ATmega8(L)
Power
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.
Management
and Sleep
Modes
To enter any of the five sleep modes, the SE bit in MCUCR must be written to logic one and a
SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the MCUCR Register
select which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save, or Standby)
will be activated by the SLEEP instruction. See Table 13 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, it 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.
Note that the Extended Standby mode present in many other AVR MCUs has been removed in
the ATmega8, as the TOSC and XTAL inputs share the same physical pins.
Figure 10 on page 25 presents the different clock systems in the ATmega8, and their distribu-
tion. The figure is helpful in selecting an appropriate sleep mode.
MCU Control Register The MCU Control Register contains control bits for power management.
– MCUCR
Bit
7
SE
R/W
0
6
5
4
3
ISC11
R/W
0
2
ISC10
R/W
0
1
ISC01
R/W
0
0
ISC00
R/W
0
SM2
R/W
0
SM1
R/W
0
SM0
R/W
0
MCUCR
Read/Write
Initial Value
• Bit 7 – 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 set the Sleep Enable (SE) bit just before the execution of the
SLEEP instruction.
• Bits 6..4 – SM2..0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the five available sleep modes as shown in Table 13.
Table 13. Sleep Mode Select
SM2
SM1
SM0
Sleep Mode
Idle
0
0
0
0
1
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
ADC Noise Reduction
Power-down
Power-save
Reserved
Reserved
Standby(1)
Note:
1. Standby mode is only available with external crystals or resonators
33
2486AA–AVR–02/2013
ATMEL [ ATMEL ]
