inner_ideas 
Dear all,
Please update me with key difference between Atxmega128A3-AU and Atxmega128A3U micro controllers?
I have seen pins on both micro controllers are same.
While Atxmega128A3-AU data sheet is saying"Not recommended for new applications use AU series"? Is this micro controller having issues in it?
Please update. Thanks[/b]
bloody-orc 
-AU should be packaging notifier and nothing more. 128A3U is a new revision of the 128A3 series and is reccomended usually over 128A3
The 128a3 and the 128a3u are almost the same, the 128a3u additionally has a USB interface. As stated, you should use the 128a3u for new designs. In old designs it should be possible to use the 128a3u as replacement. For differences see app note AVR1020.
-AU denotes the package, -AU is TQFP. There is a 128a3u-au and a 128a3-au.
inner_ideas Dear ll,
Can you please guide me regarding code;
Is code written and compiled for ATxmega12A3U can easily program into Atxmega128A3-AU?
If not, Is i need to recompile the code for ATxmega12A3? because AVR studio did not have any device with the name of ATxmega128A3-AU in device selection list.
clawson 
Is code written and compiled for ATxmega12A3U can easily program into Atxmega128A3-AU?
I don't know if you know this but the original models of Xmega had some very serious flaws in the silicon design (the ADC in particular was close to unusable) so what Atmel are tending to do is phase out a lot of the earlier models and replace them with a virtually identical device but one that (a) has the flaws fixed and (b) often has USB added for "nothing". In fact these later devices tend to actually be cheaper than the originals. So when you see "U" added to the model number (on the left of the hyphen not the right!) it means that device has had USB added but otherwise it is pretty much identical to the previous model. (and it actually works!).
If you were already looking to use USB (perhaps UART via a virtual COM port) to your Xmega design you no longer need an additional chip but get the feature for free.
On the whole this is all "win, win" and there is no downside.
In Studio the only relevant bit of the model number is the part to the left of the hyphen. As you have already been told above "-AU" just tells you something about the package type and the industrial/commercial temperature range of the device and has no influence on how you program it.
If you have an ATxmega128A3-AU then in Studio the device selection to build for is simply "ATxmeag128A3". If you have an ATxmega128A3U then the device you build for is "ATxmega128A3U". About the only thing that changes is that the latter "knows" about registers that control its USB interface.
BTW if you do have an ATxmega128A3U the chances are it's really something like ATxmega128A3U-AU, even these devices have order numbers with something like "-AU" to the right of the hyphen that say something about their package and temperature range.
Dear ll,
Can you please guide me regarding code;
Is code written and compiled for ATxmega12A3U can easily program into Atxmega128A3-AU?If not, Is i need to recompile the code for ATxmega12A3? because AVR studio did not have any device with the name of ATxmega128A3-AU in device selection list.
I would not advise that. Choose Atxmega128A3 instead of Atxmega128A3U in your IDE/Compiler and recompile. Check wether you use features that a different among these MCUs.
the -AU doesent matter, its just the package.
Currently using an Atxmega256a3 and looking to transition to an Atxmega256a3u. There are some name changes in the header file (iox256a3.h vs iox256a3u.h). So when you swap and recompile you will have to make a few name changes in your program. For example, OSC_RC32MCREF_bm in 256a3 becomes OSC_RC32MCREF0_bm and OSC_RC32MCREF1_bm in 256a3u. These are not major changes but results in some tweaks to the code to make it compile with the new board.
