34
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
6.
Power Management and 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.
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, ADC noise reduction, Power-
down, Power-save, or Standby) will be activated by the SLEEP instruction. See
Table 6-1 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.
Figure 5-1 on page 25 presents the different clock systems in the Atmel ATmega16/32/64/M1/C1, and their distribution. The figure is helpful in selecting an appropriate sleep mode.
6.1
Sleep Mode Control Register
6.1.1
Sleep Mode Control Register – SMCR
The Sleep Mode Control Register contains control bits for power management.
Bits 3..1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the five available sleep modes as shown in
Table 6-1.Note:
1.
Standby mode is only recommended for use with external crystals or resonators.
Bit 1 – SE: Sleep Enable
The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP instruction is executed. To
avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended to write the sleep enable
(SE) bit to one just before the execution of the SLEEP instruction and to clear it immediately after waking up.
Bit
76543
210
–
SM2
SM1
SM0
SE
SMCR
Read/Write
R
R/W
Initial Value
00000
000
Table 6-1.
Sleep Mode Select
SM2
SM1
SM0
Sleep Mode
0
Idle
0
1
ADC noise reduction
0
1
0
Power-down
0
1
Reserved
1
0
Reserved
1
0
1
Reserved
1
0
1
Reserved