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According to the datasheet for the BR25Hxxx-2C series SPI EEPROMs, OP[3] at the 4th bit from the bottom of the opcode is written as '0', '*', or 'A8', depending on the part number, but why is this?
The operation code OP[3] of SPI EEPROMs is fixed at '0' as an industry standard if the capacity is 8Kbits or more. For the 4Kbit BR25H040-2C, the 8bit address (A[7:0]) of 1Kbit and 2Kbit products is expanded by one and OP[3] is allocated as the 9th bit (A[8]). 1Kbit and 2Kbut products do not require address expansion, so OP[3] is '*', since it doesn't matter if it's '0' or '1'.