I/O pin(s) and Pull up resistors

Hello All,

I've always stuck by the book and used the 10 k / 220 ohms combo. Works Great. But my current project (Sensor with 2 relays built in) is having me layout/think about my PCB differently. So far in my mind it is not working.

Short of testing and maybe blowing out some I/O pins to figure it out. How would/does and I/O pin respond without the 10 K resistor bypassed and only the 220 ohms resistor receiving the 5 VDC to I/O pin? I know it there for protection, but a constant 5 VDD may be to much....don't know. I've gone through the section in "Whats a MC" and other locations but can't find anything directly related. MY original concept still may not work but this will be a learning curve getting me their.

1) Can a Parallax BS2 family handle VDD on a I/O pin with ONLY a 220 ohm resistor in place?

Comments

  • 9 Comments sorted by Date Added Votes
  • kwinnkwinn Posts: 7,446
    Short answer is yes. The 220 ohm series resistor is there to protect the pin from program errors. If the pin was connected directly to +5V or GND and made an output it may be damaged if it was low and connected to +5 or high and connected to GND.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • MikeDYurMikeDYur Posts: 1,845
    edited May 15 Vote Up0Vote Down
    hmlittle59 wrote: »
    1) Can a Parallax BS2 family handle VDD on a I/O pin with ONLY a 220 ohm resistor in place?



    My take on this is that the BS2 is a 5V microcontroller, and the pins in theory can handle five volts, the 220 ohm resistor is for current limiting, and the 10k is to keep the pin floating between states.
  • Here is some important info to know about the BS2.
    1920 x 1080 - 338K
  • MikeDYur wrote:
    ... and the 10k is to keep the pin floating between states.
    The 10k pull-up is to keep the pin from floating between states. IOW, when the pin is not being driven internally or externally, the resistor will hold it at 5V. As long as the pin is being driven one way or another, you can omit the pull-up.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • The 10k pull-up is to keep the pin from floating between states. IOW, when the pin is not being driven internally or externally, the resistor will hold it at 5V. As long as the pin is being driven one way or another, you can omit the pull-up.

    -Phil

    Yes my fault, you want the pin to be one state or the other, not oscillating between the two.
  • Thanks to All for the clarification.

    I guess that I will have to use a Current Comparator after all. Gonna have to make two more chips fit.

    Thanks again
  • ercoerco Posts: 17,854
    A current comparator? Gotta see that.

    If you share more details about what you're trying to do, someone will probably have a workable solution for you.

    Yes, I speak in rhyme, all the time.
    "When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

    - Pablo Picasso
  • kwinnkwinn Posts: 7,446
    erco wrote: »
    A current comparator? Gotta see that.

    If you share more details about what you're trying to do, someone will probably have a workable solution for you.

    Yes, I speak in rhyme, all the time.

    You are the man. The rhyming robotocist and poetic propellerist.
    In science there is no authority. There is only experiment.
    Life is unpredictable. Eat dessert first.
  • Q: what rhymes on Propeller?
    A: no, it doesn't.

    Enjoy!

    Mike
    I am just another Code Monkey.

    A determined coder can write COBOL programs in any language. -- Author unknown.

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this post are to be interpreted as described in RFC 2119.
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