28
2588F–AVR–06/2013
ATtiny261/461/861
When the PLL output is selected as clock source, the start-up times are determined by SUT fuse
6.2.3
Calibrated Internal 8 MHz Oscillator
By default, the Internal Oscillator provides an approximately 8 MHz clock signal. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See
Tablemore details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 6-6. If selected, it will operate with no external components. During reset, hardware loads
the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
Notes:
1. The device is shipped with this option selected.
2. If the oscillator frequency exceeds the specification of the device (depends on VCC), the
CKDIV8 Fuse can be programmed to divide the internal frequency by 8.
When this oscillator is selected, start-up times are determined by SUT fuses as shown in
TableNote:
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.
2. The device is shipped with this option selected.
Table 6-5.
Start-up Times for the PLLCK
SUT1:0
Start-up Time from
Power Down
Additional Delay from
Power-On-Reset (V
CC = 5.0V)
Recommended
usage
00
14CK + 1K (1024) + 4 ms
4 ms
BOD enabled
01
14CK + 16K (16384) + 4 ms
4 ms
Fast rising power
10
14CK + 1K (1024) + 64 ms
4 ms
Slowly rising power
11
14CK + 16K (16384) + 64 ms
4 ms
Slowly rising power
Table 6-6.
Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0
Nominal Frequency
0010
8.0 MHz
Table 6-7.
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1:0
Start-up Time
from Power-down
Additional Delay from
Reset (VCC = 5.0V)
Recommended
Usage
00
6 CK
BOD enabled
01
6 CK
14CK + 4 ms
Fast rising power
6 CK
14CK + 64 ms
Slowly rising power
11
Reserved