6.6Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator provides an 8.0 MHz clock. The frequency is the nominal value at 3V and 25°C. If the frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 during start-up. See “System Clock Prescaler” on page 29. for more 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 calibration byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this calibration gives a frequency within ± 1% of the nominal frequency. 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 “Calibration Byte” on page 142.
In addition the calibrated internal RC Oscillator provides a 6.4 MHz clock that is chosen by writing the CKSEL fuses to “0011” as shown in Table 7. When this CKSEL setting is written the nominal frequency of the calibrated internal RC Oscillator is calibrated down to 6.4 MHz. This clock frequency is needed for the ATtiny15 compatibility mode. In the ATtiny15 compatibility mode the 6.4 MHz internal RC oscillator is used as a reference clock for the PLL that is generating a 4x multiplied frequency from the reference clock. The resulting frequency is 25.6 MHz and it is needed for supporting the same PWM frequencies as in ATtiny15.
Table 6-6. |
Internal Calibrated RC Oscillator Operating Modes |
||
|
CKSEL3..0 |
|
Nominal Frequency |
|
|
|
|
|
0010(1) |
|
8.0 MHz |
|
0011 |
|
6.4 MHz |
|
|
|
|
Note: 1. |
The device is shipped with this option selected. |
|
|
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in Table 6-7 and the start-up times in ATtiny15 compatibility mode in Table 9.
Table 6-7. |
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection |
||
|
Start-up Time |
Additional Delay from |
|
SUT1..0 |
from Power-down |
Reset (VCC = 5.0V) |
Recommended Usage |
00 |
6 CK |
14CK |
BOD enabled |
|
|
|
|
01 |
6 CK |
14CK + 4 ms |
Fast rising power |
|
|
|
|
10(1) |
6 CK |
14CK + 64 ms |
Slowly rising power |
11 |
|
Reserved |
|
|
|
|
|
Note: 1. |
The device is shipped with this option selected. |
|
|
Table 6-8. |
Start-up Times for the Internal Calibrated RC Oscillator Clock Selection |
||
|
Start-up Time |
Additional Delay from |
|
SUT1..0 |
from Power-down |
Reset (VCC = 5.0V) |
Recommended Usage |
00 |
6 CK |
14CK + 64 ms |
|
|
|
|
|
01 |
6 CK |
14CK + 64 ms |
|
|
|
|
|
10(1) |
6 CK |
14CK + 4 ms |
|
11 |
1 CK |
14CK |
|
|
|
|
|
26 ATtiny25/45/85 Auto 
7598C–AVR–09/06