Datasheet

29
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming
mode can be entered.
6.2.5 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 6-5. To find suit-
able load capacitance for a 32.768 kHz crysal, please consult the manufacturer’s datasheet.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in Table 6-10.
Note: 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 6-11 at each TOSC pin.
6.2.6 Crystal Oscillator / Ceramic Resonator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as
an On-chip Oscillator, as shown in Figure 6-5. Either a quartz crystal or a ceramic resonator may be used.
Figure 6-5. 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 environ-
Table 6-10. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
SUT[1:0]
Start-up Time from
Power Down
Additional Delay from
Reset (V
CC
= 5.0V) Recommended usage
00 1K (1024) CK
(1)
4 ms Fast rising power or BOD enabled
01 1K (1024) CK
(1)
64 ms Slowly rising power
10 32K (32768) CK 64 ms Stable frequency at start-up
11 Reserved
Table 6-11. Capacitance of Low-Frequency Crystal Oscillator
Device 32 kHz Osc. Type Cap (Xtal1/Tosc1) Cap (Xtal2/Tosc2)
ATtiny25/45/85 System Osc. 16 pF 6 pF
XTAL2
XTAL1
GND
C2
C1