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Serial Connection between Prop and SX — Parallax Forums

Serial Connection between Prop and SX

SailerManSailerMan Posts: 337
edited 2007-03-15 19:26 in Propeller 1
I have sucessful comms between the Prop and SX, but my question is this.. I have the prop and the SX connected with two wires an RX and a TX line.. the RX line on the prop has a resistor to lower the current. When I remove power from the SX the power light still seems to glow faintly like there is power coming from some where, when I disconnect the serial connection the power light goes out completely. Is this normal? or is there some way to prevent this.

I hope that made sense.

Thanks for any insight,
Eric
·

Comments

  • T ChapT Chap Posts: 4,223
    edited 2007-03-15 12:34
    A highspeed opto on the SX board is one idea for protection. You could isolate the Tx and Rx with relays when the SX board is off as another option. What is the LED attached to? Vcc? Make sure you have a good common ground between the boards when the SX power is off(if multiple supplies).

    Post Edited (originator) : 3/15/2007 12:56:02 PM GMT
  • BeanBean Posts: 8,129
    edited 2007-03-15 12:47
    Eric,
    The SX is getting power from the propeller though the serial line. The SX has protection diodes on the inputs to limit the voltage, so when the input is higher than Vdd the diode conducts the input pin TO Vdd. So your circuit gets powered by 3.3V - 0.7(for the diode voltage drop).

    Bean.

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  • SailerManSailerMan Posts: 337
    edited 2007-03-15 12:49
    Makes sense... but is there a good way to prevent this?· Or should I leave it alone? I just want to build the circut properly.
  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-03-15 17:01
    Depends on what your aim is, typically not enough current is provided to actually run the chip, its just leakage current. Though I was able to power a Propeller solely off an actively supplied clock signal under certain circumstances.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • SailerManSailerMan Posts: 337
    edited 2007-03-15 18:32
    I guess my aim is to figure out if it's bad designing to allow that leakage current to remain. If I want to eliminate the leakage do you have any suggestions?
  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-03-15 19:06
    Isolate it, optically or otherwise.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • T ChapT Chap Posts: 4,223
    edited 2007-03-15 19:26
    Have the Prop send a query every so often, if no response from the SX, pull the pins low. They can pull back up to query again, then drop back down if no SX found. However, if the SX were to go live and need to talk, you'd need a way to signal the Prop to go to ready mode. The diodes already offer protection from Propeller levels.

    Post Edited (originator) : 3/15/2007 7:47:19 PM GMT
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