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Topic: How to enable the External Occillation. (Read 1 time) previous topic - next topic


Hello Guys,
I am confused if, atmega328  run around 100mhz with the external crystal Oscillator. is it possible? if is it possible How to enable the External Oscillation in Arduino Register and used external crystal oscillator ?


Udo Klein

No, according to the datasheet the maximum frequency is 20 MHz. The datasheet also describes the fuse setup required for external oscialltors.
Check out my experiments http://blog.blinkenlight.net


The atmega328 will run up to about 20Mhz with a crystal oscillator, depending on version and voltages.  You need to read the
processor specification for the exact parameters.

It will not run at 100MHz.

The arduino board already uses an external crystal oscillator.  To have a standalone atmega328 with an external oscillator you need
to have a 16Mhz crystal, and 2 22pF capacitors.  Look at the microprocessor part of this forum for more details.

You also need to program the fuse bits correctly - if you have an in-circuit programmer this is easy to do, again in the forums, loading a standard bootloader will set the fuse bits correctly on a brand new processor chip.

If you buy a chip with a bootloader already burnt it will be set for an external crystal oscillator already.


Hi Thanks for quick response.  so is there other way to speed the clock speed of atmega328?


so is there other way to speed the clock speed of atmega328?

Is there some part of no that you don't comprehend?


Hi Thanks for quick response.  so is there other way to speed the clock speed of atmega328?

Running a 328p chip at greater then 20MHz via an external crystal resonator or an external crystal oscillator is called 'overclocking'. Some people have experimented with doing that and maybe reached 30Mhz or maybe a little higher, I forget the speed they got to, but it's certainly not a recommended practice to run faster then ATMEL ratings and the chip will run hotter and may need some kind of cooling to prevent chip damage when playing with 'overclocking'.



What are you trying to do?  Are you sure that speed is needed?


Hi Guys, Thank you, I appreciated for those answer,I am taking the experiment of "overclocking"  and let see. ;)  i will be back, please put more suggestion here.

Tom Carpenter

It won't go much beyond 20MHz.

I tried running an atmega1284 at 24MHz (bought the wrong crystal by mistake) and while it ran, anything that tries to use the SRAM for more than a couple of bytes causes it to crash.

If you want a faster processor, get a Due (when it comes out) or an atxmega chip (someone made a core for them i think).

Udo Klein

If you really need an Atmega at higher speeds use an FPGA that can execute an atmel core at the required speed. However almost anything else is a simpler solution. Get a faster processor and that's it.
Check out my experiments http://blog.blinkenlight.net

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