NOTE: Only the values 16000000 and 8000000 are supported by the Arduino core. If you use any other values the timing functions (millis(), delay(), micros(), and delayMicroseconds()) will not be accurate... and different functions will be off by different amounts.
Thanks, found it after a bit of searching. I am yet to work out how to compile with the make file so if you have a
simple way to do this and can let me know if would be much appreciated.
The comments on the crystal frequency are particularly useful although I only need to work at 8MHz for the moment
so not a problem.
A quick work-around might be to create a new boards.txt entry to set the 8MHz clock rate and use half the upload baud rate. The bootloader is built for 57600 baud on a 16MHz clock. With an 8MHz clock it will run at half speed: 28800 baud.
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mega8M.name=Arduino Mega (ATmega1280) at 8MHz