How do I cause a watchdog reset of my embedded Linux device

If you have a watchdog on your system and a driver that uses /dev/watchdog, all you have to do is kill the process that is feeding it; if there is no such process, then you can touch /dev/watchdog once to turn it on, and if you don't touch it again, it will reset.

You also might be interested in resetting the device using the "magic sysrq" way. If you have a kernel with the CONFIG_MAGIC_SYSRQ feature compiled in, then you can echo 1 > /proc/sys/kernel/sysrq to enable it, then echo b > /proc/sysrq-trigger to reboot. When you do this, it reboots immediately, without unmounting or or syncing filesystems.

If you think the watchdog is running OK, and want to test that it really is capable of recovering a crashed system, then you can do one better than Shawn's answer by using the "magic sysrq" to crash the system with a kernel panic. Syncing your file system first is a good idea, so do something like this as root:

sync; sleep 2; sync; echo c > /proc/sysrq-trigger

That should cause a kernel panic if the sysrq is enabled. Then if you wait around 60 seconds (typical time-out for the watchdog module) you should see the machine reboot. Please note this will only work with hardware watchdogs and not the 'softdog' module.