I was checking out a FM25VN10 FRAM (an SPI device) using a Raspberry Pi, trying to put together some code for a project which will need to access it in a design. I satisfied myself that I could read and write FRAM memory locations, as well as read out the serial number and device ID.
A few days later I ran the same code and found that all the serial number and device ID bytes read out as zero (the device ID 0x7f continuation bytes remained for some reason). Importantly, I saw that my previously-written data was still there. I changed out the chip and ran the same code again on the new chip, and read out a normal serial number and correct device ID.
Can there be an undocumented SPI command which might cause the zeroing of these fields? If so, is there some way to lock-out that functionality?
