ATmega640/1280/1281/2560/2561
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 21 for a sum-
mary. 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 con-
tents 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 21 on page 39 presents the different clock systems in the
ATmega640/1280/1281/2560/2561, and their distribution. The figure is helpful in select-
ing an appropriate sleep mode.
Sleep Mode Control Register – The Sleep Mode Control Register contains control bits for power management.
SMCR
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, 2, 1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the five available sleep modes as shown in Table 21.
Table 21. Sleep Mode Select
SM2
SM1
SM0
Sleep Mode
Idle
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
ADC Noise Reduction
Power-down
Power-save
Reserved
Reserved
Standby(1)
Extended Standby(1)
Note:
1. Standby modes are 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 wak-
ing up.
51
2549A–AVR–03/05
ATMEL [ ATMEL ]
