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I want to write code for another atmega8 running at 1MHz internal RC oscillator using the Arduino IDE and its libraries. I have an ISP programmer to burn the code. What are the things that I should care about? How can I compile the code for 1MHz instead of 16MHz?

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You would need to add a new hardware configuration. Here the procedure is explained for an ATMega328: http://arduino.cc/en/Tutorial/ArduinoToBreadboard but you would need to adjust the example hardware config or maybe you'll find a working one.

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It depends specifically on what compiler/library you're using to write the AVR code, but in general, so long as you compile the code with the proper preprocessor macros #define'd for the CPU speed, you should be good to go.

For example, in the avr-libc documentation, the delay functions and UART baud rates rely on the definition of the F_CPU macro. The proper way to set the CPU speed in such a case is:

#define F_CPU 1000000UL     // 1 MHz, 16MHz is 16000000UL
#include <util/delay.h>     // Delay functions library, delay values based on F_CPU.
#include <util/setbaud.h>   // Automatically sets UART baud rate registers based on F_CPU.

I have also noted the use of #define FOSC to set the CPU speed in some architectures/libraries (in the same way as above, so you could also follow with #define FOSC F_CPU). For this reason, note that the particular macro you need to define/re-define may differ.


If you're using an integrated development environment, setting the clock frequency in your project settings may automatically define the above variables during compile-time. In that case, no settings should need to be made to the code, unless your code is using the wrong preprocessor macro (or hard-coded value) for the processor frequency. To be sure, I would double-check what macro is being set by the IDE, and ensure your code uses the correct values.

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