Datasheet

29
2586N–AVR–04/11
ATtiny25/45/85
6.2.4 Internal 128 kHz Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3V and 25°C. This clock may be select as the system clock by
programming the CKSEL Fuses to “0100”.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-9.
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 suitable 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.
Table 6-9. Start-up Times for the 128 kHz Internal Oscillator
SUT[1:0]
Start-up Time from
Power-down
Additional Delay from
Reset Recommended Usage
00 6 CK 14CK
(1)
BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
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