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Tri-State Digital input

Semih YELKEN
Prodigy 10 points

Hello
I want to read DC 24V status GND status and open circuit (high resistance) digitally from microcontroller. I am waiting for your suggestions for this.

  • 0
    TI__Guru*** 135355 points

    Hello Semih,

    Are you looking for a microcontroller recommendation?

    Any MCU will require external circuitry to handle such a high DC voltage.

    Any other requirements you have could help narrow down your options on what a good fit would be. Or if you know which MCU you are going to use, then include that information.

  • 0 in reply to Ralph Jacobi
    Guru 117855 points

    Hello Ralph,

    Indeed poster's request is not especially clear.   And - just as you well note - external circuitry IS required...

    Young staff presents "their" ideas on how (this) poster (and others) may introduce outside world signals (beyond) any MCU's input capabilities:

    • Introduction of external signals which, "Exceed the MCU's specified "Vin(max)" (referenced to ground)

    Should the "high level" of such external signals exceed MCU spec - imposition of:

      • Voltage Divider
      • Opto-Coupler
      • Level Shifter (may be a dedicated IC or an "open-collector/open-drain" device "pulled-up" to ~3V

    The intent is that the external signal will be constrained to arrive at the MCU input at either ~3V or Gnd signal level.   (Never driving below Gnd, (i.e. negative) as well).   This well addresses poster's 24V & Gnd. requirement.   We shall name this GPIO as, "Protected Signal Input and/or buffered input "

    • open circuit detection

    This proves more complex due to the "added devices" (as above) which prevent the external signal from arriving directly at the MCU.   

    Case 1: The MCU's buffered input is at/near ground.   (which indicates that the external signal is either @ Gnd or is Open Circuit.)   This is resolved via a 2nd GPIO which drives a "small signal transistor" (normally held "Off") - which when "switched on" will apply a (current limited) test voltage to the external signal node.  The presence of this "briefly switched on (new) voltage" - as detected by the original GPIO employed as "Protected Signal Input" (which swings now to logic high) - indicates the Open Circuit Condition.   (Had the external signal been at Gnd potential - the (weak) newly switched test voltage would not have been able to create the logic high @ buffer input.)   Less obvious - but clearly beneficial - if the added transistor switches in (this user's) 24V - then (both) Open Circuit AND 24V presence are "detectable!"

    Case 2: The MCU's buffered input is at/near logic high.    With the 2nd GPIO in idle (switched Off) - it must be that the external signal is at/near "high" - thus the Circuit is Not Open!

    These techniques may be employed w/MCUs and/or other ICs - which have similar "input Signal Restrictions."    It is usual that "multiple such signals must be input."   There exist "multi-channel" voltage translators as well as (standard) 4 channel Op-Amps or Analog Comparators - which voltage translate 4 channels via 1 chip...