AT90CAN128
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 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 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, 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 19 on page 34 presents the different clock systems in the AT90CAN128, and
their distribution. The figure is helpful in selecting an appropriate sleep mode.
Sleep Mode Control Register – The Sleep Mode Control Register contains control bits for power management.
Bit
7
6
5
4
3
2
1
0
SE
R/W
0
SMCR
–
–
–
–
SM2
R/W
0
SM1
R/W
0
SM0
R/W
0
SMCR
Read/Write
Initial Value
R
0
R
0
R
0
R
0
• 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 17.
Table 17. Sleep Mode Select
SM2
SM1
0
SM0
Sleep Mode
Idle
0
0
0
0
1
1
1
1
0
1
0
1
0
1
0
1
0
ADC Noise Reduction
Power-down
Power-save
Reserved
1
1
0
0
Reserved
1
Standby(1)
1
Reserved
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 purpose, it is recommended to write the Sleep Enable (SE) bit to one
43
4250E–CAN–12/04
ATMEL [ ATMEL ]
