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Help with power on reset

Discussion in 'Electronic Design' started by galapogos, Jan 9, 2007.

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  1. galapogos

    galapogos Guest

    I have a chip that has a reset input that is currently connected with a
    simple RC circuit, according to reference design. It is an active high
    reset signal, i.e. when power is applied, the chip resets. However, I
    wish to control the actual reset of the chip with an external signal
    via a MCU's reset pin. I have tried to connect a MCU's GPIO pin to the
    chips reset, and it works, but I'm wondering if it's a good idea to do
    that with the existing RC circuit in place? Since my MCU will be
    holding the reset signal low until it decides to reset the chip,
    wouldn't I be better off with a pulldown instead of a pullup for the RC
    circuit(pulldown because on system startup the MCU's IO pin is high,
    and only a split second later does my firmware run and set it to low,
    so for a split second the chip reset could be high instead of low,
    triggering a reset when I don't want it to happen)?

    I'm sorry if this is a stupid question. I don't have any experience
    with reset circuits. Thanks!
  2. I do not understand why use a GPIO to control reset?

    If you really want to do like this,RC is no need.

    "galapogos дµÀ£º
  3. Guest

    I can't follow this since at times you indicate reset is high, and
    other times reset is low. If reset is low, you would call it reset bar.
  4. mkaras

    mkaras Guest

    If the RESET to your microcontroller is high active then that is what
    it is and it will require a high level signal to assert the reset.
    Whatever pulldown/pullup you try to apply will have to take this into
    account. If you intend to have the MCU able to reset itself from the
    use of an I/O pin that pin will need to be set high as an output to
    cause the reset. You do have some issues to consider however.

    First off you will still need to provide for a valid initial reset at
    power up. Whatever you do for this will have to somehow combine
    logically with the port pin logic you want to use and whatever other
    reset sources you may have such as a manual reset switch. The app note
    reset circuit consisting of the resistor and capacitor may be suitable
    for this initial power-on reset or it may not. Many designers opt to
    use one of the more predictable reset generator chips (also known as
    microprocessor supervisory chips) that are made by the likes of TI,
    Maxim, National, Philips or Analog Devices.

    Secondly you need to consider what happens to the microcontroller when
    its firmware programs an I/O pin to output and drives the reset line to
    the same microcontroller to an active assertion. This will almost
    instantly stop the program that is running and return the
    microcontroller back to almost the intial start up conditions. This of
    course will include returning the I/O pin back to its default state
    which is most likely as a high impedance input. This subsequently has
    the effect of causing the firmware asserted reset signal to be taken
    away. For a great many microcontrollers the time the reset signal stays
    asserted in this senario is shorter than the minimum reset assertion
    time that the MCU manufacturer recommends for their device. So you may
    very well need to arrange the circuitry so that the I/O pin triggers a
    reset assertion through some type of pulse extension scheme via a
    resistor/capacitor circuit or through the use of the "reset switch
    input" that can be found on many MCU supervisory chips.

    - mkaras
  5. Guest

    Hi Here your capacitor will take of that short unwanted glitch. Pull up
    or pull down may not matter much. your RC timing should be designed in
    such a way that the unwanted spike or glith pulse width is less than
    your RC timing

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