Datasheet

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in the
following Table. 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.
By changing the OSCCAL register from SW, 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 calibration value.
Table 10-10. Internal Calibrated RC Oscillator Operating Modes
Frequency Range
(1)
[MHz] CKSEL[3:0]
7.3 - 8.1 0010
(2)
Note:
1. 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.
2. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses:
Table 10-11. Start-Up Times for the Internal Calibrated RC Oscillator Clock Selection - SUT
Power Conditions Start-Up Time from Power-down
and Power-Save
Additional Delay from Reset (V
CC
=
5.0V)
SUT[1:0]
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4ms 01
Slowly rising power 6 CK 14CK + 65ms 10
(1)
Reserved 11
Note:
1. The device is shipped with this option selected.
Related Links
Clock Characteristics on page 398
System Clock Prescaler on page 51
Calibration Byte on page 366
OSCCAL on page 53
10.7. 128kHz Internal Oscillator
The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. This clock may be
select as the system clock by programming the CKSEL Fuses to '0011':
Table 10-12. 128kHz Internal Oscillator Operating Modes
Nominal Frequency
(1)
CKSEL[3:0]
128kHz 0011
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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