Sleep Modes

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power. The AVR provides various sleep modes allowing the user to tailor the power consumption to the application’s requirements.

Figure Clock Distribution in section Clock Systems and their Distribution presents the different clock systems in the ATmega8A, and their distribution. The figure is helpful in selecting an appropriate sleep mode. The table below shows the different clock options and their wake-up sources.

Table 1. Active Clock Domains and Wake-up Sources in the Different Sleep Modes
  Active Clock Domains Oscillators Wake-up Sources
Sleep Mode clkCPU clkFLASH clkIO clkADC clkASY Main Clock Source Enabled Timer Osc. Enabled INT1/ INT0 TWIAddress
Match Timer2 SPM/ EEPROM Ready ADC Other I/O
Idle     X X X X X(2) X X X X X X
ADC Noise
Reduction       X X X X(2) X(3) X X X X  
Power-down               X(3) X        
Power-save         X(2)   X(2) X(3) X X(2)      
Standby(1)           X   X(3) X        
  1. 1.External Crystal or resonator selected as clock source.
  2. 2.If AS2 bit in ASSR is set.
  3. 3.Only level interrupt INT1 and INT0.

To enter any of the five sleep modes, the SE bit in MCUCR must be written to logic one and a SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the MCUCR Register select which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save, or Standby) will be activated by the SLEEP instruction. See the table above for a summary.

If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for four cycles in addition to the start-up time, it executes the interrupt routine, and resumes execution from the instruction following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes up from sleep. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.

Note that the Extended Standby mode present in many other AVR MCUs has been removed in the ATmega8A, as the TOSC and XTAL inputs share the same physical pins.