PCIF1 datasheet(28/294 Pages) ATMEL

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9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

System Clock and Clock Options

The Atmel

® AVR® provides a large number of clock sources. They can be divided into two cat-

egories: internal and external. Some external clock sources can be shared with the

asynchronous timer. After reset, the clock source is determined by the CKSEL Fuses. Once

the device is running, software clock switching is possible to any other clock sources.

Hardware controls are provided for clock switching management but some specific proce-

dures must be observed. Clock switching should be performed with caution as some settings

could result in the device having an incorrect configuration.

4.1

Clock Systems and their Distribution

Figure 4-1 presents the principal clock systems in the Atmel AVR and their distribution. All of

the clocks may not need to be active at any given time. In order to reduce power consumption,

the clocks to modules not being used can be halted by using different sleep modes or by using

features of the dynamic clock switch circuit (See “Power Management and Sleep Modes” on

page 46 and “Dynamic Clock Switch” on page 35). The clock systems are detailed below.

Figure 4-1.

Clock Distribution

Modules

clk

I/O

clk

ASY

AVR Clock

Control Unit

clk

CPU

clk

FLASH

Source Clock

Watchdog Timer

Watchdog

Oscillator

Reset Logic

Prescaler

Multiplexer

Watchdog Clock

Low-frequency

Crystal Oscillator

Crystal

Oscillator

External Clock

clk

ADC

Asynchronous

Timer/Counter0

General I/O

ADC

CPU Core

RAM

Flash and

EEPROM

Calibrated RC

Oscillator

PB5 / XTAL2 / CLKO

PB4 / XTAL1 / CLKI

CKOUT

Fuse

Clock Switch

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