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7647H–AVR–03/12
Atmel ATmega16/32/64/M1/C1
5. System Clock
5.1 Clock Systems and their Distribution
Figure 5-1 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 unused
modules can be halted by using different sleep modes, as described in “Power Management and
Sleep Modes” on page 40. The clock systems are detailed below.
Figure 5-1. Clock Distribution
5.1.1 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 performing
general operations and calculations.
5.1.2 I/O Clock – clk
I/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, UART. 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.
General I/O
Modules
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
(Crystal
Oscillator)
External Clock
Fast Peripherals
PLL
CLK
PLL
Multiplexer
PLL Input
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