Datasheet

25
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
7. System Clock and Clock Options
7.1 Clock Systems and their Distribution
Figure 7-1 on page 25 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 “Power Management and Sleep Modes” on page 34. The clock systems are detailed below.
Figure 7-1. Clock Distribution
7.1.1 CPU Clock – clk
CPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core.
Examples of such modules are the General Purpose Register File, the Status Register and the
Data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from perform-
ing general operations and calculations.
7.1.2 I/O Clock – clk
I/O
The I/O clock is used by the majority of the I/O modules, like the Timer/Counter. The I/O clock
is also used by the External Interrupt module, but note that some external interrupts are
detected by asynchronous logic, allowing such interrupts to be detected even if the I/O clock is
halted.
7.1.3 Flash Clock – clk
FLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active
simultaneously with the CPU clock.
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