14.6 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 14-12. 128kHz Internal Oscillator Operating Modes
Nominal Frequency(1) CKSEL[3:0]
128kHz0011
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 14-13. Startup Times for the 128kHz Internal Oscillator
Power ConditionsStartup Time from Power-down and Power-saveAdditional Delay from ResetSUT[1:0]
BOD enabled6 CK19CK(1)00
Fast rising power6 CK19CK + 4ms01
Slowly rising power6 CK19CK + 65ms10
Reserved11
Note:
  1. If the RSTDISBL fuse is programmed, this startup time will be increased to 
19CK + 4ms to ensure programming mode can be entered.