My bootloader is in SD card card? I want to check the feasibility to test DDR3 by booting primary boot loader from sd card into internal SRAM and testing DDR3. Downloading the next boot loader into DDR3
That should be possible. I believe i later versions of u-boot, special "plugin" function is used during boot to initialize memory. You may add your DDR3 test to that part of code, and still leave internal ROM to load the rest of the bootloader to memory after you've done your tests.
U-boot code related to this should be in (approximately): board/freescale/mx53_loco/flash_header.S.
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