Datasheet

28
8006K–AVR–10/10
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.
The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table 6-8 at
each TOSC pin.
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
00 1K CK
(1)
4 ms Fast rising power or BOD enabled
01 1K CK
(1)
64 ms Slowly rising power
10 32K CK 64 ms Stable frequency at start-up
11 Reserved
Table 6-8. Capacitance for the Low-Frequency Crystal Oscillator
Device
32 kHz Osc.
Type
Cap
(Xtal1/Tosc1)
Cap
(Xtal2/Tosc2)
ATtiny24/44/84 System Osc. 16 pF 6 pF
XTAL2
XTAL1
GND
C2
C1