29
2588F–AVR–06/2013
ATtiny261/461/861
It is possible to reach a higher accuracy than factory calibration by changing the OSCCAL regis-
When this oscillator is used as device clock, the Watchdog Oscillator will still be used for the
Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed cali-
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 depends on supply voltage, temperature and batch variations. This clock may be select
as the system clock by programming the CKSEL Fuses to “0011”.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
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 ‘0100’. The crystal should be connected
as shown in
Figure 6-4. To find suitable capacitors please consult the manufacturer’s datasheet.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in
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 6-10 at
each TOSC pin.
Table 6-8.
Start-up Times for the 128 kHz Internal Oscillator
SUT1:0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset
Recommended Usage
00
6 CK
BOD enabled
01
6 CK
14CK + 4 ms
Fast rising power
10
6 CK
14CK + 64 ms
Slowly rising power
11
Reserved
Table 6-9.
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
4 ms
Fast rising power or BOD enabled
01
64 ms
Slowly rising power
10
32K (32768) CK
64 ms
Stable frequency at start-up
11
Reserved
Table 6-10.
Capacitance for Low-Frequency Crystal Oscillator
Device
32 kHz Osc. Type
Cap (Xtal1/Tosc1)
Cap (Xtal2/Tosc2)
ATtiny261/461/861
System Osc.
16 pF
6 pF