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Input pins P16..P23 on QuickStart Board — Parallax Forums

Input pins P16..P23 on QuickStart Board

MJHanaganMJHanagan Posts: 189
edited 2014-01-26 17:15 in Propeller 1
I apply 3.3V to any of the P16..P23 pins on the QuickStart board (#40000 Rev B) and the corresponding LED lights. However, if I apply 4.8V to any of these pins all 16..23 LEDs light up. Can anyone explain why this happens?

Comments

  • JonnyMacJonnyMac Posts: 9,107
    edited 2014-01-25 17:48
    You're breaking something?

    The Propeller and 74HC541 are powered with 3.3v -- applying anything greater is looking to do damage to the board. I suspect that the 4.8 is causing some bleed in the x541 output structure.
  • MJHanaganMJHanagan Posts: 189
    edited 2014-01-25 19:49
    JonnyMac wrote: »
    You're breaking something?

    The Propeller and 74HC541 are powered with 3.3v -- applying anything greater is looking to do damage to the board. I suspect that the 4.8 is causing some bleed in the x541 output structure.

    I have a 200 ohm resistor in series to protect the input on the Propeller and I assumed the 74HC541 would be tolerant with the overvoltage (being a buffer). I am surprised that the 1.5V overage causes so much chaos with the other inputs. I suppose "hardware" does have its limits. :lol:
  • Duane DegnDuane Degn Posts: 10,588
    edited 2014-01-25 19:57
    MJHanagan wrote: »
    I have a 200 ohm resistor in series to protect the input on the Propeller and I assumed the 74HC541 would be tolerant with the overvoltage (being a buffer). I am surprised that the 1.5V overage causes so much chaos with the other inputs. I suppose "hardware" does have its limits. :lol:

    The Propeller pins need a larger value of a resistor to safely protect them from higher voltages than 3.3V. This recent thread mentions the values (3.3K or higher for 5V).

    If you make the Propeller I/O pins P16-P23 outputs then you could force the LEDs on or off. With the pins set as inputs they are left floating an small disturbances can cause a state change. Many of us have the experience of seeing the LEDs turn on or off as the board is touched. If you want an LED to on or off, you need to set the I/O pin connected to it an output and set the pin high or low to turn the LED on or off.

    The 1.5V isn't causing chaos with inputs. It's the floating pins causing the chaos. We normally don't get a visual indication of why floating inputs are pretty useless. Add a pull-up or pull-down resistor to the pins and their states won't be so easily changed.

    With the I/O pins set as inputs the LEDs make a very effective ghost detector. It's not mentioned often but the QuickStart board is better at detecting ghosts than any other ghost detecting device on the market. Or more accurately, there is not a device more effective at detecting ghosts on the market than the QuickStart board.

    Edit: I almost forgot to mention this thread where I started collecting QuickStart demos and links. I like the QuickStart servo tester, the QuickStart controlling 32 servos and the QuickStart remote controlled Roomba.
  • MJHanaganMJHanagan Posts: 189
    edited 2014-01-26 06:19
    Duane Degn wrote: »
    The Propeller pins need a larger value of a resistor to safely protect them from higher voltages than 3.3V. This recent thread mentions the values (3.3K or higher for 5V).

    If you make the Propeller I/O pins P16-P23 outputs then you could force the LEDs on or off. With the pins set as inputs they are left floating an small disturbances can cause a state change. Many of us have the experience of seeing the LEDs turn on or off as the board is touched. If you want an LED to on or off, you need to set the I/O pin connected to it an output and set the pin high or low to turn the LED on or off.

    The 1.5V isn't causing chaos with inputs. It's the floating pins causing the chaos. We normally don't get a visual indication of why floating inputs are pretty useless. Add a pull-up or pull-down resistor to the pins and their states won't be so easily changed.

    With the I/O pins set as inputs the LEDs make a very effective ghost detector. It's not mentioned often but the QuickStart board is better at detecting ghosts than any other ghost detecting device on the market. Or more accurately, there is not a device more effective at detecting ghosts on the market than the QuickStart board.

    Edit: I almost forgot to mention this thread where I started collecting QuickStart demos and links. I like the QuickStart servo tester, the QuickStart controlling 32 servos and the QuickStart remote controlled Roomba.

    I just replaced the 200 with a 5k resistor. With the input on P22 it now lights LEDs 22 and 23. A bit of an improvement but the 74HC541 still seems to be activating adjacent outputs. Is the input LED in the 74HC541 being driven so much that it is activating adjacent photodiode(s)??
  • Duane DegnDuane Degn Posts: 10,588
    edited 2014-01-26 08:40
    MJHanagan wrote: »
    I just replaced the 200 with a 5k resistor. With the input on P22 it now lights LEDs 22 and 23. A bit of an improvement but the 74HC541 still seems to be activating adjacent outputs. Is the input LED in the 74HC541 being driven so much that it is activating adjacent photodiode(s)??

    It's because input side of the 74HC541 is not pulled high or low. This floating state is easily influenced by nearby circuits. If the input side wanders high, then it will actively drive the output high. The 74HC541 is in limbo when not connected to something actively driving it.

    If you connected P23 to something either high or low, the state of P22 won't have much of an influence on the adjacent input state. As it is, the LED associated with P23 just reflects the state of the floating I/O pin.
  • JonnyMacJonnyMac Posts: 9,107
    edited 2014-01-26 09:05
    Instead of using a simple series resistor, you could you a voltage divider ( 6.8K + 10K ) -- that combo would drop 5v down to 2.9v.
  • MJHanaganMJHanagan Posts: 189
    edited 2014-01-26 17:15
    JonnyMac wrote: »
    Instead of using a simple series resistor, you could you a voltage divider ( 6.8K + 10K ) -- that combo would drop 5v down to 2.9v.

    Yes, I had to use a voltage divider to get the adjacent LEDs to stop ghost triggering. Wow, sensitive little things.

    Thanks everyone!
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