Calibrated Internal RC Oscillator

By default, the internal RC oscillator provides an 8.0MHz 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.

Table 1. Internal Calibrated RC Oscillator Operating Modes
Frequency Range⁽¹⁾ [MHz]	CKSEL[3:0]
7.3 - 8.1	0010⁽²⁾

Note:

If 8MHz frequency exceeds the specification of the device (depends on V_CC), the CKDIV8 fuse can be programmed in order to divide the internal frequency by 8.
The device is shipped with this option selected.

When this oscillator is selected, start-up times are determined by the SUT fuses:

Table 2. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT
Power Conditions	Start-up Time from Power-down and Power-save	Additional Delay from Reset (V_CC = 5V)	SUT[1:0]
BOD enabled	6 CK	14CK	00
Fast rising power	6 CK	14CK + 4ms	01
Slow rising power	6 CK	14CK + 65ms	10⁽¹⁾
Reserved			11

Note:

The device is shipped with this option selected.