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WAM newbie: Resistors — Parallax Forums

WAM newbie: Resistors

KB6WIJKB6WIJ Posts: 9
edited 2005-08-30 18:52 in Learn with BlocklyProp
I'm working on WAM (v2.1), chapter 3. Regarding pull-up and pull-down resistors , why not simply connect the pin in question to Vdd or Vss? Should there always be a resistance between a pin and a voltage? If so, how much?

BTW, why isn't Vss simply called "ground" or "negative"?

Thanks in advance! Joe

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-30 01:24
    Joe,

    ·· Pull Up-Down resistors are there to keep the input from floating or oscillating between states when it's not tied high/low by the button or input device.· It's possible to see inputs change states by leaving them floating and waving your hand near the pins.

    ·· For this reason the pin is pulled (not tied) to the opposite of it's active state.· The in-line resistor is there in case of a programming mistake where you accidentally make an input pin an output.· By putting that resistor there it prevents the I/O pin from being damaged if you do make that mistake.· 220 ohm resistors are used on the Homework Board for this purpose, and often used in our circuit diagrams/schematics for various input/output devices, especially switches.

    ·· By sticking to the industry standard designations, such as Vdd and Vss, we ensure that you learn the terminology used in all standard schematics and circuit diagrams everywhere.· That cuts down on some confusion later on if you pursue a career in electronics or microcontrollers.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Martin HebelMartin Hebel Posts: 1,239
    edited 2005-08-30 02:38
    Joe,
    The reason for not directly going to Vdd or Vss is that when the switch is closed, it would cause a short-circuit between Vss and Vdd, which is not good. The resistor limits current between those 2 points when the switch is closed.

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    Martin Hebel

    Electronic Systems Technologies
    Southern Illinois University Carbondale
    Personal Links - ·Lot of BASIC Stamp info
    and
    SelmaWare Solutions
    StampPlot Pro Version 3 Release 4
    Graphical Data Acquisition for your Micro and Imagination!
    Now allows additional controls to be added, or developed.
    ·
  • KB6WIJKB6WIJ Posts: 9
    edited 2005-08-30 02:53
    I haven't gotten to using pins as outputs. Can I assume that pushing Vdd into a output is a bad idea?

    Thanks, Chris and Martin! This really helps a lot!

    Joe
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-30 03:00
    KB6WIJ said...(trimmed)
    I haven't gotten to using pins as outputs. Can I assume that pushing Vdd into a output is a bad idea?
    Unless you want to see a one-time pretty light & smoke show, it's probably not a good idea!· =)

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • KB6WIJKB6WIJ Posts: 9
    edited 2005-08-30 16:34
    Chris Savage (Parallax) said...

    Unless you want to see a one-time pretty light & smoke show, it's probably not a good idea! =)
    Hey, Chris, I'm an Extra class ham radio operator. I already know how to let the smoke out of equipment that's much more expensive that a Basic Stamp! lol.gif

    But thanks for the warning anyway!

    Joe
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-30 18:52
    Joe,

    ·· All of us working with electronics have smoked something at one time or another.· The trick is to not tempt fate.· I don't!· =)· That's not to say I'm not an experimenter at heart, because I am...But a more careful one now.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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