[avr-gcc-list] Power consumption when dividing the MCU clock

[John Butera]

I am implementing a bootloader based on AVR230 using a mega128 which 
runs off the internal RC oscillator at 4MHz.  The application is pretty 
large (around 75k) and the resulting encrypted firmware file is about 
170k.  I first tried 3DES at 9600bps, which was taking an obscene 
amount of time to complete.  Using regular DES at 9600bps took about 15 
minutes.  I recompiled the supplied Update program to transmit at 
57.6kbps instead of the default 9600bps, and updating took about 10 
minutes.

What I could do is change the RC oscillator to run at 8MHz for the 
bootloader, and then use the XDIV register to divide the clock down to 
4MHz in the application section.  The datasheet recommends the usage of 
XDIV to save power when less processing speed is needed.  However, the 
RC oscillator would still be running at 8MHz.  How would that impact 
the power consumption?  I'm guessing that the RC oscillator's power 
consumption is negligible compared to the rest of the MCU.  Am I way 
off here?  I guess 10 minutes for updating the firmware isn't terribly 
bad, especially with a startup CRC check which I have drop back into 
the bootloader on failure.

TIA,
John