ATtiny44 datasheet(28/236 Pages) ATMEL

28 / 236 page

8006H–AVR–10/09

ATtiny24/44/84

6.2.4

Low-Frequency Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency crystal

oscillator must be selected by setting CKSEL fuses to ‘0110’. The crystal should be connected

as shown in Figure 6-3. To find suitable capacitors please consult the manufacturer’s datasheet.

For this oscillator start-up times can be set with the SUT fuses, as shown in Table 6-7.

Notes:

1. These options should be used only if frequency stability at start-up is not important.

6.2.5

Crystal Oscillator / Ceramic Resonator

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.

Figure 6-3.

Crystal Oscillator Connections

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 6-9 below. For ceramic resonators, the capacitor values

given by the manufacturer should be used.

Table 6-7.

Start-up Times for the Low-Frequency Crystal Oscillator Clock Selection

SUT1..0

Start-up Time

from Power Down

Additional Delay

from Reset

Recommended usage

1K CK(1)

4 ms

Fast rising power or BOD enabled

1K CK(1)

64 ms

Slowly rising power

32K CK

64 ms

Stable frequency at start-up

Reserved

Table 6-8.

Capacitance for the Low-Frequency Crystal Oscillator

Device

32 kHz Osc.

Type

Cap

(Xtal1/Tosc1)

Cap

(Xtal2/Tosc2)

ATtiny24/44/88

System Osc.

16 pF

6 pF

XTAL2

XTAL1

GND

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