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 1. 128kHz Internal Oscillator Operating Modes
Nominal Frequency(1) CKSEL[3:0]
128kHz 0011
Note:
  1. The 128kHz oscillator is a very low power clock source, and is not designed for high accuracy.

When this clock source is selected, startup times are determined by the SUT fuses:

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