ATMEGA8L ATMEL [ATMEL Corporation], ATMEGA8L Datasheet - Page 23

• Manufacturer Part Number: ATMEGA8L
• Description: 8-bit AVR with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

System Clock and
Clock Options
Clock Systems and their
Distribution
CPU Clock – clk
I/O Clock – clk
Flash Clock – clk
2486M–AVR–12/03
I/O
CPU
FLASH
Figure 10 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 31. The clock systems
are detailed Figure 10.
Figure 10. Clock Distribution
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 Reg-
ister and the Data memory holding the Stack Pointer. Halting the CPU clock inhibits the
core from performing general operations and calculations.
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
tion in all sleep modes.
The Flash clock controls operation of the Flash interface. The Flash clock is usually
active simultaneously with the CPU clock.
Timer/Counter
Timer/Counter
Asynchronous
Oscillator
General I/O
External RC
Modules
Oscillator
clk
clk
ASY
I/O
External Clock
Control Unit
AVR Clock
Multiplexer
Clock
ADC
clk
Source Clock
ADC
I/O
CPU Core
Reset Logic
is halted, enabling TWI address recep-
clk
Oscillator
clk
Crystal
CPU
FLASH
Watchdog Clock
Crystal Oscillator
Low-Frequency
Watchdog Timer
ATmega8(L)
Watchdog
Oscillator
RAM
Calibrated RC
Flash and
EEPROM
Oscillator
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