Hardware watchdogs ?

Hi all we have a number of Nano Pi Neo Air boards all running openSUSE Leap 15.x - the software watchdog with the sunxi H3 watchdog works very well. The other day, after a board got locked up hard, heartbeat LED on solid, board getting quite hot (after about 12 hours until someone noticed), I started thinking of an external hardware circuit - I was just wondering if anyone here might have looked at that already ? -- Per Jessen, Zürich (1.1°C)