Datasheet

46
7682C–AUTO–04/08
AT90CAN32/64/128
6. 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 6-1 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 exe-
cutes from the Reset Vector.
Figure 5-1 on page 37 presents the different clock systems in the AT90CAN32/64/128, and their
distribution. The figure is helpful in selecting an appropriate sleep mode.
6.0.1 Sleep Mode Control Register – SMCR
The Sleep Mode Control Register contains control bits for power management.
• Bit 7..4 – Reserved Bits
These bits are reserved for future use.
• 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 6-1.
Note: 1. Standby mode is only recommended for use with external crystals or resonators.
• Bit 1 – 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
Bit 7 6 5 4 3 2 1 0
– – – – SM2 SM1 SM0 SE SMCR
Read/Write R R R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 6-1. 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 Reserved