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Power-On Reset

Discussion in 'Electronic Design' started by Jim Thompson, Feb 9, 2005.

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  1. Rich Grise

    Rich Grise Guest

    I wondered, "WTF MEMS???" so I looked it up.

    Sh*t man f*ck - it ain't that far from reality! Nanomachines! Buckytubes!
    The one-atom motor!

    Or is that a whole nother level of reality?

    Thanks,
    Rich
     
  2. MEMS is already reality. AFAIUI, the first killer app was
    accelerometers for automotive air bags.


    Best regards,
    Spehro Pefhany
     
  3. It only works just fine some of the time- when you control the power
    on/off sequencing just so. If your 6502 is doing something critical,
    and especially if it's suppose to be running unattended 24/7 with no
    supervision you *need* a real reset circuit.


    Best regards,
    Spehro Pefhany
     
  4. I chose .25V because one POR circuit requirement was for an active low
    to be guaranteed above a .25V VCC, until the power supply was within
    spec and stable for some period. Forcing a low voltage with a .25V
    rail, under worst case conditions, wasn't an easy thing to guarantee.
     
  5. Jim Thompson

    Jim Thompson Guest

    A low VTH N-Channel?

    My usual way is to generate a LOW for the region VTH < VDD < BG-Ref*
    plus a time-out.

    * BG-Ref = bandgap reference

    In this case I need a POR to last until the charge-pump has reached
    final value, WITHOUT loading the charge-pump with any current.

    If you think the obvious, you will guess my solution, but I can't
    confirm or deny it ;-)

    ...Jim Thompson
     
  6. Jim Thompson

    Jim Thompson Guest

    My circuit that needed the POR has a MEMS... can't say what however
    ;-)

    ...Jim Thompson
     
  7. Well, it was 30 years ago, so no low Vt's. It wasn't my circuit, but
    IIRC Germanium NPNs were the only solution that worked, barely. (and
    what a PITA to get them approved, even then). ;-)
    Seems reasonable. I guess with a 0Vt you should be able to get a
    decent POR circuit.
    Well, I'm one of those humorous digital guys, so what do I know? ...but
    if I had to guess, I'd guess a mirror.
     
  8. Jim Thompson

    Jim Thompson Guest

    I had a SMPS on a GenRad portable tester that I bragged couldn't be
    killed.

    So a smart-aleck came along and did a machine-gun staccato with the
    ON/OFF switch, and flame erupted ;-)

    So I fixed his ass... I added some circuitry to the ON/OFF switch path
    that limited the recycle time to a safe value.

    ...Jim Thompson
     
  9. Tease!


    Best regards,
    Spehro Pefhany
     
  10. Jim Thompson

    Jim Thompson Guest

    You forgot the smoke ;-)

    ...Jim Thompson
     
  11. Rich, it was a 6502 which is not a recent part. It was popular in
    the early '80s. They also used the 6522 and several other chips I could
    swipe from an unwanted VIC-20 computer. In fact, I lifted the reset
    circuit from the VIC-20 service manual.
     
  12. Guy Macon

    Guy Macon Guest

    (Why did you crosspostthis to sci.electronics.cad? There is a
    reason why different newsgroups have different names; it is so
    that someone interested in electronics CAD does not have to wade
    through off-topic posts about electronics design.)

    I believe that you are incorrect. An RC circuit is a bad reset
    generator in particular applications, not in general. It depends
    on the device. Many devices do just fine with an RC. Others do
    well with a resistor/capacitor/diode circuit. A few reset just
    fine with no external components. Others really need a dedicated IC.

    In general, the reset circuit recommended in the datasheet is a
    good reset circuit. There are exceptions, but that's the general
    rule.
     
  13. Guy Macon

    Guy Macon Guest

    It says that at some time in the past it was easier to design a
    dedicated chip that behaves correctly at all voltages, and it was
    easier to design a microprocessor that expects a reset after the
    power supply becomes stable.
     
  14. Guy Macon

    Guy Macon Guest

    I have worked on systems (an I don't mean ICs) that have features that
    are less than a hundred atoms across, and seen them into mass production.
     
  15. Guy Macon

    Guy Macon Guest

    So what happened when he did the same thing with the power cord?
     
  16. Robert

    Robert Guest

    I assume you haven't seen the collections of MEMS images at:

    http://mems.sandia.gov/scripts/index.asp

    Take a look at their "image gallery" section at the bottom of the page for a
    group of micro photos of MEMS devices with some microscopic bugs included
    with for comparison. Or their "Gears and Transmissions" section for some
    lovely photos of microscopic gears and matching linkages.

    The "Movie section" is also good but I don't have a high speed connection
    for that.

    Robert
     
  17. keith

    keith Guest

    Oh, no! We silly digital folk package our magic smoke on the
    inside too!
     
  18. Ken Smith

    Ken Smith Guest

    This sounds like a job for a JFET.
     
  19. Ken Smith

    Ken Smith Guest

    I had the same problem without the smart-aleck. It was a bad contact in a
    customer supplied winch slip-ring assembly. It went open-connect-open at
    maybe 10Hz. I had to add a circuit that turned the power off if the
    current stopped.
     
  20. Ken Smith

    Ken Smith Guest

    We already have the electron-proton based M-G set so we have already
    passed the one atom point. Hint: Overhauser
     
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