Thus for naming an internal watchdog useful, it must meet the following conditions:

1. it must already be enabled by power on reset

2. there exist no way to disable it by software

3. it must have a different clock source that it never sleeps, even if the CPU enters the power down state.
comments

ad 1) irrelevant, you can make the first instruction start the WD.

ad 2) absolutely. A watchdog that can be turned off may be turned off by runaway software and thus be rendered irrelevant. My preference of internal WDs is due to this very fact. An extrenal WD require a bit toggled, which can easily happen in runaway software.

ad 3) if it does, it will wake up the processer at timeout intervals which can be desirable in some cases but in other cases it will be unwanted.

Erik