I will include the screen shot of each area which is to be explained?

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avr freaky

Last Edited: Tue. Aug 13, 2019 - 02:26 PM
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Forget about ceramic resonator, don't even look at it.  Ceramic resonators are lower cost/less accurate replacements for crystals...before China came along, crystals used to be $$$ and you might save 50 cents by using a resonator.  Now crystals are a nickel, so who cares.

 

Many of the entries are concerned with how long it takes the clock to get up to speed and stabilize...for some critical apps that is important (such as coming out of sleep and performing some timing operation), for many & yours it likely doesn't matter...you can always put in a  delay at the begiinning, or select the longest startup option. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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2) 20MHz (and indeed anything over 8MHz) = Full Swing

 

3) crystal resonation is a bit like that thing you can do when you run a wet finger around the rim of a cut crystal glass and after a while the class starts to "sing". As you will know you have to drag your finger around a few times before the glass starts to resonate. Crystals on micros are like this too (also think of swinging a heavy pendulum like a child's swing to get it to "oscillate"). These things take time to get going. So you want to say to the AVR - start the crystal going but give it a while to "sing". That is "Start Up Time" (SUT). Unless you can think of a reason why it must be otherwise (unlikely) then set this to the longest option possible which, in all likelihood, is 64 milliseconds.