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Power on Reset

Discussion in 'Electronic Design' started by Benjamin David Lunt, Oct 25, 2012.

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  1. Hi guys,

    I am new to electronic design and have been building a few
    small devices. I am currently studying the TUSB2046B 4-port
    hub controller.

    One of the pins is the RESET pin and from what I have read,
    this pin needs to be held high, as soon as power is asserted,
    for 100uS to 1ms, then held low, maybe using a Schmitt Trigger.

    However, I haven't been able to find any documentation on how
    one would go about making this POR circuit.

    Would someone here please point me to a schematic or circuit
    diagram of a POR circuit that would be used for the TUSB2046B?

    Is there a chip or device, maybe a TO-92 case device that already
    exists to do this?

    Anyway, any help would be appreciated.

    Thank you,

    Forever Young Software
    To reply by email, please remove the zzzzzz's

    Batteries not included, some assembly required.
  2. John,

    Thank you for the response. This help greatly.

    May I verify that I got it correctly? I believe that if
    I use an MAX809, giving 5v0 at Vcc, Grounding the GND pin,
    and sending the RESET pin to the RESET pin of the TUSB2046B
    chip, also putting a 100k resistor on this RESET between
    the MAX809 and TUSB2046B to hold reset low due to any
    leakage that may take place, as with Figure 2, Page 6, at

    On the "Selector Guide" Table on page 5 of that same document,
    do I want a low reset threshold, MAX809Z, or a high threshold,

    Thanks again for your help,

    Forever Young Software
    To reply by email, please remove the zzzzzz's

    Batteries not included, some assembly required.
  3. And the problem with a resistor and a capacitor is....?

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  4. Guest

    it will work most of the time, but some ICs are picky about slow
    rising reset

  5. rickman

    rickman Guest

    Really? You need to ask this? What there is a power glitch and the
    power supply voltage dips below proper values, but not to ground? An RC
    circuit will typically hold up and let the device be corrupted. This is
    why many customer service folks tell you to unplug a device for a number
    of seconds, to let crappy reset circuit completely discharge.

    A proper reset chip is a good thing to use in any product that has to
    work reliably.

  6. Got it, thank you. I appreciate your help, I have learned a bit
    about Power-on Reset circuits today, thank you.


    Forever Young Software
    To reply by email, please remove the zzzzzz's

    Batteries not included, some assembly required.
  7. garyr

    garyr Guest

    How about a TPS77833 regulator, it has a POR output.
  8. Jamie

    Jamie Guest

    +-----------+-o/ o+--------+----------+
    | | .-.
    | | | | 5k
    | | | |
    + .1u --- '-'
    --- 3.3V --- +----------+ RESET PIN
    - + |
    | | ||-+
    | | ||<- 2N7000
    === +------+||-+
    GND .-. +
    | | |
    470 | | |
    '-' |
    | |
    GND ===
    (created by AACircuit v1.28.6 beta 04/19/05

    That works most of the time ;)

    Selection of a mosfet with lower Vgs(th) would be nicer.

  9. Jamie

    Jamie Guest

    Only if the chip does not have a hysteresis input. Most do.

    You simply need to calculate the raise time to make sure the hysteresis
    window falls with in the spec's of the reset time.

    But, it never hurts to add a little speed I guess and set a time gap
    externally with a square pulse.

  10. miso

    miso Guest

    Products that don't POR cleanly are a total pain in the ass. However,
    POR is a common place to skimp.

    POR is really a big deal if you have some part of the circuit that can
    draw significant power without control circuitry functioning. A SMPS is
    a prime example, where you could be "charging" the inductor while
    powering up. This alone is one reason why SMPS controller chips are far
    better to put in a product than those roll your own power supplies,
    which usually skimp or POR and/or UVL.
  11. miso

    miso Guest

    It is possible that the power supply can itself can be rising slowly, or
    even dropping as parts of the system turns on and load the supply.
    Typically chips under evaluation (pre-production) have to handle any
    voltage from 0 to abs max without exhibiting unusual behavior.

    I've gone as far in designs to include POR, UVL, and a watch dog timer
    should the product be clocked by one-shots. Pre-production evaluation
    should include shorting any two pins together, to ground, or to the
    supply voltage, without causing the chip to go haywire. This is to avoid
    the situation where the tech or design engineer poking around with a
    scope probe.
  12. Well, because it only works "most" of the time, which is a bit like
    having a spouse who is "mostly" faithful.

    Das ist absolutely verboten for any application where decent
    reliability is required, and, sadly, a typical newbie mistake.

    The only application where I'd consider it is a battery-powered type
    where the user will see it not working and will cycle power if it
    doesn't start up right, and a bad startup won't kill something like
    non-volatile memory.

    I'm retrofitting this onto an $8K-per-unit product that will
    occasionally start up with an output enabled that does some very, very
    bad things. Typical sort of setup, a microcontroller controlling stuff
    via a bunch of HC latches- the latch states are more-or-less random
    after power has been off long enough, and the micro is counted on to
    reset them during initialization- so on a less-than-clean startup
    (eg. blip during power-up) you can get the outputs to stay on
    continuously because they left out a 50-cent reset chip (one occurence
    will pay for a truckload of chips). Shockingly bad design. Oh, and
    unplug the PLCC micro (or let it fall out of the socket) and sometimes
    you'll get outputs stuck on. I would have put both a proper hardware
    reset circuit to both the micro and latches, AND an external watchdog
    timer in place (both of which are in something like an ADM805). And
    more, but that gets into system level stuff.

    The OP should pick a chip that is guaranteed to assert "reset" for any
    conditions under which bad things could happen, and is guaranteed not
    to come out of reset until the chip can operate to spec (usually it's
    a threshold with hysteresis and a time delay of some milliseconds for
    clocks to spin up, PLLs to stabilize and that sort of thing). On-chip
    reset circuits, when present, are often inadequate for the task. The
    second criteria is usually well specified, but it's often up to the
    engineer to define the first criteria-- for example at what worst-case
    _minimum_ voltage could a non-volatile memory be corrupted or at what
    worst-case minimum voltage (over temperature and unit-to-unit
    variations and vibration etc.) could a relay pull in to turn on a
    heater. Semi manufacturers will usually not tell you the guaranteed to
    "not" operate conditions (relay manufacturers usually do).

    Other than the xx805, I've often used Microchip's MCP10x series, which
    is 3-pin supervisor- a precision voltage detector and time delay of a
    few hundred msec. Simple, cheap, and works fine in many cases. There
    are better choices if you need really low power consumption or other
    features (eg. Seiko for low power). The MAX809 (eg. On-semi) that JL
    pointed to is a really popular choice that's pretty good all-round,
    3-pin, inexpensive, and currently my favorite.

    Digikey lists about 35,000 different part numbers of supervisors so
    obviously there's a recognized need! ;-)


    Best regards,
    Spehro Pefhany
  13. Argh, terrible. No hysteresis, no time delay, and assert / release
    reset levels are not controlled. Reset is never asserted if the supply
    comes up slowly, a short blip down that disrupts the digital stuff may
    not generate a reset pulse.

    In fact, if the reset input is a ST, this might actually be worse than
    an RC directly on the reset pin.

    About a dime in quantity. Check it out, if it works, spec it, sleep
    well at night.

    Best regards,
    Spehro Pefhany
  14. The switching levels of Schmitt trigger inputs are never specified in
    such a way as to be useful. Not only are they uncontrolled (in terms
    of guarantees of chip operation or benign non-operation) but they
    change with power supply voltage. With all that unspecified flipping
    back and forth they should be called Schmitt Romney triggers.

    A bulletproof POR circuit does need an absolute voltage reference in
    If you can guarantee that the power supply will always be either zero,
    within spec or transitioning in a specified way between the two that's
    true. For every other (almost every real) situation, you need a proper
    POR circuit. Digital circuits can get into untold mischief in brownout
    situations, for example.
    Time + tightly specified absolute voltage levels for on/off.
  15. He did supposedly generate a reset with Russia.
  16. Guest

    mean while the tea party gate would be desperately trying to short
    supply hoping something explodes they can blame on the Schmidtt-Obama

  17. It's even a sh*t design with just one IC that needs reliable reset
    under all conditions.
  18. miso

    miso Guest

    The bullet proof POR has crude and precise references. A bandgap may not
    be awake yet, so usually you have a crude sensor that flips when there
    is enough supply voltage to support the operation of the precise
    reference. Then you compare the bandgap to a crude reference like a VTN,
    just to make sure it is on its way to waking up, then use a one shot to
    give the bandgap time to settle. Then the POR uses the precise reference.

    There is often a crude lockout that insures the supply is a VTN at
    least, if not a VTN plus VTP.

    Timing circuits will have a parasitic diode from the supply rail to pull
    the timing cap down to at least a diode drop from ground. This is to
    insure the supply didn't get slammed to ground then suddenly brought
    back up again. That is, you want to insure the timing cap has as little
    residual charge on it as possible.
  19. Jamie

    Jamie Guest

    Then I must be the lucky one, I do PIC, AVR and now looking at some TI
    stuff at work. All which uses a simple RC reset . They all reset

    This is dealing with a supply that is there when switched on, like
    a battery for example or ready operating source via a switch..

    I just think some designs are over killed for such a simple approach
    that works in most cases. It gets to the point where people just think
    that if you had to do it once, then it means it's un holy to not ever do
    it again.

    And yes, I explore the options of a design, I just don't assume we
    need to through in the kitchen sink to start with, just because that is
    what we did last time and the time before that etc..

    I get the idea that some designers are accustom to practices only
    because it was done elsewhere but never really dig down deep to find out

    WHat ever..

  20. Jamie

    Jamie Guest

    I can't believe what you're saying..

    I bet you haven't tried that type of circuit..

    It goes to show how people open their mouth and insert..

    That circuit is a classic used in many and I mean many of reset circuits.

    I'm glad I don't take a good lot of you here seriously..

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