I have a product that includes an SPI EEPROM connected to a Microcontroller.
Address 0 if the EEPROM contains what we call the status word. In production the value of the status word is set to
0x2152 which indicates that the EEPROM is "alive" and the rest of the data stored in the EEPROM is sane.
If a erase/write/read/verify failure occurs we mark the status word as
0xDEAD. We also mark the status word as
0xDEAD if we detect corrupt data at boot. Note,
0xDEAD == ~0x2152
I've noticed on small population of our units when I write a value of
0x2152 to the EEPROM's status word and read it back immediately it is still
0x2152, but if I then perform a read several seconds later the value seems to "decay" to
0x2102. On a particular unit I read the value back five minutes later and it was
0x0000. All of the other locations in the EEPROM can be written to and appear to retain the proper values for long periods of time.
We do not think we write/erase to that EEPROM location frequently, nominally just once ever. We have identified a situation though where we could perform a lot of writes/erases to that location in production if some steps are not performed correctly. The endurance is a million writes and we could be hitting that.
We perform frequent reads from this location over the life of the product, we generally read every give minutes.
Previously in my career I've always seen write endurance failures look like sticking bits that seem to never take on a new value. Could this "decay" phenomenon that I am seeing also be a explained by excessive writes? Or is there another way EEPROM could become damaged that could explain this failure mode?
Answers to questions in the comments, and some tangential things:
- I am deliberately not including the part number or data sheet because we have an open case with the vendor and I do not want to disclose too much if we end up uncovering a quality issue.
- The SPI clock speed is 1MHz.
- Writes are self timed by the part. We confirm the part is done with its write before attempting any other operations or powering it down (the part signals it is done on is MISO line)
- We're using a hardware SPI peripheral with software control of CS.
- This is a bare metal system.
- We have adequate delays on power up before attempting to communicate with the part.
- We always enable writes before writing.
- Interrupts are not factor, we do a blocking write in the main thread.
- The minimum erase/write block is one 16 bit word, this part is word addressable.
- This part has a erase/write endurance or 1M cycles per word.
- The power supply to the system is very stable, the system is powered by a lithium thionyl chloride battery that has tab welded leads. It is connected to the PCB with a robust connector that is potted over so vibration/contact bounce isn't possible. The system is "always on", the microcontroller is in control of when it goes to sleep.
- The voltage at the VCC pin of the EEPROM is stable and within spec throughout the duration of a write. This was measured with an o'scope.