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IO pin protection

roverxroverx Posts: 24
edited 2010-02-21 13:22 in Propeller 1
Moving from basic stamp to propeller, with the basic stamp it is/was common practice to use a 220 ohm resistor in series with IO pins , is this still the case with the propeller?

I could not find an answer during numerous searches

Comments

  • TappermanTapperman Posts: 319
    edited 2010-02-20 18:38
    I don't remember where I saw it, but I recall that in one of the labs for reading RCTIME's .. the circuit was missing the 220 ohm resister? I thought this was a mistake, but after reading further .. the lab explained the propeller has a 40ma limit souce/sink in all 32 I/O pins. It might have been in the PE Lab Kit? But I can't find it now.
  • roverxroverx Posts: 24
    edited 2010-02-20 18:44
    Thanks for the response.

    I'm designing a breakout board to use with a USB proto board. Basically an easier way to to connect IO Pins ect using pc board screw type type terminals , since I have a habbit of switching stuff around a lot. Was wondering if I should wire 220 ohm resitors as a small protection for the IOs?. (I have done similar stuff with the stamps)
  • TappermanTapperman Posts: 319
    edited 2010-02-20 19:24
    No Problem, what are you doing with your stamp? Mine is collecting dust, since I purchased the propeller. I just cant get enough of it!
  • roverxroverx Posts: 24
    edited 2010-02-20 19:27
    Still in use... while I configure the prop. Stamp is attached to max186 ADC, an RTC, 912 mhz transmitter and is used for data logging my solar panels and wind turbines.

    Gonna do the dame with the prop, but using 32 bit math and higher sampling rates
  • HumanoidoHumanoido Posts: 5,770
    edited 2010-02-21 13:22
    Both BASIC Stamps and Propellers are good tools to work with.
    About the resistor, you need it for protecting the I/O pin.
    Lets say you don't use a resistor and the line is 1 ohm with
    3.3 volts. A direct output pin is high and shorts to ground.
    According to ohms law,

    E = I R I = E/R

    I = 3.3/1 = 3.3 AMPS

    it would draw 3.3 amps which is beyond the safe operating
    range. You're likely to see smoke.

    If you keep the pin as an input, in a typical pushbutton circuit
    (for example, page 44 - Propeller Education Kit Labs: Fundamentals)
    can go from the pin on one leg to 3.3 volts on the other leg. The pin
    also routes to a 10K ohm resistor and ground. Read the information
    on pullup and pulldown circuits.

    humanoido
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