until today I have believed changing Watchdog prescaler in AVRs need the timed sequence - setting WDE and WDCE and in 4 clock cycles change the prescaler. But I did not use it today on ATTiny85 by mistake and it worked anyway! So I started looking for information. My ATTiny25/45/85 Datasheet says the WDT have 2 safety levels: when WDTON fuse is not programmed, timed sequence is needed for clearing WDE bit only. When WDTON fuse is programmed, timed sequence is needed for changing prescaler and changing WDE is impossible. The same information is i.e. in ATTiny2313 Datasheet but most other AVRs have this formulation:
The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System
Reset mode. With the fuse programmed the System Reset mode bit (WDE) and Interrupt
mode bit (WDTIE) are locked to 1 and 0 respectively. To further ensure program security, alterations
to the Watchdog set-up must follow timed sequences. The sequence for clearing WDE and
changing time-out configuration is as follows: (...)
But otherwise the WDT looks very similar. In AVR132 appnote in section 2.5 they say:
If the WDTON fuse is unprogrammed on ATtiny13 and ATtiny2313, it is possible to
change the WDT timeout period without following the timed sequence.
But both ATTiny13 and ATTiny13A have the more common formulation quoted above. So - are there 2 very similar versions of WDT in AVR devices or is the timed sequence not needed when changing prescaler with WDTON unprogrammed?