The environment is an ATmega64 with a 32 KHz watch crystal running a real time clock on AVR timer0.
The problem is with an occasional watch dog reset that happens every few days or longer is resetting the AVR processor and wiping out the current real time clock values. The long term fix it to find what is causing the watchdog resets, but saving the real time clock through a watchdog reset is still a desirable program enhancement.
What I would like to do is use the MCUSR register reset flags to detect the watchdog (WDRF) flag only and treat this as a warm start (i.e. a warm start as in the SRAM real time clock variables still have valid data in them).
In order to do this, I need to read the MCUSR register and copy the real time clock variables before these real time clock variables are initialized, before main() is run. I think using a custom startup file is the correct place to access the MCUSR and the real time clock variables. The problem is I cannot see a straight forward way of creating a protected memory storage area just for the copies of the real time clock variables and the MCUSR flag.
Of course, if it is not a warm start then the normal real time clock initialization would still apply.
The idea is roughly: STARTUP- read MCUSR save MCUSR to a protected temporary location clear MCUSR (as per data sheet instructions) if MCUSR == WDRF read the real time clock values save the real time clock values in a protected temporary location
memory will be initialized before main() is run, except for the protected memory-
main()- read the MCUSR flag from the protected temporary location if MCUSR == WDRF read the real time clock values from the protected temporary location restore the real time clock values (i.e. overwrite the ICC initialized values)
I'm open to any other suggestions on how to do this.