14.5 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.

Table 14-10. Internal Calibrated RC Oscillator Operating Modes
Frequency Range(1) [MHz] CKSEL[3:0]
7.3 - 8.10010(2)
Note:
  1. If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8.
  2. The device is shipped with this option selected.

When this Oscillator is selected, start-up times are determined by the SUT Fuses:

Table 14-11. Start-Up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT
Power ConditionsStart-Up Time from Power-down and Power-SaveAdditional Delay from Reset (VCC = 5.0V)SUT[1:0]
BOD enabled6 CK19CK(1)00
Fast rising power6 CK19CK + 4.1ms01
Slowly rising power6 CK19CK + 65ms(2)10
Reserved11
Note:
  1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 
19CK + 4.1ms to ensure programming mode can be entered.
  2. The device is shipped with this option selected.

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.