ATtiny25-15ST datasheet(24/196 Pages) ATMEL

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7598H–AVR–07/09

ATtiny25/45/85

6.3

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.

6.4

Crystal Oscillator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-

figured for use as an On-chip Oscillator, as shown in Figure 6-3. Either a quartz crystal or a

ceramic resonator may be used.

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the

capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the

electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for

use with crystals are given in Table 6-3. For ceramic resonators, the capacitor values given by

the manufacturer should be used.

Figure 6-3.

Crystal Oscillator Connections

The Oscillator can operate in three different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 6-3.

Notes:

1. This option should not be used with crystals, only with ceramic resonators.

The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table

6-4.

Table 6-2.

Number of Watchdog Oscillator Cycles

Typ Time-out

Number of Cycles

4 ms

512

64 ms

8K (8,192)

Table 6-3.

Crystal Oscillator Operating Modes

CKSEL3..1

Frequency Range (MHz)

Recommended Range for Capacitors C1 and

C2 for Use with Crystals (pF)

100(1)

0.4 - 0.9

–

101

0.9 - 3.0

12 - 22

110

3.0 - 8.0

12 - 22

111

8.0 -

12 - 22

XTAL2

XTAL1

GND

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