Axiomtek FDK172-834 - Manual (Page 65)

FDK172-834 User’s Manual
Watchdog Timer 59
Appendix A
Watchdog Timer
About Watchdog Timer
Software stability is major issue in most application. Some embedded systems are not
watched by human for 24 hours. It is usually too slow to wait for someone to reboot when
computer hangs. The systems need to be able to reset automatically when things go wrong.
The watchdog timer gives us solution.
The watchdog timer is a counter that triggers a system reset when it counts down to zero from
a preset value. The software starts counter with an initial value and must reset it periodically. If
the counter ever reaches zero which means the software has crashed, the system will reboot.
How to Use Watchdog Timer
The I/O port base addresses of watchdog timer are 2E (hex) and 2F (hex). The 2E (hex) and
2F (hex) are address and data port respectively.
Assume that program A is put in a loop that must execute at least once every 10ms. Initialize
watchdog timer with a value bigger than 10ms. If the software has no problems; watchdog
timer will never expire because software will always restart the counter before it reaches zero.
Begin
Enable and Initialize
Watchdog Timer
Program “A”
Disable Watchdog
Timer
Next
Next
Next
Next
Begin
Enable and Initialize
Watchdog Timer
Program “A”
Reset Watchdog
Timer
Next
Next
Next
Next