Now I am using STM32F769NI Discovery Board. The board uses MX25L51245GZ2I-10G chip and I can't buy this chip for prototyping.
Please advise me a direct replacement chip so that I don't need to change the code.
RS seems to carry some MX25L51245GMI-10G
The 512 MBit flash chips are still somewhat rare, indeed. But as long as you don't need some fancy special features of a particular chip, almost any vendor's chip will do: ISSI, Micron, Macronix, Winbond, Cypress. Although there are minor differences (IDs, configuration register settings for dual/quad mode, drive strength), firmware changes should be rather limited.
And, I'd suggest to be prepared to use different chips in your final product anyway ...
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