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1477K–AVR–08/10
ATtiny26(L)
System Clock
and Clock
Options
Clock Systems
and their
Distribution
Figure 20 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 Manage-Figure 20. Clock Distribution
CPU Clock – clkCPU
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 performing
general operations and calculations.
I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, and USI. 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.
Flash Clock – clkFLASH The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-
taneously with the CPU clock.
ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks
in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion
results.
General I/O
modules
Timer/Counter1
ADC
CPU Core
RAM
clk
I/O
AVR Clock
Control Unit
clk
CPU
Flash and
EEPROM
clk
FLASH
clk
ADC
Source clock
Watchdog Timer
Watchdog
Oscillator
Reset Logic
Clock
Multiplexer
Watchdog clock
Calibrated RC
Oscillator
PLL
Crystal
Oscillator
Low-Frequency
Crystal Oscillator
External RC
Oscillator
clk
PLL
clk
PCK
External clock