I'm using Raspbian Linux on 2 Pi's and I was successful in enabling the hardware watchdog after a lot of failed attempts because of wrong manuals on the net.

Edit the Raspbian boot config file to enable the watchdog

root@raspbianpi:~# vi /boot/config.txt

Add the following somewhere at the end:

dtparam=watchdog=on

Reboot your Pi

root@raspbianpi:~# reboot

After the reboot install the watchdog software

root@raspbianpi:~# apt install watchdog

Edit the watchdog configuration file

root@raspbianpi:~# vi /etc/watchdog.conf

and add:

watchdog-device = /dev/watchdog
watchdog-timeout = 15

Start and make sure the watchdog daemon starts at boot

root@raspbianpi:~# systemctl start watchdog
root@raspbianpi:~# systemctl enable watchdog