We have encountered a strange issue on certain 56F84789 chips. The last 4KW of RAM (0x3000-0x3FFF) is unusable. On read, each word is 0xFFFF. Attempting to write a word in that range has no effect.
We only find this problem with two specific date codes so far, all other date codes appear normal.
1) Is this a silicon fault?
2) Are there any configuration bits that could stop part of RAM not working as expected?
3) Has anyone seen this type of problem?