Electronic – reading EEPROM at startup only versus reading everytime from EEPROM

Yes, an EEPROM is a type of nonvolatile memory which can hold data even without any power for a significant amount of time (the actual time depends on temperature and other conditions).

There are a few other non volatile solutions for you to consider:

1) Internal Flash memory - Most microcontrollers nowdays have Flash for code storage and some provide it for user data and logging as well. This also has limited write cycles and requires block erase before changing bits from '1' to '0' (you store data elsewhere in the meantime and then rewrite it).

2) Internal FRAM memory - TI has devices with FRAM memory which is nonvolatile, can be accessed very simply (like any other memory location. Reads are also destructive as writes (though in your case this might not matter since the number of cycles is 10^13 or so at 85 degrees celsius).

3) External Flash memory - You can add memory externally to hold a few megabits of data rather inexpensively.

Depending on the rate at which you want to save (and whether you can shutdown nicely) you might be able to increase the effective number of cycles available for you.

Sounds fine. The way this is generally done is to have the timer rollover throw and interrupt, which sets a flag. In your main loop, handle everything that needs handling, and then wait for the flag to be set. Once set, reset the flag to zero, call your read routine, increment your index, then repeat.

This assumes you have time to do everything you need to do between timer rollovers. If you don't, it's a more complicated issue.

Pseudocode below. Each compiler will have different ways to define interrupt routines.

int1 flag=0;


null timer_isr(){
flag=1;
preload_your_timer_to_correct_value; // there are more exact ways to do this
}    


main {
            int8 x=0;
            setup_and_start_your timer;
            preload_your_timer_to_correct_value;
            x=0;  //This is your index


                while (1) {
                while (flag==0) {} //program will hold here
                flag =0 ;
                read_data_and_put_in_location x // pseudocode, obviously
                x +=;




                }

        }