Saving a variable in between runs.

[Rickertsen2]

Originally Posted by Fat Alex

so... uh, where would i learn how to do that?

Unless you are writing a bootloader(program to allow the pic to program itself without a programmer), i can't really see why it would ever be a good idea to store anything to flash. The only advantage is that it is big... If you need to store alot of data, I would use an external EEPROM..

*Anything you write to flash will be erased if you upload a new program
*It is difficult to both read and write from
*It can cause VERY odd errors if you are not careful (you are writing to the same space as your program is executing from and if you overlap bad things will happen)
*you can only write blocks of 8 bytes at a time
*you can only erase in blocks of 64 bytes at a time
*Flash memory is only rated for 100,000 write/erase cycles. EEPROM is rated for 1,000,000

If you really have your heart set on it, the datasheet will tell you how.

Originally Posted by Mark McLeod

2) "rom const int x=10;" uses ROM or program space. You cannot change the value of x from within your program as it's running (the compiler will reject "x=y+2;" as an error). The value at program load is permanent and unchanging, only by downloading a new program can x be modified.

you CAN write to ROM at runtime but its difficult and i wouldnt' advise it.

[Mark McLeod]

Originally Posted by Rickertsen2

you CAN write to ROM at runtime but its difficult and i wouldnt' advise it.

Yes. I'm guilty of posting the "accepted" techniques rather than what is potentially possible. I've done my experimenting with self-modifying code, but I don't believe it has a place on FIRST robots, so I advise against it as well.
The big issue isn't how to do the physical write, but how to avoid clobbering your code while doing so. Your code changes size and placement in memory every time you modify it. Even if you place the bytes to be modified at the high-end of memory there is no self protect mechanism that will prevent the linker from claiming that space for code on some future date. Just wait until you try executing your variables!