Datasheet

47
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
10.7 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the user. See Table 31-1
on page 371 and “Internal Oscillator Speed” on page 404 for more details. The device is shipped
with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 49 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 10-9. 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 cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
Table 31-1 on page 371.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 50, 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 31-1 on page 371.
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 338.
Notes: 1. The device is shipped with this option selected.
2. 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 10-10.
Note: 1. The device is shipped with this option selected.
10.8 128 kHz Internal Oscillator
The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The fre-
quency is nominal at 3V and 25C. This clock may be select as the system clock by
programming the CKSEL Fuses to “11” as shown in Table 10-11.
Note: 1. Note that the 128kHz oscillator is a very low power clock source, and is not designed for high
accuracy.
Table 10-9. Internal Calibrated RC Oscillator Operating Modes
(1)(2)
Frequency Range (MHz) CKSEL3:0
7.3 - 8.1 0010
Table 10-10. 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 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(1)
10
Reserved 11
Table 10-11. 128kHz Internal Oscillator Operating Modes
(1)
Nominal Frequency CKSEL3:0
128kHz 0011