ATMEGA649V_14 Datasheet, PDF (30/392 Pages) ATMEL Corporation – High Endurance Non-volatile Memory Segments

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-7 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 329.
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 329.
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 296.
Table 8-7.
Internal Calibrated RC Oscillator Operating Modes⁽¹⁾⁽³⁾
Frequency Range⁽²⁾ (MHz)
CKSEL3..0
7.3 - 8.1
0010
Note: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values.
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 8-8 on page 30.
Table 8-8. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
Start-up Time from Power-
down and Power-save
6 CK
6 CK
6 CK
Reserved
Additional Delay from
Reset (V_CC = 5.0V)
14CK
14CK + 4.1ms
14CK + 65ms⁽¹⁾
SUT1..0
00
01
10
11
Note: 1. The device is shipped with this option selected.
30 ATmega329/3290/649/6490
2552K–AVR–04/11