9.2.1 Default Clock SourceThe device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed,resulting in 1.0MHz system clock. The startup time is set to maximum and time-outperiod enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures thatall users can make their desired clock source setting using any available programming interface.
9.11 System Clock PrescalerThe ATmega48A/PA/88A/PA/168A/PA/328/P has a system clock prescaler, and the systemclock can be divided by setting the "CLKPR - Clock Prescale Register" on page 387. This featurecan be used to decrease the system clock frequency and the power consumption when therequirement for processing power is low. This can be used with all clock source options, and itwill affect the clock frequency of the CPU and all synchronous peripherals. clkI/O, clkADC, clkCPU,and clkFLASH are divided by a factor as shown in Table 29-12 on page 324.
10. Power Management and Sleep ModesSleep modes enable the application to shut down unused modules in the MCU, thereby savingpower. The AVR provides various sleep modes allowing the user to tailor the power consumptionto the application's requirements.
10.3 Idle ModeWhen the SM2...0 bits are written to 000, the SLEEP instruction makes the MCU enter Idlemode, stopping the CPU but allowing the SPI, USART, Analog Comparator, ADC, 2-wire SerialInterface, Timer/Counters, Watchdog, and the interrupt system to continue operating. This sleepmode basically halts clkCPU and clkFLASH, while allowing the other clocks to run.Idle mode enables the MCU to wake up from external triggered interrupts as well as internalones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from theAnalog Comparator interrupt is not required, the Analog Comparator can be powered down bysetting the ACD bit in the Analog Comparator Control and Status Register - ACSR. This willreduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automaticallywhen this mode is entered.
10.10 Minimizing Power ConsumptionThere are several possibilities to consider when trying to minimize the power consumption in anAVR controlled system. In general, sleep modes should be used as much as possible, and thesleep mode should be selected so that as few as possible of the device's functions are operating.All functions not needed should be disabled. In particular, the following modules may needspecial consideration when trying to achieve the lowest possible power consumption.
I don't really need speed with this project, just a low battery draw.