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eprom failure modes

Discussion in 'Electronic Basics' started by Fritz Oppliger, Aug 17, 2004.

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  1. I came across a new one today:
    eprom (27c512) programs fine,
    verifies fine,
    runs like crap. lots of errors.
    Same code in an emulator runs fine.
    Same code read back into the programmer and verified again fine.
    Same code in another physical 27c512 chip runs fine (same target board).
    What could the chip be doing differently when plugged into the target
    board vs. the programmer?
     
  2. Try verifying with slight variations in the supply voltages. Also see if
    it's giving you good logic-level voltages on the outputs.
     
  3. Is this one of those chips that have a window for erasure?
    Is light shining on the chip?
     
  4. Assuming your board is designed well, just throw the darned thing away. They
    are cheap to replace.

    Jon
     
  5. John Larkin

    John Larkin Guest

    Could be slow access time.

    John
     
  6. Yes, UV erasable, with window. But there really is not much light in here.
    There is a plain white paper label stuck on the window, proudly
    proclaiming the version number I gave it.
    I have done many hundreds of Eproms but so far they either worked or
    failed to program/ verify.
    I'll chalk this one up to some subtle (ESD?) degradation that affects its
    speed.
    And I may throw it away. Or I may keep it for some future forensic need
    (with all the other stuff awaiting the same unlikely day...)

    Are you suggesting trying it out with a light-impermeable label on it?
    I can try that.
     
  7. CFoley1064

    CFoley1064 Guest

    Subject: eprom failure modes
    Hi, Fritz. First look at the numbers after the 27C512. You might find
    something like -15 (meaning 150 ns access time). If the number is different on
    the two ICs you have, that would be the primary suspect. If the numbers are
    the same, look at replacing with a faster access time 27C512. If the design is
    right on the edge with data access times, one IC could work and another not,
    but both of them meet the spec. This is the type of problem that requires
    muscling through all the data sheets for worst case specs on processor, glue
    logic if present, and EPROM on memory reads, then doing the maths. Not easy,
    but there it is. You'd be astounded, but sometimes engineers spec memory chips
    based on typical times and crossed fingers rather than worst-case. If that
    solves the problem, you're good to go. Also look to see if one IC in the
    CPU-to-memory chain has been replaced by another possibly slower one on the
    board.

    If none of these work, it could just be a defective EPROM.

    Good luck
    Chris
     
  8. James Beck

    James Beck Guest

    I'm voting for this explaination. I have had it happen in the past. I
    have yet to see a programmer that actually tests the part ANYWHERE near
    the speed the part should be capable of. The programmer is probably
    operating in the high microseconds not nanoseconds.

    Jim
     
  9. If you have a label over the window, then light is probably not the
    problem.
    But you should be aware that light definitely messes up the operation
    of these devices, and they should be kept in dim light or darkness
    during the programming and verification process, as well as when in
    use.
     
  10. Dana Raymond

    Dana Raymond Guest

    Another possibility is that the part has been stressed with ESD or some
    other operating condition. EPROM programmers do not run the part at full
    speed so it may pass in the programmer and fail in the target. It sounds
    like we're talking about a single EEPROM here - chuck it out!
     
  11. my conclusion, my response. THanks

    EPROM.
    clunk.
     
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