I asked a question from Atmel support:
Accoring to ATXMEGA128A1 datasheet, SDRAM start address (base address) can be from 0x4000. Also the maximum SDRAM size is written 16MB. The main question is the maximum SDRAM size.
Is it 16MBytes or 16MBytes-0x4000=0xFFC000?
With regards to the implementation of the SRAM/SDRAM interface to the XMEGA, we have an application note AVR1312: Using the XMEGA External Bus Interface and the driver interface written in C is included. This application note details the usage.
I have read AVR1312 and there is noting about my question in this text. I repeat the question:
which one of the following is the maximum reachable SDRAM size (ATXMEGA128A1):
0xFFC000 = 16760832 = 0x1000000 - 0x4000 Bytes (0x4000 is the next address after internal SRAM)
0x1000000 = 16777216 Bytes?
Please refer to section `4.9 External Memory' in the XMEGA A manual.The external memory address space will always start at the end of internal SRAM.