Hello Vignesh,
It depends on your safety requirments (if you want to use internal reference or external, timeout value, etc.), but basically using a clock source other than the bus clock ensures that the watchdog counter continues to run if the bus clock is somehow halted (you can check Backup reset feature on page 220 from Reference manual for more details).
Next table also sumarizes different time-out values according the selected reference clock (both enabling or disabling prescaler) so it could help you to meet your requirements:
I hope this can help you!
Regards,
Isaac
