11.6 128 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 11-12. 128 kHz Internal Oscillator Operating Modes
Nominal Frequency(1) CKSEL[3:0]
128 kHz0011
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:

Table 11-13. Start-Up Times for the 128 kHz Internal Oscillator
Power ConditionsStart-Up Time from Power-Down and Power-SaveAdditional Delay from ResetSUT[1:0]
BOD enabled6CK19CK(1)00
Fast rising power6CK19CK + 4 ms01
Slowly rising power6CK19CK + 65 ms10
Reserved11
Note:
  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.