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.
Bit
7
6
5
4
3
2
1
0
SE
R/W
0
–
–
–
–
SM2
R/W
0
SM1
R/W
0
SM0
R/W
0
SMCR
Read/Write
Initial Value
R
0
R
0
R
0
R
0
• 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.
Table 6-1. Sleep Mode Select
SM2
0
SM1
SM0
Sleep Mode
Idle
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
ADC noise reduction
Power-down
Reserved
0
0
1
Reserved
1
Reserved
Standby(1)
1
1
Reserved
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.
34
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
