Question about strange flash problem

Hi all,

I'm seeing a strange bug with a flash chip. So far it's isolated but I was
wondering if anyone has opinions about what might be happening here:

Chip: Spansion 32MBit
http://www.spansion.com/datasheets/s29al032d_00_a8_e.pdf

When I program certain sectors (3f8000,3fa000,3fc000,3fe000) they act like
RAM. I can write data and read data back but when I power cycle the device
the data is back to 0xFFFF...

I can think of a few explanations but I don't know enough about the inner
workings of flash to know which ones to eliminate:

1) The chip is defective and acts like a RAM chip on those sectors
2) The sectors in question are write-protected and act therefore act like
RAM
3) There's a defect on our PCB and I actually am writing to the SRAM chip
sharing the address bus
4) There's a defect on the chip that causes the sectors to be erased each
time the device powers up

Some background:
- It's an FPGA based system with flash and SRAM sharing the bus.
- We don't have any high voltages hooked up to the flash chip so our only
way of programming are the in-system CFI routines.
- The programming routines include a normal SRAM-like write cycle at the end
so that would explain how it could look like SRAM
- So far only one of several devices exhibit the problem, all others work
fine.

Thanks,
Andrew


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