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Active Low or Active High

Discussion in 'Embedded' started by Harry, Oct 17, 2007.

  1. Harry

    Harry Guest

    Hi all,

    We all know that microprocessors/microcontrollers have active low &
    active high signals.Is there any specific advantage for active low
    signals?Can't it be all active high or active low?
    Harry, Oct 17, 2007
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  2. Most digital logic is edge-triggered or edge-sensitive,
    meaning one edge is the trigger and the other isn't. The
    important edge must be the fast one, and some technologies,
    such as TTL, pull faster edges in one direction than the other.

    As a result, TTL tends to use active-low, since it's the high
    to low transition that's the fast one.

    Clifford Heath.
    Clifford Heath, Oct 17, 2007
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  3. For microcontroller outputs that drive things in the real world, one
    thing you might want to look at is any internal pullup / pulldown
    resistors. It can be an advantage if you design your outputs so they
    are automatically pulled to the "off" state during reset.
    John Devereux, Oct 17, 2007
  4. Broadly yes.
    In the details it can matter :
    If you are driving a LED, the LED polarity dictates your active level.
    Ditto for if you are driving a NFET.
    If you want to "OR Wire" a BUS, that usually points to Active-Low,
    Open-Collector topology in the simplest form.
    NFETs drive more than PFETs on a same-area basis, so if you want highest
    drive, into relays, or Solenoids, that's usually NFET active low.

    Jim Granville, Oct 17, 2007
  5. Yes, the Select lines, Reset lines and many others are active low.
    Especially the control signals are active low.
    This is because, most logic families can sink more current than they
    can source.
    That is, because of this, the logic gate output will be high to drive
    a number of inputs of other logic gates of the same type.

    Karthik Balaguru
    karthikbalaguru, Oct 17, 2007
  6. Harry

    Guy Macon Guest

    Read the datasheet and figure out whether the pins are high, low,
    or floating during and right after reset. Then design so that is
    the safe condition (motors off, etc.).
    Guy Macon, Oct 17, 2007
  7. It is the deep historical and religious question. It has to do with the
    fact that for the ancient 74xx series the outputs normaly act as drains,
    and the inputs act as sources.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    Vladimir Vassilevsky, Oct 17, 2007
  8. I always thought that it went way back, in relay circuits. I have an
    idea that it was so you could not easily short the supply to ground
    and pop fuses.

    Martin Griffith, Oct 17, 2007
  9. Back in the day, TTL inputs floated high. Therefore interface
    signals were often made active low so that they would float to
    the inactive state.

    These days, CMOS logic inputs don't typically float high, so
    there's no overriding reason to use active low. Active high is
    more "common sense" so it tends to get used more on new
    designs, but there are still alot of interfaces that were
    spec'ed in the TTL days.

    So, you'll see both active high and active low.
    Grant Edwards, Oct 17, 2007
  10. Harry

    Tauno Voipio Guest

    There were other causes, too.

    The noise margin of a TTL high is much
    larger than the noise margin of a low.

    Selecting low active minimized the risk
    of data corruption in inactive parts
    by noise.

    Also, before three-state TTL chips, a
    bus was built using open-collector
    elements, which made passive high
    a necessary selection.

    Been there - done that (about 40 years ago).
    Tauno Voipio, Oct 17, 2007
  11. Harry

    msg Guest

    Mustn't forget other open-collector families such as DTL...


    msg, Oct 17, 2007
  12. The active low was very common on old bipolar logic families, such as
    RTL, DTL or TTL.

    Since in those days it was not practical to make both NPN and PNP
    transistors on the same chip, so only NPN transistors were used. The
    output stage was a Common emitter NPN stage and it was easy to drive
    current into the base of the NPN, putting the transistor into
    saturation, with a large current flowing from collector to emitter
    with a low voltage drop, thus the output is low, when current flows
    into the base of the output transistor.

    It would be very hard to make an active high output stage, since an
    emitter follower transistor with the collector at Vcc and the output
    at the emitter. You would actually need an additional supply voltage
    higher than Vcc in order to be able to drive sufficient current into
    the base of the NPN transistor in order to get the output voltage even
    close to Vcc.

    Of course ECL used NPN emitter followers with negative Vee at -5.2 V,
    but that was an oddity.

    Paul Keinanen, Oct 17, 2007
  13. Harry

    Guy Macon Guest

    The original relay circuits were mostly used in positive ground
    telephone circuits, so it is the high condition that is less likely
    to short out and blow fuses.

    It's an interesting story as to how it came to be that telephones
    are mostly positive ground and modern cars are mostly negative
    ground. When Michael Faraday did his pioneering work on electrolysis
    he determined that what we now call the positive electrode (anode)
    lost material and what we now call the negative electrode (cathode)
    gained material. So he called the electrode that was putting material
    into the liquid positive and the electrode that was taking material
    out of the liquid negative. He had no way of knowing that there were
    electrons moving from negative to positive and ions moving from
    positive to negative.

    Because whichever part of the system is negative gains metal and
    whichever part of the system is positive loses metal, the designers
    of the telephone system made the ground positive, because in a
    telephone system, ground stakes are a lot easier to replace than
    wires are. In automobiles, body and frame parts are harder to
    replace than wires are, so the automotive designers eventually
    settled on negative ground.

    Telegraphs had a different system: a battery on each end. This
    resulted in negative ground at one end and positive ground at
    the other end, and ground halfway between positive and negative
    somewhere in between.
    Guy Macon, Oct 17, 2007
  14. Harry

    CBFalconer Guest

    Actually the immediate predecessor (to NPN DTL gates) were PNP DTL
    gates, which swang between a negative voltage and ground. However,
    the ground level was still the power direction. They went out
    about 50 years ago, but reappeared in early PMOS designs.
    CBFalconer, Oct 18, 2007
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