ATtiny25 datasheet(31/196 Pages) ATMEL

31 / 196 page

7598H–AVR–07/09

ATtiny25/45/85

Power Management and Sleep Modes

The high performance and industry leading code efficiency makes the AVR microcontrollers an

ideal choise for low power applications.

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 three sleep modes, the SE bit in MCUCR must be written to logic one and a

SLEEP instruction must be executed. The SM1..0 bits in the MCUCR Register select which

sleep mode (Idle, ADC Noise Reduction, or Power-down) will be activated by the SLEEP instruc-

tion. 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 21 presents the different clock systems in the ATtiny25/45/85, and their dis-

tribution. The figure is helpful in selecting an appropriate sleep mode.

7.1

MCU Control Register – MCUCR

The MCU Control Register contains control bits for power management.

• Bit 7 – BODS: BOD Sleep

BOD disable functionality is available in some devices, only. See “Limitations” on page 33.

In order to disable BOD during sleep (see Table 7-2 on page 33) the BODS bit must be written to

logic one. This is controlled by a timed sequence and the enable bit, BODSE in MCUCR.

First,both BODS and BODSE must be set to one. Second, within four clock cycles, BODS must

be set to one and BODSE must be set to zero. The BODS bit is active three clock cycles after it

is set. A sleep instruction must be executed while BODS is active in order to turn off the BOD for

the actual sleep mode. The BODS bit is automatically cleared after three clock cycles.

In devices where Sleeping BOD has not been implemented this bit is unused and will always

read zero.

• Bit 5 – 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.

• Bits 4, 3 – SM1..0: Sleep Mode Select Bits 2..0

These bits select between the three available sleep modes as shown in Table 7-1.

Bit

765

432

BODS

PUD

SM1

SM0

BODSE

ISC01

ISC00

MCUCR

Read/Write

R/W

Initial Value

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