
27
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
6.2.3
Calibrated Internal Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage and temperature
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 6-6 on page27. If selected, it will operate with no external components. During reset, hardware loads the pre-programmed cali-
bration value into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The accuracy of
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
When this Oscillator is used as the chip 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 calibration value, see the section
“Cali-The internal oscillator can also be set to provide a 6.4 MHz clock by writing CKSEL fuses to “0011”, as shown in
Table 6-6 below. This setting is reffered to as ATtiny15 Compatibility Mode and is intended to provide a calibrated
clock source at 6.4 MHz, as in ATtiny15. In ATtiny15 Compatibility Mode the PLL uses the internal oscillator run-
four, providing a 1.6 MHz system clock.
Note:
1. The device is shipped with this option selected.
2. This setting will select ATtiny15 Compatibility Mode, where system clock is divided by four, resulting in a 1.6 MHz
clock frequency.
01
14CK + 16K (16384) CK + 4 ms
4 ms
Fast rising power
10
14CK + 1K (1024) CK + 64 ms
4 ms
Slowly rising power
11
14CK + 16K (16384) CK + 64 ms
4 ms
Slowly rising power
Table 6-5.
Start-up Times for the High Frequency PLL Clock
SUT[1:0]
Start-up Time from
Power Down
Additional Delay from
Power-On Reset (V
CC = 5.0V)
Recommended
usage
Table 6-6.
Internal Calibrated RC Oscillator Operating Modes
CKSEL[3:0]
Nominal Frequency
0010
8.0 MHz
6.4 MHz