LEDs on Eval Board

I had my PropScope attached to pins 56 and 57. The PropScope was clipped to the male end of a male-female jumper. I then noticed that those LEDs were lit even if I was not using those pins. A high would turn the LED off. I disconnected the PropScope and investigated. With the jumper connected to P58, I can turn on the LED dimly by placing my fingers close to the jumper. After about a second the LED turns off. If I touch the insulation the LED turns on brighter and stays on as long as hold the jumper. With the jumper connected to P56 or P57 the LED is on all the time. I don't understand.

John Abshier

Comments

  • I can't find the exact thread were this was mentioned, but this is from another thread:
    I saw somebody post about the LEDs... It seems the natural inclination to grab the board by the left side, top and bottom. Unfortunately, fingers tend to touch the P60 header that turns the P60 and P61 leds off.

    I know Chip said some led pins are floating and so sensitive to touch...
    Infernal Machine
  • TubularTubular Posts: 3,605
    edited 2019-02-21 - 18:54:41
    The LEDs are connected to a buffer, and light when the pin is below about 1.6V* (ie they light when Low rather than high)

    Unconnected, floating prop inputs naturally float up to around 1.7v*. They are really high input impedance.

    It doesn't take much loading to bring the pin voltage down below the 1.6V threshold - connecting my 10 Megohm Fluke 75 in volts measuring range drops it to around 0.35V, and a CRO probe can load things sufficiently to change state.

    There are lot of things that can be done, eg set unused pins to "output a 150 kohm pull up", or connect the multimeter between 3v3 and the pin and invert the result mentally

    The reason the LEDs sometimes take a few seconds to extinguish after a pin has been 'low' is because the voltage is gradually climbing back towards the asymptote of 1.7V, and it takes quite a while before it crosses the 1.6v* threshold

    * The 1.7v and 1.6v are illustrative/approximate. From what I've measured so far, the pin float voltages are around 1.64 to 1.7v, I haven't spent enough time accurately checking the thresholds. Need to find the right feedback smartpin mode to help do it automatically

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