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Topic: Arduino force 12Mhz crystal (Read 92 times) previous topic - next topic

eadmaster

Oct 21, 2014, 09:13 am Last Edit: Oct 21, 2014, 09:19 am by eadmaster
is it possible to reduce the frequency of the external oscillator to 12Mhz without replacing the onboard crystal entirely?
I've read it is possible to reduce the frequency by adding a small capacitor in parallel with the crystal.

I need this to use an Arduino Mini with a V-USB project.


Riva

To try putting a small C across the built in resonator you may as well just replace the resonator.
Arduino AVR cores tend to only support 8 & 16MHz so if you alter the speed to 12MHz delay and other timing related things won't work as expected.
I'm sure the arduino port of V-USB supports 16MHz
http://forum.arduino.cc/index.php?action=unread;boards=5,67,10,11,66,12,15,17,21,22,23,24,25,29;ALL

mrburnette

I've done a lots of V-USB work on the 328P... the 16MHz crystal and the USBkeyboard.h library work just fine.
http://forum.arduino.cc/index.php?topic=135623.0
http://code.rancidbacon.com/ProjectLogArduinoUSB
http://code.rancidbacon.com/files/UsbKeyboard.h

Ray

PS: Trinket and Digispark are t85 implementations of VUSB.  There are significant libraries available for play.

mrburnette

Oh, and if you really want to use a 12MHz and follow the old guidelines, you may find everything you need here:
http://eleccelerator.com/usnoobie/
I've breadboarded the above and just used a 12MHz and 22pF load caps... works great.

eadmaster

Basically i need to flash in V-USB project that was written (not by me) for 12Mhz.

The usnoobie looks a nice alternative, but i prefer a more compact and pre-assembled board like the Arduino Mini.

mrburnette

is it possible to reduce the frequency of the external oscillator to 12Mhz without replacing the onboard crystal entirely?
I've read it is possible to reduce the frequency by adding a small capacitor in parallel with the crystal.

I need this to use an Arduino Mini with a V-USB project.
You can "pull" a crystal to a lower frequency using a small amount of capacitance, but the percentage from fundamental is rather small, typically being only a few kilohertz.  From 16MHz to 12MHz will require a crystal/resonator replacement.

Ray

eadmaster

Ok, thank you for the tips!

Do you know any other pre-built board as small as the Arduino Mini but without the external oscillator ?

Riva

What about the Adafruit Pro Trinket here. It's 3.3V though so your hardware may need tweaking if you was expecting to use 5V.
http://forum.arduino.cc/index.php?action=unread;boards=5,67,10,11,66,12,15,17,21,22,23,24,25,29;ALL

smeezekitty

Keep in mind the standard Arduino core doesn't work right at 12 MHz. Only 8 or 16
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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