Datasheet

37
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
System Clock and
Clock Options
This section describes the clock options for the AVR microcontroller.
Overview 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 Management and Sleep Modes” on page 50. The clock systems
are detailed below.
Figure 20. Clock Distribution
General I/O
Modules
Asynchronous
Timer/Counter
CPU Core RAM
clk
I/O
clk
ASY
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
Timer/Counter
Oscillator
Crystal
Oscillator
Low-frequency
Crystal Oscillator
External Clock
ADC
clk
ADC
System Clock
Prescaler