ATMEGA406 datasheet(25/263 Pages) ATMEL

25 / 263 page

2548E–AVR–07/06

ATmega406

System Clock and Clock Options

7.1

Clock Systems and their Distribution

Figure 7-1 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 Manage-

ment and Sleep Modes” on page 31. The clock systems are detailed below.

Figure 7-1.

Clock Distribution

7.1.1

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.

7.1.2

TWI Clock - clk

TWI

The TWI module is provided with a dedicated clock domain. This is because the TWI module

requires a 4 MHz clock to achieve the specified Data Transfer Speed. It also allows power

reduction by halting the clk

TWI clock when TWI communication is not used. Note that address

match detection in the TWI module is carried out asynchronously when clk

TWI is halted, enabling

TWI address watch detection in all sleep modes except Power-off.

7.1.3

I/O Clock – clk

I/O

The I/O clock is used by the majority of the I/O modules. The I/O clock is also used by the Exter-

nal 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.

7.1.4

Flash Clock – clk

FLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-

taneously with the CPU clock.

Ultra Low Power

RC Oscillator

Watchdog Timer

Battery Protection

& FET Control

Reset Logic

CPU

CORE

RAM

FLASH and

EEPROM

Voltage

ADC

Other I/O

Modules

Coulomb Counter

ADC

Wake-up

Timer

1/4

AVR

Clock Control

AVR

Clock Control

Slow RC

Oscillator

Fast RC

Oscillator

clk

CPU

clk

FLASH

clk

VADC

clk

I/O

clk

CCADC

clk

WUT

AVR

Clock Control

TWI

1/4

Sync

Delay

clk

TWI

32 kHz Crystal

Oscillator

Run-Time

Selection

Clock

Multiplexer

TWI Disconnect

Delay

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