Running 2560 at 8MHz no bootloader

Using Arduino Project Guidance
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I figured this would be something I could answer quickly with a search but just turned up a lot of bootloader related stuff. I have a 2560 on a custom board. No bootloader, no external crystal, programmed via ICSP. How can I tell Arduino IDE to compile for 8MHz, not 16MHz? Everything works fine, just at half speed right now.
Thanks!

EDIT: I think I need to add something to "boards.txt" ?

2

Yes.
You answered everything already.

Did you set the fuses for internal 8MHz ? Then the sketch runs at half it's speed, and the baudrate is half.
This is a handy tool : AVR® Fuse Calculator – The Engbedded Blog
You can retrieve the current fuse settings with avrdude, or use a sketch that reads its own fuses.

The boards.txt defines all the possible boards.
It has also the fuses to set when a bootloader is written, and the lock bits.

Look for the section for "Mega 2560" and make a copy of that.
It is different for version 1.0.6 and 1.5.8 BETA.
Set the frequency to 8000000L, and check everything careful.
The frequency and the build.mcu together will make the compiler to create 8MHz code for the ATmega2560.

Copy that added section of boards.txt into your sketch between /* and */ , so you keep it in your project.
When you update the Arduino IDE, you might have forgotten that change and it might be overwritten with a new boards.txt

3

I was working in tandem with you...here is what I added. BUT I can't get any new board options to show up in the IDE even after a restart.

##############################################################

mega25608.name=Mega 2560 8MHz

mega25608.upload.protocol=wiring
mega25608.upload.maximum_size=258048
mega25608.upload.speed=57600

mega25608.bootloader.low_fuses=0xC2
mega25608.bootloader.high_fuses=0xD8
mega25608.bootloader.extended_fuses=0xFD
mega25608.bootloader.path=stk500v2
mega25608.bootloader.file=stk500boot_v2_mega2560.hex
mega25608.bootloader.unlock_bits=0x3F
mega25608.bootloader.lock_bits=0x0F

mega25608.build.mcu=atmega2560
mega25608.build.f_cpu=8000000L
mega25608.build.core=arduino
mega25608.build.variant=mega

##############################################################
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OK I got it. There are two boards.txt files. At least I have two anyway. One in hardware>arduino which is the one I was editing with no luck and one in hardware>arduino>avr. After making the addition to the later, my board list looks way different...not just the addition of my one item. It seems to have gotten rid of the processor sub menu and made them all one list like older versions.

5

That did not go very well. Can you download a new Arduino IDE and install that ?
If you download the *.exe and install it in "Program Files (x86)\Arduino", you might first have to remove the old one.
I think I have only one boards.txt
Make a copy of the original boards.txt, and try again.

Can you tell which Arduino IDE version you use ?
In Arduino IDE 1.5.8 BETA, you could add another processor, but it might be easier to add a complete new board at the bottom of boards.txt.

Are the fuses already set for you ?
You can read them in a sketch:

boot_lock_fuse_bits_get(GET_LOW_FUSE_BITS);
boot_lock_fuse_bits_get(GET_HIGH_FUSE_BITS);
boot_lock_fuse_bits_get(GET_EXTENDED_FUSE_BITS);
boot_lock_fuse_bits_get(GET_LOCK_BITS);
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Everything works fine, just at half speed right now.
Thanks!

Well, of course, since you are running at half speed. (8 MHz compared to 16 MHz).

This isn't a programming question, since you haven't posted any code.

7