21
7598H–AVR–07/09
ATtiny25/45/85
5.4
I/O Memory
The I/O space definition of the ATtiny25/45/85 is shown in
“Register Summary” on page 182
.
All ATtiny25/45/85 I/Os and peripherals are placed in the I/O space. All I/O locations may be
accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32
general purpose working registers and the I/O space. I/O Registers within the address range
0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the
value of single bits can be checked by using the SBIS and SBIC instructions. Refer to the
instruction set section for more details. When using the I/O specific commands IN and OUT, the
I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using
LD and ST instructions, 0x20 must be added to these addresses.
For compatibility with future devices, reserved bits should be written to zero if accessed.
Reserved I/O memory addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI
instructions will only operate on the specified bit, and can therefore be used on registers contain-
ing such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and Peripherals Control Registers are explained in later sections.
6.
System Clock and Clock Options
6.1
Clock Systems and their Distribution
presents the principal clock systems in the AVR and their distribution. All of the clocks
need not be active at a given time. In order to reduce power consumption, the clocks to modules
not being used can be halted by using different sleep modes, as described in
ment and Sleep Modes” on page 31
. The clock systems are detailed below.
Figure 6-1.
Clock Distribution
General I/O
Modules
CPU Core
RAM
clk
I/O
AVR Clock
Control Unit
clk
CPU
Flash and
EEPROM
clk
FLASH
Source clock
Watchdog Timer
Watchdog
Oscillator
Reset Logic
Clock
Multiplexer
Watchdog clock
Calibrated RC
Oscillator
Calibrated RC
Oscillator
External Clock
ADC
clk
ADC
Crystal
Oscillator
Low-Frequency
Crystal Oscillator
System Clock
Prescaler
PLL
Oscillator
clk
PCK
clk
PCK
