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I dont understand this... — Parallax Forums

I dont understand this...

Lee MarshallLee Marshall Posts: 106
edited 2007-07-27 08:39 in Propeller 1
Hi, i was just looking over the datasheet for the propeller, and i cant understand the alternate serial port connection. specifically DTR to RESn.
as i understand it:
DTR 1 = -10v
DTR 0 = 10v
Maximum voltage on all pins with respect to VSS: VDD+0.3v
the capacitor there would mean that a voltage swing of +-20v would be created.

i simulated this in ktechlab, just to make sure i hadnt missed anything(im 15, i miss a lot)

would this not kill the micro?

Comments

  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-07-26 22:07
    It's not the voltage that kills, it is the power (current multiplied by voltage). Capacitors do not permit DC current to flow through them. The amount of current the capacitor will source is the change in voltage times it's capacity divided by the amount of time it takes to deliver the charge or A = CV/t, if you do the math it ends up not being much current.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.

    Post Edited (Paul Baker (Parallax)) : 7/26/2007 10:13:00 PM GMT
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2007-07-26 22:10
    Hi Mr C,
    You're right that it would kill the micro if it were connected directly and if it could supply enough current. Trouble is there is a capacitor in the way plus all RS-232 drivers are current limited by an internal 300R resistor. So there would be a little current when the dtr level changed but that would be clamped by the internal diode structure which can handle that current especially for a short period.

    Trouble with simulations is that you need an accurate model. Hooking up -10v is not an accurate representation of an RS-232 driver.

    *Peter*
  • Lee MarshallLee Marshall Posts: 106
    edited 2007-07-26 22:20
    ah, i see, thanks ppl.

    also i think i read somewhere that you can hook up a 5v input to an input pin, as long as you use a 1k resistor or something.
    is this true?

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    The Z80 Rocks!

    In a world without walls and fences, who needs windows and gates?
  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-07-26 22:48
    Yes it is.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • HarleyHarley Posts: 997
    edited 2007-07-26 22:50
    Mr Crowley,

    Please read the last thread at top of the forum in darker blue titled

    HOW TO SAFELY INTERFACE A 5V SIGNAL TO THE PROPELLER?

    for a fairly complete discussion on the 'interfacing' concerns. Yes, it's several pages long, but worth reading once at least.

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    Harley Shanko
    h.a.s. designn
  • RinksCustomsRinksCustoms Posts: 531
    edited 2007-07-27 05:57
    MR C., would you like a Z80 dev kit for S+H, i bought one before i discovered the stamp, it just sits in the closet collecting dust.

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    E3 = Thought

    http://folding.stanford.edu/·- Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.
  • Shane De CataniaShane De Catania Posts: 67
    edited 2007-07-27 08:39
    Hey Rinks, is that Z80 kit a MicroProfessor by any chance?
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