Thanks technik3k,
I'm just trying this out, and syntax for the arduino is actually:
CLKPR = (1<<CLKPCE);
CLKPR = B00000011;
I'm trying to get power consumption down on a project here, so I just ran some tests, including various boards I had lying around.
These were all tested running off 4 x AA NiMH cells, measured at 5.22V, using the code above in setup() and running an empty loop().
Arduino Pro Mini 3.3V/8MHz (power connected to RAW pin)
default clock: 4.8mA
CLKPR mod: 1.85mA
Duemilanove w/ ATmega328
default clock: 12.7mA
CLKPR mod: 7.8mA
Diecimila w/ ATmega168
default clock: 13.3mA
CLKPR mod: 8.2mA
Freeduino v1.16 w/ ATmega168
default clock: 16.2mA
CLKPR mod: 10.mA
According to the datasheet I'm looking at here for the 48/88/168, the prescaler is actually 4 bits and can go up to 256... (B00001000).
Setting it to B00000011 divides by 8, which would result in 1MHz on the Arduino Pro and 2MHz on all the other boards, right?
Divisors are (p38, bottom nybble)
0000 - 1
0001 - 2
0010 - 4
0011 - 8
0100 - 16
0101 - 32
0110 - 64
0111 - 128
1000 - 256