Datasheet

Table Of Contents
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
Table 9-7 on page 30. 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. The accuracy of this calibration is shown as Factory
calibration in Table 28-2 on page 300.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 32, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 28-2 on page 300.
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-
bration value, see the section “Calibration Byte” on page 268.
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
Table 9-8 on page 30.
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 Figure
9-3 on page 31. To run the device on an external clock, the CKSEL Fuses must be programmed
to “0000” (see Table 9-9 on page 30).
Table 9-7. Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz) 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 (V
CC
= 5.0V) SUT1..0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1ms 01
Slowly rising power 6 CK 14CK + 65ms
(Note:)
10
Reserved 11
Table 9-9. Crystal Oscillator Clock Frequency
Frequency Range CKSEL3..0
0 - 16MHz 0000