If you are using an external watch dog chip it can be problematic trying to use an internal SRAM location as a flag for detecting if a reset was due to power-up or from the watch dog. The reason for this is that at power-up the SRAM location could have any pattern in it so it could come up matching the special flag value you have selected in your code!! That may not be a reliable enough scheme!!

I have seen some people try to solve this problem by using multiple memory locations but that just lowers the chances and does not really solve the problem.

So for you you really have two choices for a reliable design. One way is to select a microcontroller that has an on-board WDT function. These devices very often have a register that you can read at the start of your code that indicate what as the source of the last RESET signal.

On the other hand if you need to use an external WDT then you either need to have extra circuits that latch the WDT reset and make it available on a port pin or find a supervisor that has this functionality built in.

Michael Karas