Rainbow Electronics ATtiny43U - Manual (Page 34)

34
8048B–AVR–03/09
ATtiny43U
7.4.6 Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The
most important thing 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 (clk
ADC
) 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
enabled. See the section
“Digital Input Enable and Sleep Modes” on page 66 for details on
which pins are enabled. If the input buffer is enabled and the input signal is left floating or has 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 Register (DIDR0). See
“DIDR0 – Digital Input Disable Register 0” on page 130 for details.
7.5 Register Description
7.5.1 MCUCR – MCU Control Register
The MCU Control Register contains control bits for power management.
• Bit 7 – BODS: BOD Sleep
The BODS bit must be written to logic one in order to turn off BOD during sleep, see Table 7-1
on page 31 . Writing to the BODS bit is controlled by a timed sequence and an enable bit,
BODSE in MCUCR. To disable BOD in relevant sleep modes, both BODS and BODSE must first
be set to one. Then, to set the BODS bit, BODS must be set to one and BODSE must be set to
zero within four clock cycles.
The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed
while BODS is active in order to turn off the BOD for the actual sleep mode. The BODS bit is
automatically cleared after three clock cycles.
• 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 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.
• Bits 4, 3 – SM[1:0]: Sleep Mode Select Bits 2:0
These bits select between the three available sleep modes as shown in Table 7-2 below.
Bit 7 6 5 4 3 2 1 0
0x35 (0x55) BODS PUD SE SM1 SM0 BODSE 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
Table 7-2. Sleep Mode Select
SM1 SM0 Sleep Mode
0 0 Idle