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Basic Electronics » Using a 555 IC instead of a Crystal Oscillator?

February 26, 2011
by Twarter369
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Basically, I am wanting to know what the pro/con were for using an astable 555 chip instead of the crystal oscillator might be? I am building a permanent NKP board and I have 3 555 IC's but only the one Crystal.

How would I set up the 555 to oscillate at the same frequency as the crystal?

February 26, 2011
by Rick_S
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I wouldn't use a 555 as an oscillator for the micro. The 555 bases it's timebase on R/C circuit and would fluctuate with temperature. You'd be better off getting some more crystals.

February 26, 2011
by mongo
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As much as I like the 555 and its brothers, Rick is right. Unless you run a crystal oscillator, you will not be able to attain any real accuracy in the circuit. Temperature does affect the capacitors and other components where they are not really all that stable, especially at higher rates where crystals are usually used.

I do use 555's for things like baudrate generators and other applications but not for MCU timing.

February 26, 2011
by Twarter369
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Ahhh, okay. Well I haven't been able to find 14.74 mhz crystals at my local shack. Could I use a 10, or 20 mhz? am I wrong in thinking the higher the mhz the faster each clock cycle happens? would that mean that an instruction that takes one cycle would now take 1.5? I am a little confused how all of that works together! Thanks for your help guys!

February 26, 2011
by Rick_S
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While those crystal values would work with the micro, they won't work on the default NK bootloader. If you live in the US, try ebay, they almost always have them available.

Rick

February 26, 2011
by Ralphxyz
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You could have one Nerdkit breadboard setup with the the 14.7mhz crystal for programming and have another with a different crystal to run your program. Just be sure to change the #define F_CPU. Or of course you could modify the bootloader, if you take that route make sure and post your code so we will all know how to do it.

Ralph

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