ATMEGA8 datasheet(33/331 Pages) ATMEL

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2486AA–AVR–02/2013

ATmega8(L)

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 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

– MCUCR

The MCU Control Register contains control bits for power management.

• 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.

Note:

1. Standby mode is only available with external crystals or resonators

Bit

765

4321

SM2

SM1

SM0

ISC11

ISC10

ISC01

ISC00

MCUCR

Read/Write

R/W

Initial Value

000

0000

Table 13. Sleep Mode Select

SM2

SM1

SM0

Sleep Mode

000

Idle

ADC Noise Reduction

010

Power-down

011

Power-save

100

Reserved

101

Reserved

110

Standby(1)

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