
30
2570N–AVR–05/11
ATmega325/3250/645/6450
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby
automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory
page 32, 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 cali-
Note:
1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. 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.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Note:
The device is shipped with this option selected.
9.6
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure9-3 on page 31. To run the device on an external clock, the CKSEL Fuses must be programmed
Table 9-7.
Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
7.3 - 8.1
0010
Table 9-8.
Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
BOD enabled
6 CK
14CK
00
Fast rising power
6 CK
14CK + 4.1ms
01
Slowly rising power
6 CK
10
Reserved
11
Table 9-9.
Crystal Oscillator Clock Frequency
Frequency Range
CKSEL3..0
0 - 16MHz
0000