PCINT9 datasheet(29/294 Pages) ATMEL

29 / 294 page

9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

4.1.1

CPU Clock – clk

CPU

The CPU clock is routed to parts of the system concerned with the Atmel

® AVR® core opera-

tion. 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.

4.1.2

I/O Clock – clk

I/O

The I/O clock is used by the majority of the I/O modules, like synchronous Timer/Counter. 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.

4.1.3

Flash Clock – clk

FLASH

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

simultaneously with the CPU clock.

4.1.4

Asynchronous Timer Clock – clk

ASY

The asynchronous timer clock allows the asynchronous Timer/Counter to be clocked directly

from an external clock or an external low frequency crystal. The dedicated clock domain

allows using this Timer/Counter as a real-time counter even when the device is in sleep mode.

4.1.5

ADC Clock – clk

ADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O

clocks in order to reduce noise generated by digital circuitry. This gives more accurate ADC

conversion results.

4.2

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits (default) or

by the CLKSELR register (dynamic clock switch circuit) as shown below. The clock from the

selected source is input to the Atmel AVR clock generator, and routed to the appropriate

modules.

Notes:

1. For all fuses “1” means unprogrammed while “0” means programmed.

2. Flash Fuse bits.

3. CLKSELR register bits.

Table 4-1.

Device Clocking Options Select

(1) vs. PB4 and PB5 Functionality

Device Clocking Option

CKSEL3..0 (2)

CSEL3..0 (3)

PB4

PB5

External Clock

0000 b

CLKI

CLKO - I/O

Calibrated Internal RC Oscillator 8.0 MHz

0010 b

I/O

CLKO - I/O

Watchdog Oscillator 128 kHz

0011 b

I/O

CLKO - I/O

External Low-frequency Oscillator

01xx b

XTAL1

XTAL2

External Crystal/Ceramic Resonator (0.4 - 0.9MHz)

100x b

XTAL1

XTAL2

External Crystal/Ceramic Resonator (0.9 - 3.0MHz)

101x b

XTAL1

XTAL2

External Crystal/Ceramic Resonator (3.0 - 8.0MHz)

110x b

XTAL1

XTAL2

External Crystal/Ceramic Resonator (8.0 - 16.0MHz)

111x b

XTAL1

XTAL2

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