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Reading pot and button. — Parallax Forums

Reading pot and button.

potatoheadpotatohead Posts: 10,261
edited 2010-12-07 22:30 in Propeller 1
I've got a old VCS game controller I want to read.

There is one switch, and one pot, which is 1 mega ohm.

I'm confused about a few basic things:

1. Can I simply connect the button to a pin, through a 1K resistor, and the other side of the button to VSS on the demoboard? (assuming the pin is input)

Is there any real worry about where that resistor is? Can I just mount it inside the game controller, for the purpose of getting a quick and dirty setup for programming?

2. How to determine a good cap, and protection resistor for the pot?

Found the attached circuit on a forum search here. Can the right cap be installed inside the game controller as well, assuming the 1 state circuit shown?

pot.bmp

Secondly, I need a sample time of somewhere around 1/120 sec, with about 8 bits precision. Honestly, this can vary some.

Is this just C = time / resistance to approximate charge, discharge times?
bmp
bmp
pot.bmp
337K
pot.bmp 337.3K

Comments

  • potatoheadpotatohead Posts: 10,261
    edited 2010-12-07 21:00
    Bump...

    I did battle on this last weekend, and just didn't have a lot of joy. If somebody could straighten me out on this stuff, I would appreciate it. I think I've got the code part down, just am not sure about the hardware part.

    I want to use the set pin state, count until pin state changes method on the Pot, BTW. Not using the PWM method, which I understand requires components in close proximity to the Prop. Perhaps the other method does too, thus the question.

    If there is something I need to add, let me know. (I'll take a RTFM with pointer too, no worries)

    Kind of wanting to take another stab at this over the coming weekend, combined with the fun VGA / TV / Sprite driver we've recently built!
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-12-07 21:12
    There is one switch, and one pot, which is 1 mega ohm.

    That might be a bit tricky as the example circuit is 10k. All the maths will be wrong.

    re C = time / resistance to approximate charge, discharge times? Capacitor charge and discharge times are non linear http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Capacitor_Square_wave_charge-discharge.svg/800px-Capacitor_Square_wave_charge-discharge.svg.png

    (at least, that is true for a constant voltage and a fixed resistor. The curve is linear if you charge with a constant current).

    I've not quite got my head around how the propeller reads voltages. Maybe you could use an op amp buffer as 1 meg is a fairly high impedance. Is it possible to open the controller and change the 1 meg to a 10k pot - then you could use all the standard circuits?

    The button will be easy though and it would not matter where the resistor is.
  • potatoheadpotatohead Posts: 10,261
    edited 2010-12-07 21:18
    Yeah, that was a warm fuzzy for me on the button. Cool. one down, and I should know better. Now I do, and I won't break something. (hate that)

    Yeah, the maths will be wrong, which is why it all kind of died?

    Can I scale a 1 meg ohm pot with a resistor in parallel, or something?

    (somebody is gonna say, just try it, and I will barring some input here)
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2010-12-07 21:19
    potatohead,

    RTFM (Read The 'Fine' Manual) :smile:

    Look at the Propeller Education Labs Fundamentals v1.1 ... http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/PELabsFunBook-v1.1.pdf

    In particular look at 'Measuring RC Decay' for reading your POTS, and 'Reading an Input' for the Switches

    'Can I scale a 1 meg ohm pot with a resistor in parallel, or something?' - You 'can' , but it won't have a linear response ... depending on the application the non-linearity could be desired.
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-12-07 21:33
    Leaving aside propeller pin input impedances, as a rough first test, if the 1 meg pot is 100x the value of a 10k pot, to get the same RC time constant, make the capacitor 100x smaller. It would be interesting to see the result.
  • potatoheadpotatohead Posts: 10,261
    edited 2010-12-07 22:30
    @Beau: Thanks!

    I had forgotten that was in there. One gets used to a certain mode of operating, and sort of ends up in "the bubble". That reference is just what I needed actually. Appreciated. At some point, early on, I had noted that for when this time came. Completely got side tracked, and didn't go down that path initially. As soon as I saw the chapter pages, I knew "been there before". Funny how that works.

    --->"fine", yes. Exactly.
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