Datasheet

13. System Clock and Clock Options
13.1 Clock Systems and their Distribution
The figure below 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. The clock systems are detailed in the following figure.
Figure 13-1. Clock Distribution
General I/O
Modules
Asynchronous
Timer/Counter
ADC CPU Core RAM
clk
I/O
clk
ASY
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
Timer/Counter
Oscillator
Crystal
Oscillator
Low-Frequency
Crystal Oscillator
External RC
Oscillator
External Clock
13.1.1 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.
13.1.2 I/O Clock – clk
I/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. 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. Also note that
address recognition in the TWI module is carried out asynchronously when clk
I/O
is halted, enabling TWI
address reception in all sleep modes.
AVR 8-Bit Microcontroller
System Clock and Clock Options
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 46