ATmega325PA Atmel Corporation, ATmega325PA Datasheet - Page 29

• Manufacturer Part Number: ATmega325PA
• Manufacturer: Atmel Corporation

9. System Clock and Clock Options
9.1
9.1.1
9.1.2
9.1.3
8285D–AVR–06/11
Clock Systems and their Distribution
CPU Clock – clk
I/O Clock – clk
Flash Clock – clk
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P
I/O
Figure 9-1 on page 29
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
Figure 9-1.
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.
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 inter-
rupts 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 asynchro-
nously when clk
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-
taneously with the CPU clock.
CPU
FLASH
Asynchronous
Timer/Counter
Timer/Counter
Oscillator
Clock Distribution
I/O
is halted, enabling USI start condition detection in all sleep modes.
General I/O
presents the principal clock systems in the AVR and their distribution. All
Modules
External Clock
clk
clk
ASY
I/O
System Clock
Control Unit
AVR Clock
Multiplexer
Prescaler
Clock
Oscillator
Crystal
Source clock
39. The clock systems are detailed below.
CPU Core
clk
clk
Reset Logic
CPU
FLASH
Crystal Oscillator
Watchdog clock
Low-frequency
Watchdog Timer
RAM
Watchdog
Oscillator
Calibrated RC
Flash and
EEPROM
Oscillator
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