Rainbow Electronics ATmega128L - Manual (Page 41)

41
ATmega128(L)
2467B–09/01
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 consumption to the application’s requirements.
To enter any of the six 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, Standby, or Extended Standby) will be activated by the SLEEP instruction.
See
Table 17 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 instruc-
tion 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 17 on page 33 presents the different clock systems in the ATmega128, and their
distribution. 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 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 programmers 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 wak-
ing up.
• Bits 4..2 - SM2..0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the six available sleep modes as shown in Table 17.
Note: 1. Standby Mode and Extended Standby Mode are only available with external crystals
or resonators.
Bit 7 6 5 4 3 2 1 0
SRE SRW10 SE SM1 SM0 SM2 IVSEL IVCE MCUCR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Table 17. Sleep Mode Select
SM2 SM1 SM0 Sleep Mode
0 0 0 Idle
0 0 1 ADC Noise Reduction
0 1 0 Power-down
0 1 1 Power-save
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Standby (1)
1 1 1 Extended Standby (1)