ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 25

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with 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
Asynchronous Timer Clock –
clk
2570A–AVR–09/04
ASY
I/O
CPU
FLASH
Figure 12 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 33. The clock systems
are detailed below.
Figure 12. 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 start condition detection in the USI
module is carried out asynchronously when clk
detection in all sleep modes.
The Flash clock controls operation of the Flash interface. The Flash clock is usually
active simultaneously with the CPU clock.
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked
directly from an external clock or an external 32 kHz clock crystal. The dedicated clock
domain allows using this Timer/Counter as a real-time counter even when the device is
in sleep mode.
Timer/Counter
Timer/Counter
Asynchronous
Oscillator
General I/O
Modules
External Clock
clk
clk
ASY
I/O
Control Unit
AVR Clock
Multiplexer
Clock
ATmega325/3250/645/6450
Source clock
Oscillator
Crystal
I/O
CPU Core
clk
clk
Reset Logic
is halted, enabling USI start condition
CPU
FLASH
Crystal Oscillator
Watchdog clock
Low-frequency
Watchdog Timer
RAM
Watchdog
Oscillator
Calibrated RC
Flash and
EEPROM
Oscillator
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