ATmega48/88/168
7. 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 consump-
tion 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 7-1 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 6-1 on page 25 presents the different clock systems in the ATmega48/88/168, and their
distribution. The figure is helpful in selecting an appropriate sleep mode.
7.0.1
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 7..4 Res: Reserved Bits
These bits are unused bits in the ATmega48/88/168, and will always read as zero.
• 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 7-1.
Table 7-1.
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)
Reserved
Note:
1. Standby mode is only recommended for use with external crystals or resonators.
• Bit 0 – 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
37
2545E–AVR–02/05
MICROCHIP [ MICROCHIP ]
