Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0MHz clock. All frequencies are nominal values at 5V and 25°C. This clock may be selected as the system clock by programming the CKSEL Fuses as shown in the next table. If selected, it will operate with no external components. The CKOPT Fuse should always be unprogrammed when using this clock option. During reset, hardware loads the 1MHz calibration byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0MHz Oscillator frequency selected, this calibration gives a frequency within ± 3% of the nominal frequency. Using run-time calibration methods as described in application notes available at it is possible to achieve ± 1% accuracy at any given VCC and Temperature. 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.

Table 1. Internal Calibrated RC Oscillator Operating Modes
CKSEL3:0 Nominal Frequency (MHz)
0001(1) 1.0
0010 2.0
0011 4.0
0100 8.0
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 the table below. PB6 (XTAL1/TOSC1) and PB7(XTAL2/TOSC2) can be used as either general I/O pins or Timer Oscillator pins:

Table 2. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1:0 Start-up Time from Power-down and Power-save Additional Delay from Reset 
(VCC = 5.0V) Recommended Usage
00 6 CK BOD enabled
01 6 CK 4.1ms Fast rising power
10(1) 6 CK 65ms Slowly rising power
11 Reserved
Note: 1. The device is shipped with this option selected.