ATmega8U2/16U2/32U2
9. Power Management and Sleep Modes
9.1
Overview
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 consump-
tion to the application’s requirements.
9.2
Sleep Modes
Figure 8-1 on page 26 presents the different clock systems in the ATmega8U2/16U2/32U2, and
their distribution. The figure is helpful in selecting an appropriate sleep mode. shows the differ-
ent sleep modes and their wake up sources.
Table 9-1.
Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Active Clock
Domains
Oscillators
Wake-up Sources
Sleep Mode
Idle
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Power-down
Power-save
Standby(1)
X(2)
X(2)
X(2)
X
X
Extended
Standby
X(2)
X
X
Notes: 1. Only recommended with external crystal or resonator selected as clock source.
2. For INT[7:4], only level interrupt.
3. Asynchronous USB interrupt is WAKEUPI only.
To enter any of the five sleep modes, the SE bit in SMCR must be written to logic one and a
SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the SMCR Register select
which sleep mode (Idle, Power-down, Power-save, Standby or Extended standby) will be acti-
vated by the SLEEP instruction. See Table 9-2 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, 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.
9.3
Idle Mode
When the SM2:0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode,
stopping the CPU but allowing the USB, SPI, USART, Analog Comparator, Timer/Counters,
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ATMEL [ ATMEL ]
