This AT24C256 library ? https://github.com/Polohpi/AT24C256
That library uses default 0x50 as I2C address and your chip is at 0x51.
If the device does not exist, then the library returns 0xFF.
This AT24C256 library ? https://github.com/Polohpi/AT24C256
That library uses default 0x50 as I2C address and your chip is at 0x51.
If the device does not exist, then the library returns 0xFF.