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SRAM 'OE' pin query

Discussion in 'Electronic Basics' started by mjones, Dec 24, 2004.

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  1. mjones

    mjones Guest

    Okay, just showing my ignorance here ...... the OE signal is an input.
    The OE pin (22) on a 41256SJ surface mount (J-Lead) SRAM on a board I
    have is stuck high *even when the pin is lifted* (ie the problem isn't
    originating elsewhere).

    I'd guess that means the SRAM is at fault, correct?


    Thanks
     
  2. Not necessarily. The OE pin is usually "active low" meaning that an
    external circuit must drive the pin to a low state to enable the chips
    outputs. It may have a weak internal pullup resistor that you see as a
    voltage present on the pin. Try grounding the pin and see if your
    circuit starts working.
     
  3. mjones

    mjones Guest

    I see, thanks. Well I tried grounding the pin and that pulled it low
    but made no difference.

    I should also have mentioned that this particular chip gets rather hot
    compared to the others that are the same as it on the PCB.



    Cheers
     
  4. Rich Webb

    Rich Webb Guest

    What does the data sheet say about this pin?

    Tying it low may mean that it's trying to drive its outputs at
    inappropriate times. Are there chip select and read/write strobes that
    you're also controlling?

    FWIW, the only 41256 parts that I could locate datasheets for were all
    dynamic RAMS and none had OE pins.
     
  5. mjones

    mjones Guest

    If I could understand it all I'd tell you. :)

    I got it from:

    http://www.farnell.com

    Part no: 3005392

    Just key in that number and the search result will come back.

    Then click on the Orange order code to the left, and on the next page
    click on the datasheet (PDF format).
    Not sure I'm afraid - I don't have any schems for this board.

    I do know though that this (and another RAM) is being driven by the
    output of a 7414 (Hex schmitt inverter - pin 10 (this 7414 is also
    connected to an oscillator via another pin). In fact, it was looking
    at this 7414 with my logic probe that highlighted something being
    wrong - the probe would NOT light up high or low with the probe on pin
    10 of the 7414, but on lifting pin 10 it then went low as expected.

    Re-connected pin 10 of the 7414 and traced it to pin 22 of the
    aforementioned RAM and found that lifting pin 22 (OE) of the RAM also
    caused pin 10 of the 7414 to go low (as expected) and that pin 22 of
    the RAM, when lifted, was still high (which seemed unusual, especially
    when it's also noted that this particular RAM is getting noticeably
    hotter than the others of the same type).
    Curious.

    Thanks
     
  6. Rich Webb

    Rich Webb Guest

    Ahhh... It's an AS7C256. Different critter.
    It's always possible that it's faulty. CMOS is sensitive to static
    discharge damage. I'd guess that even in the UK it gets a *bit* dry in
    the winter months... ;-)

    Otherwise, CMOS typically won't dissipate energy unless it's switching,
    so perhaps it's more active than it should be. Are all of the address
    and control lines connected to a driver or pulled high? Also, there's a
    note that "CE or WE must be High during address transitions."

    If you don't have an o-scope or logic analyzer handy, consider one of
    the gizmos from www.rockylogic.com. I've had one of their 8-channel
    analyzers since they first came out and still carry it with me in my
    notebook PC bag. Eight (even sixteen) channels aren't enough to see
    *everything* but are enough to get a better idea of what's happening.
     
  7. mjones

    mjones Guest

    Ah, good. :)
    Well, *sometimes* ............ :)
    From what I recall, I think they're all high.
    Hmmm, I see. Worth swapping it out though do you think?
    Thanks for the tip, will take a look.

    Happy Christmas. :)
     
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