By default, the internal RC oscillator provides an 8.0 MHz clock. Though voltage and temperature dependent, this clock can be very accurately calibrated by the user. The device is shipped with the CKDIV8 Fuse programmed.
This clock may be selected as the system clock by programming the CKSEL fuses as shown in the following table. 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 calibrates the RC oscillator.
By changing the OSCCAL register from SW, it is possible to get a higher calibration accuracy than by using the factory calibration.
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 section Calibration Byte.
| Frequency Range(1) [MHz] | CKSEL[3:0] |
|---|---|
| 7.6 - 8.4 | 0010(2) |
Warning: The oscillator frequency is not guaranteed to be monotonic within the given range as the oscillator calibration contains discontinuity (see figure 8 MHz RC Oscillator Frequency vs. OSCCAL Value in chapter Typical Characteristics.)
When this oscillator is selected, start-up times are determined by the SUT fuses:
| Power Conditions | Start-Up Time from Power-Down and Power-Save | Additional Delay from Reset (VCC = 5.0V) | SUT[1:0] |
|---|---|---|---|
| BOD enabled | 6 CK | 19CK(1) | 00 |
| Fast rising power | 6 CK | 19CK + 4 ms | 01 |
| Slow rising power | 6 CK | 19CK + 65 ms(2) | 10 |
| Reserved | 11 | ||