Input Voltage

ElectronegativityElectronegativity Posts: 311
edited 2006-02-28 - 22:15:53 in Propeller 1
Is it possible to use a Propeller with 5V memory?

Would 5V damage the input pin?

Do you think 3.3V would be enough that a 5V memory would read it as high?


I wonder if this wire is hot...


  • inakiinaki Posts: 262
    edited 2006-02-27 - 20:36:03
    I have read somewhere that the Propeller is 5V tollerant.

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-02-27 - 20:37:44
    The Propeller is a 3.3v part -- we've "accidentally" tested the I/O pins at 5v and no harm seems to have been done. Still, this is out of spec and you should not do it deliberately.

    Jon Williams
    Applications Engineer, Parallax
  • GadgetmanGadgetman Posts: 2,436
    edited 2006-02-27 - 20:53:05
    Maybe Parallax could start stocking something like THIS chip?

    If so, I think many of those who have old 5V APPMods might order a few together with the Propeller...

    Don't visit my new website...
  • ElectronegativityElectronegativity Posts: 311
    edited 2006-02-27 - 21:12:07
    What happens if you run current backwards through a 2-resistor voltage divider?

    would it come through unchanged?

    I am thinking that I could divide the voltage from the 5V memory down to 3.3V at the propeller.

    I wonder if this wire is hot...
  • GadgetmanGadgetman Posts: 2,436
    edited 2006-02-27 - 21:17:48
    DO NOT use a resistor-based divider to drive the Propeller!

    Those kinds of dividers only works when the current stays constant. Any variation(the Propeller draws from microamps, into the milliAmp range... ) will translate into a variation in the voltage.
    A decent regulator will keep your Propeller much happier(and intact)...

    If you're thinking of level-matching for the IO-pins, I'd rather use a chip like those I just linked to.

    Don't visit my new website...
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-02-27 - 22:07:54
    Gagetman, the chip you provided is for LV to ULV translation (3.3V or low voltage to < 3.3V or ultra low voltage), though Im sure there is an equivalent SV (standard voltage) to LV translation chip.

    Electro, you cannot use voltage dividers for power, always use regulators for that purpose. For signal lines, voltage dividers will work when you have a low impedence source termined in a high impedance sink (fancy talk for one pin on the line is output, being read by a pin which is an input). Now for any 5V->3.3V single direction signal, the voltage divider should be used, for a 3.3V->5V single direction signal you would use no voltage divider, and the 3.3V seen by the 5V device should still interpret it as a high level. Now for bi-directional signals (such as the data bus on a memory chip) you would have the voltage divider step down the voltage in the 5V->3.3V direction, it still works for the reverse direction because you are pegging the node between the resistors to 3.3V, after the small capacitance of the 5V device's input pin is charged, there is no current flowing through the resistor between the 3.3V output pin and the 5V input pin, so the 3.3V seen on the 5V input pin should be interpreted as a high value.

  • GadgetmanGadgetman Posts: 2,436
    edited 2006-02-27 - 22:23:35
    I guess I didn't study the datasheets, just did a quick browse...

    But it is the right class of chips, though, as trying to use the output of 3.3V logic directly on 5V logic is to ask for trouble.
    (Yes, dividers can be used for inputs. )

    Don't visit my new website...
  • gibbmangibbman Posts: 98
    edited 2006-02-27 - 23:10:51

    Maybe this one?··

    There are many others at varying price ranges. I think this one will do about 400ma with 3.3 input.


    In the end, it seems that it's all about getting the LEDs to blink....
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-02-28 - 02:18:00

    ·· No that is a regulator, not a tranlsator...At a quick glance I would say this would be a closer match.

    Chris Savage
    Parallax Tech Support
  • ElectronegativityElectronegativity Posts: 311
    edited 2006-02-28 - 02:36:17
    Thanks guys, you convinced me to do the right thing.

    This looks like it will take care of the problem:*ram*%2b*memory*&crc=false&Ns=SField&N=51122


    I wonder if this wire is hot...
  • TigerTiger Posts: 105
    edited 2006-02-28 - 02:44:26
    You really don't need a "voltage divider" to drive the chip from 5v. All you need is a series resistor to limit the current so you don't pour a bunch of current into a substrate diode and latch the world up or something. A simple series resistor will provide forgiveness for 5v and still allow 3.3v operation. I think you'll find that most 5v parts will drive just fine with the 3.3v since it swings rail to rail. Can't think of anything right off that wouldn't work.

  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-02-28 - 03:57:02
    Thanks tiger, what values would be good from your experience?

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-02-28 - 05:59:03
    The solution that Jen's Altenburg used in his Parallax Prize winning autopilot is quite clear and simple. You have a 5 volt regulator feeding a 3.3 volt regulator.· A dual supply for a variety of chips.· He goes further to buffer the 3.3v I/O from the 5v. accessories.

    Regarding quesitons of timing and bridging, The Art of Electronics convieniently has a whole section of discussion on it.· It covers nearly all the permutations of voltages and chip generatons. See Chapter 9, sections 9.0 through 9.14.· Excellent stuff!

    The question is why have the second 3.3 v·regulator fed already regulated? I suppose for better regulation, less noise. But nonetheless, Tiger's solution is very appealing as you begin to fill up a board with regulators and heatsinks rather than more central components.

    I also would like to point ou the the FRAM memory [noparse][[/noparse]which I personally think will be highly useful for the Propeller due to it's fast writing speed and duriblity (see comes in an SPI package at 2.7 to 5.5 volts.

    We are entering a phase where the hobbyist will have to be more aware of power regulation. One of the greatest innovations of the BasicStamp was to eliminate·many concerns about the voltage regulation, allow a begineer to just plug in a battery, and go.

    Whatever come about needs to have similar fault tolerance and duriblity to become extremely popular.· Maybe modular adds-on will be a solution.· Or proto-type area with both regulated voltages.

    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

    Post Edited (Kramer) : 2/28/2006 6:12:48 AM GMT
  • TigerTiger Posts: 105
    edited 2006-02-28 - 22:15:53
    Paul - I'd probably make the series resistor 1K if I really needed speed and 2.7K if I didn't. If you go up to 10K, you could even run RS232 signals into the Propeller port (+/-10v!) without a problem.

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