39
8008H–AVR–04/11
ATtiny48/88
7.3.6
Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The
most important is then to ensure that no pins drive resistive loads. In sleep modes where both
the I/O clock (clk
I/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will
be disabled. This ensures that no power is consumed by the input logic when not needed. In
some cases, the input logic is needed for detecting wake-up conditions, and it will then be
which pins are enabled. If the input buffer is enabled and the input signal is left floating or have
an analog signal level close to V
CC/2, the input buffer will use excessive power.
For analog input pins, the digital input buffer should be disabled at all times. An analog signal
level close to V
CC/2 on an input pin can cause significant current even in active mode. Digital
input buffers can be disabled by writing to the Digital Input Disable Registers (DIDR1 and
7.3.7
On-chip Debug System
If the On-chip debug system is enabled by the DWEN Fuse and the chip enters sleep mode, the
main clock source is enabled and hence always consumes power. In the deeper sleep modes,
this will contribute significantly to the total current consumption.
7.4
Register Description
7.4.1
SMCR – Sleep Mode Control Register
The Sleep Mode Control Register contains control bits for power management.
Bits 7:3 – Res: Reserved Bits
These bits are reserved and will always read zero.
Bits 2:1 – SM[1:0]: Sleep Mode Select Bits 1 and 0
These bits select between the available sleep modes as shown in
Table 7-2.
Bit 0 – 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 just before the execution of
the SLEEP instruction and to clear it immediately after waking up.
Bit
7
654
32
10
–
SM1
SM0
SE
SMCR
Read/Write
R
R/W
Initial Value
0
Table 7-2.
Sleep Mode Select
SM1
SM0
Sleep Mode
00
Idle
0
1
ADC Noise Reduction
1
0
Power-down
11
Reserved