ATtiny261A datasheet(25/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

6.1.3

Flash Clock – clkFLASH

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

taneously with the CPU clock.

6.1.4

ADC Clock – clkADC

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.

6.1.5

Fast Peripheral Clock – clkPCK

Selected peripherals can be clocked at a frequency higher than the CPU core. The fast periph-

eral clock is generated by an on-chip PLL circuit.

6.1.6

PLL System Clock – clkADC

The PLL can also be used to generate a system clock. The clock signal can be prescaled to

avoid overclocking the CPU.

6.2

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown

below. The clock from the selected source is input to the AVR clock generator, and routed to the

appropriate modules.

Note:

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

The various choices for each clocking option is given in the following sections. When the CPU

wakes up from Power-down or Power-save, the selected clock source is used to time the start-

up, ensuring stable oscillator operation before instruction execution starts. When the CPU starts

from reset, there is an additional delay allowing the power to reach a stable level before com-

Table 6-1.

Device Clocking Options Select(1) vs. PB4 and PB5 Functionality

Device Clocking Option

CKSEL3:0

PB4

PB5

External Clock (see page 26)

0000

XTAL1

I/O

High-Frequency PLL Clock (see page 26)

0001

I/O

Calibrated Internal 8 MHz Oscillator (see page 28)

0010

I/O

Internal 128 kHz Oscillator (see page 29)

0011

I/O

Low-Frequency Crystal Oscillator (see page 29)

01xx

XTAL1

XTAL2

Crystal Oscillator / Ceramic Resonator

0.4...0.9 MHz (see page 30)

1000

1001

XTAL1

XTAL2

Crystal Oscillator / Ceramic Resonator

0.9...3.0 MHz (see page 30)

1010

1011

XTAL1

XTAL2

Crystal Oscillator / Ceramic Resonator

3...8 MHz (see page 30)

1100

1101

XTAL1

XTAL2

Crystal Oscillator / Ceramic Resonator

8...20 MHz (see page 30)

1110

1111

XTAL1

XTAL2

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