Some of the program section takes too much time when you start from u-boot or system. the system will be resetted if you haven't feed the dog,like MYD-SAM9X5. So, the internal watch dog won't be opened on SAM9X5 by default, if you want to get it opened, please refer to the follow steps.
1. Amend bootstrap
Amend file: AT91Bootstrap-5series_1.2/board/at91sam9x5ek/at91sam9x5ek.c
writel(AT91C_WDTC_WDDIS, AT91C_BASE_WDTC + WDTC_WDMR);
in hw_init function
2.Amend the core
Configuration of Linux core
Device Drivers --->
Watchdog Timer Support --->
<*> Software watchdog
<*> AT91SAM9X / AT91CAP9 watchdo
There will be a hybrid ：/dev/watchdog on the systen if you have completed it.
Attention: you should feed the dog every 16 seconds (default) when you opend it, or the system will be resetted!