ATtiny261-ESSZ datasheet(27/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

7.2

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 while “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

commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of

the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table

7-2.

7.3

Default Clock Source

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default

clock source setting is therefore the Internal RC Oscillator running at 8 MHz with longest start-up

time and an initial system clock prescaling of 8. This default setting ensures that all users can

make their desired clock source setting using an In-System or High-voltage Programmer.

7.4

External Clock

To drive the device from an external clock source, CLKI should be driven as shown in Figure

7-3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

Table 7-1.

Device Clocking Options Select

(1) vs. PB4 and PB5 Functionality

Device Clocking Option

CKSEL3..0

PB4

PB5

External Clock

0000

XTAL1

I/O

PLL Clock

0001

I/O

Calibrated Internal RC Oscillator 8.0 MHz

0010

I/O

Watchdog Oscillator 128 kHz

0011

I/O

External Low-Frequency Crystal

01xx

XTAL1

XTAL2

External Crystal/Ceramic Resonator (0.4 - 0.9 MHz)

1000

XTAL1

XTAL2

External Crystal/Ceramic Resonator (0.4 - 0.9 MHz)

1001

XTAL1

XTAL2

External Crystal/Ceramic Resonator (0.9 - 3.0 MHz)

1010

XTAL1

XTAL2

External Crystal/Ceramic Resonator (0.9 - 3.0 MHz)

1011

XTAL1

XTAL2

External Crystal/Ceramic Resonator (3.0 - 8.0 MHz)

1100

XTAL1

XTAL2

External Crystal/Ceramic Resonator (3.0 - 8.0 MHz)

1101

XTAL1

XTAL2

External Crystal/Ceramic Resonator (8.0 - 16.0 MHz)

1110

XTAL1

XTAL2

External Crystal/Ceramic Resonator (8.0 - 16.0 MHz)

1111

XTAL1

XTAL2

Table 7-2.

Number of Watchdog Oscillator Cycles

Typ Time-out

Number of Cycles

4 ms

512

64 ms

8K (8,192)

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