ATmega48 datasheet(36/340 Pages) ATMEL

36 / 340 page

7530H–AVR–02/09

ATmega48/88/168 Automotive

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 24 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.

• 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.

Reserved

Table 6-14.

Clock Prescaler Select

CLKPS3

CLKPS2

CLKPS1

CLKPS0

Clock Division Factor

Bit

76543210

––––

SM2

SM1

SM0

SMCR

Read/Write

RRRR

R/W

Initial Value

00000000

Table 7-1.

Sleep Mode Select

SM2

SM1

SM0

Sleep Mode

000

Idle

ADC Noise Reduction

010

Power-down

011

Power-save

100

Reserved

ATmega48 Datasheet(PDF) 36 Page - ATMEL Corporation

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