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Topic: Arduino Atmega Standalone - need for Capacitors & Crystal? (Read 5 times) previous topic - next topic

Andy_Cool

Thank you all for replying :)

Quote
I just don't understand why you don't want to use it in your project....( I don't know your situation... )  


@Techone Nothing like that. I was just curious to know what was the need & what prevents the internal clock from being used as is. Beginners knowledge sake :)

floresta


FYI in the future... This page will help you setup a chip for use with out the need for external mumbo jumbo.

http://arduino.cc/en/Tutorial/ArduinoToBreadboard


Unfortunately this tutorial does not set a very good example.  Two things that come to mind are the lack of decoupling capacitors at the IC power pins and the long wires between the crystal and the IC.

Don

RobDrizzle

All true... But it does have a readily available Board.Txt file with all the fuses and settings for running at 8MHz without a crystal making the bootloading and fuses easy as pie....
























mmmmm... pie.

MarkT


Hi


If I have grasped it correctly, the capacitors on the power-pins are required to remove
the DC ripple effect.



No, the power supply capacitors are for decoupling - for logic circuits they prevent the high-speed(*) switching transients from all the 10,000's of transistors adding up on the supply rail and causing circuit misbehaviour - incorrect logic switching.  Since the Arduino chip has analog inputs as well the decoupling also is very important in reducing analog noise.  With high speed decoupling its important the capacitors are right next to the chips connected via a low impedance (low inductance) path - that means short fat wires/PCB-traces.
Often voltage regulators require a certain amount of capacitance on the output rail to be able to live up to their specifications.  Typically you want 0.1uF ceramic caps next to each chip and 10uF to 100uF somewhere on the board for lower-frequency decoupling.

Inadequate decoupling shows up as unreliable behaviour, pattern sensitive and difficult to trace with a logic analyser or 'scope since monitoring the signals changes the high-speed transients that are causing the problem.  It will drive you crazy trying to work out what's happening, so never skimp on decoupling.

Incidentally DC ripple should be handled by the voltage regulator circuit/chip.

(*) modern CMOS devices switch output pins in 1 to 10 nanoseconds, but internally some might be switching in 10's to 100's of picoseconds (and have local decoupling capacitors built onto the surface of the chip!). 
[ I won't respond to messages, use the forum please ]

Andy_Cool

@MarkT Wow. That was heavy :) I will have to read it several times more & cross-reference it with the help of Google Uncle before I fully understand it. Thanks all the same for willing to share your knowledge.

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