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Hardware watchdog

Discussion in 'Electronic Basics' started by Richard, Oct 1, 2005.

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  1. Richard

    Richard Guest

    Sorta discussed before, but I have more to go on now. These are systems
    already deployed with a 5vdc Single Board computer which sometimes does not
    boot due a random issue when powering up. Looks like a DC converter timing
    issue and the SBC doing a brownout. Anyway, it's so random its hard to
    repeat and fix. Developing a small external watch dog will fix the very
    random problem and keep folks from driving several hundred miles to reset
    the stupid SBC.

    The design is a External Hardware Watchdog that will reset the SBC if it
    locks up, or if it powers up but "does not complete the boot process, POST,
    or anything else on power up".

    We have a three terminal plug coming into our Terminal Board. The Terminal
    board also has a 24vdc to 5vdc 40 watt Dc/Dc converter on board that
    supplies 5vdc to the SBC and some other items. The DC Converter has a
    remote power on pin, floating is On, grounded is Off.

    Our terminal board has 8 digital inputs and 8 digital outputs controllered
    by the SBC. Not all 8 of each are used (Good).
    We plan on sending a pulse out on DO8. Digital outs are ran thru a ULN2803
    darlington, that goes low to ground when On and floats to 24vdc when off.

    Terminal 1 is Switch - User Switch to turn power on and off (Actually only
    a signal)
    Terminal 2 is Ground
    Terminal 3 is Battery Positive (24 vdc all the time)

    I want to build a small dongle which will plug into the three terminal power
    socket, then the actual 3 pin power in plug will plug into my dongle.
    Easy.... No big deal here except space. The board just gets inserted in
    between the plugs.

    When the user turns on the switch, the circuit see's this, and starts a
    timer, if the timer expires (5 minutes), then it grounds the Remote Power On
    pin of the DC converter for 10-20ms then puts it back in float. Timer
    adjustable by a POT.

    When the system powers up and the application is running, the application
    will toggle DO8 every xx seconds. DO8 goes to ground when it's turned on,
    and floats at 24 volts when it's turned off (ULN2803). This ground should
    signal the pic that there is a heart beat and the timer is reset.

    If the user exists the application for maintenance, then the application
    will send 5 short pulses to the pic, telling the pic to go into Timer2 Mode
    (Extended period of time - 30 minutes or so). When the application opens
    again, it can send 2 quick pulses telling the pic to goback into Timer1
    mode. Timer2 Mode has it's time hard coded and not adjustable.

    Upon any power up, the Pic goes into Timer1 Mode for the boot process.

    See any problems with the logic?

    Circuit description...
    Space Limitation - .600" x .700"

    Input to detect switch on
    Input to determine if DO8 is pulsing
    Input to read a POT to set the timer - Do I really need this? Should I just
    hardcode a time?
    Output to Ground the DC Remote Power On pin.

    DC Converter - Specs state that the amperage on the remote pin requires
    about 10ma.
    This pin has 2 or 3 volts floating on it.

    1 - Output to DC Converter Power On pin - What type of tranny?

    1 - Detect Switch On - (May not even need this)
    1 - Detect DO8 Low - Transistor or Opto?

    Voltage Regulator
    1 regulator to convert 24vdc Nominal to 5 vdc or 3.3 vdc depending on pic

    See any Gotchas with the logic?
    Any favorite Pic Micro you can suggest?
    Voltage regulator - any suggestions?
    On the two inputs detecting 24 vdc or Ground, I was going to use Opto's, is
    there a better way such as resistor and zener?
    Transistor to switch the DC Power On pin - Any particular one to recommend?

    I could most likely power the pic from the switch on signal (24vdc) and then
    start the timer, instead of leaving it on all the time and detecting it on
    an input.

    I have to build a couple of hundred of these.... Trying to keep cost at a
    minimum, but keep the reliability up. Reliability being more important.

    Opinions, Advice, or suggestions please? I could use some help selecting
    the proper devices since electronics is not my strong side. I could patch
    something together that would work, but not knowing all the ins and outs and
    sizing I could end up with another unreliable device.

  2. Bob Monsen

    Bob Monsen Guest

    If it really is a brownout, there are these nifty micropower
    voltage detectors that will keep the microprocessor in reset until the
    voltage stabilizes. You could sample one of these and see if you can still
    make it freeze.

    MCP111 or MCP112. One of them is active high, the other is active low.
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