Datasheet

Table 13-11. Internal Calibrated RC Oscillator Operating Modes
Frequency Range
(1)
[MHz] CKSEL[3:0]
7.3 - 8.1 0010
(2)
Note:
1. If 8 MHz 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 13-12. 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 14 CK 00
Fast rising power 6 CK 14 CK + 4 ms 01
Slow rising power 6 CK 14 CK + 65 ms 10
(1)
Reserved 11
Note:
1. The device is shipped with this option selected.
Related Links
33.4 Clock Characteristics
13.11 System Clock Prescaler
32.4 Calibration Byte
13.12.1 OSCCAL
13.12.1 OSCCAL
13.7 128 kHz Internal Oscillator
The 128 kHz internal oscillator is a low-power oscillator providing a clock of 128 kHz. This clock may be
selected as the system clock by programming the CKSEL fuses to '0011' as shown in the following table.
Warning: Using the 128 kHz internal oscillator as the system oscillator and Watchdog timer
simultaneously is not recommended as this defeats one of the purposes of the Watchdog timer.
Table 13-13. 128kHz Internal Oscillator Operating Modes
Nominal Frequency
(1)
CKSEL[3:0]
128 kHz 0011
Note:
1. The 128 kHz oscillator is a very low-power clock source and is not designed for high accuracy.
When this clock source is selected, start-up times are determined by the SUT fuses:
ATmega48PA/88PA/168PA
System Clock and Clock Options
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 61