128 kHz Internal Oscillator

The 128 kHz internal oscillator is a low-power oscillator providing a clock of 128 kHz. This clock may be select 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 1. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency(1) CKSEL[3:0]
128 kHz 0011
  1. 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:

Table 2. Start-Up Times for the 128 kHz Internal Oscillator
Power Conditions Start-Up Time from Power-Down and Power-Save Additional Delay from Reset SUT[1:0]
BOD enabled 6CK 19CK(1) 00
Fast rising power 6CK 19CK + 4 ms 01
Slowly rising power 6CK 19CK + 65 ms 10
Reserved 11
  1. 1.If the RSTDISBL fuse is programmed, this start-up time will be increased to 
19CK + 4 ms to ensure programming mode can be entered.