Uploading to atmega328 over serial with arduino bootloader running at 2MHz

Question

I want to run an atmega328 on a breadboard at 2MHz and upload to it via the serial pins (pins 2 and 3). So I am setting the clock divide by 8 fuse bit to enable this, meaning the atmega runs at 2MHz eith the 16MHz crystal. But I can't upload, probably because the bootloader is not expecting the clock to be running at 2MHz. The baud rates are probably mismatched by 8 times, the IDE is trying to upload at 57600 but since the bootloader code is not aware its being clocked at 2MHz, its probably running 8 times too slow. I've messed around a bit with the arduino conf files changing baud rates but had no joy. I was wondering if anyone knows how to do this off the top of their head.

Thanks, Pete

Answer 1

Unprogram the CKDIV8 fuse and add the following to the sketch instead, near the beginning:

#include <avr/power.h>

void setprescaler(void) __attribute__ ((naked)) \
  __attribute__ ((section(".init1")));

void setprescaler(void)
{
  clock_prescale_set(clock_div_8);
}

This will do essentially the same thing as the fuse, but won't interfere with the bootloader.