Rainbow Electronics ATmega8515L - Manual (Page 38)

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ATmega8515(L)
2512A–AVR–04/02
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 three sleep modes, the SE bit in MCUCR must be written to logic
one and a SLEEP instruction must be executed. The SM2 bit in MCUCSR, the SM1 bit
in MCUCR, and the SM0 bit in the EMCUCR Register select which sleep mode (Idle,
Power-down, or Standby) will be activated by the SLEEP instruction. See Table 16 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 exe-
cutes 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 17 on page 31 presents the different clock systems in the ATmega8515, and
their distribution. The figure is helpful in selecting an appropriate sleep mode.
MCU Control Register –
MCUCR
• 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.
• Bit 4 – SM1: Sleep Mode Select Bit 1
The Sleep Mode Select bits select between the three available sleep modes as shown
in Table 16.
MCU Control and Status
Register – MCUCSR
• Bit 5 – SM2: Sleep Mode Select Bit 2
The Sleep Mode Select bits select between the three available sleep modes as shown
in Table 16.
Extended MCU Control
Register – EMCUCR
Bit 7 6 5 4 3 2 1 0
SRE SRW10 SE SM1 ISC11 ISC10 ISC01 ISC00 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
Bit 7 6 5 4 3 2 1 0
– – SM2 – WDRF BORF EXTRF PORF MCUCSR
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
Bit 7 6 5 4 3 2 1 0
SM0 SRL2 SRL1 SRL0 SRW01 SRW00 SRW11 ISC2 EMCUCR
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