If an atmega328 has codeing(a program) on it, what would happen to the existing program if a new program, such as an arduino sketch, was downloaded to the atmega328?
:-?
It depends. It is possible to have more than one program loaded (like the Bootloader and a Sketch).
If the Bootloader was used to load the existing program and the Bootloader is used to load the new program, the existing program is overwritten (this is the most common case).
I'm pretty sure there's a bootloader on the device that I have, which is a SparkFun OpenLog, -
(SparkFun OpenLog - DEV-13712 - SparkFun Electronics) but I don't know if its an arduino bootloader.
I do know that the device will accept Arduino sketches, since I once did that, inadvertantly (I won't go into that :-[). However I don't know what, if anything, happened to the existing program, which came with the device.
OpenLog firmware is open source and is based on Roland Riegel's FAT library. OpenLog currently supports FAT16 (upto 2GB microSD cards). We're working on FAT32 and would love your help!
I'm afraid that my infernal "tinkering" with the device screwed it up again. :
:-[
PO'T
If you upload a program to an Arduino the existing program is erased, except for the bootloader.
Thank you MikMo and Coding Badly. 
PO'T
Looks like you can download the original program.
Look at the last entry:
http://wiki.github.com/nseidle/OpenLog/datasheet
neonpolaris
Re: Changing existing program on atmega 328, what if?
Reply #5 - Today at 14:55:48 | Looks like you can download the original program.Look at the last entry:
http://wiki.github.com/nseidle/OpenLog/datasheet
Thanks neonpolaris, I've gone over and over that page.
I've given up on that thing. I've downloaded files sent to me by SparkFun. I've tried just about everything I can think of. I tried useing a parallel programmer as well as the FTDI USB to serial programmer to reload the original OpenLog codes. The thing is stuck in a neverending loop, forever.
I put the thing aside with intentions of forgetting about it.
PO'T 