The ATmega48P/88P/168P/328P has a system clock prescaler, and the system clock can be divided by setting the "CLKPR - Clock Prescale Register" on page 377. This feature can be used to decrease the system clock frequency and the power consumption when the requirement for processing power is low. This can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous peripherals.
CLKPR = (1<<CLKPCE); CLKPR = 0011b; // Divide by 8
CLKPR = (1<<CLKPCE); CLKPR = B00000011;