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Can a RF transmitter generate simultaneous pulses at the output of 16 RF receivers?

Discussion in 'Embedded' started by rogerphilips, Apr 16, 2005.

  1. rogerphilips

    rogerphilips Guest

    I want to trigger an interrupt in 16 microcontrollers at the same time
    (+/- 10 microseconds).


    The microcontrollers are indoors, within 20 meters of each other.


    Is this possible using inexpensive RF parts (Linx, Micrel, RF
    Monolithics)?


    Thanks,
    Roger
     
    rogerphilips, Apr 16, 2005
    #1
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  2. rogerphilips

    larwe Guest

    That timing is pretty tight.. you might have trouble due to component
    tolerances.
     
    larwe, Apr 16, 2005
    #2
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  3. You could have the slaves transmit a signal that is transmitted
    back by the master, and thus figuring out what the delay is,
    then calibrate the receivers with the measured delays, so that
    the real trigger fires a software one-shot, and all modules start
    their routine at exactly the same moment. This can all be done in
    software, automatically.

    If a delay between master and slaves can not be tolerated, your
    specification is actually a bit more than just "all at the same time",
    it is in fact "all at the same time with no delay". Give or take a
    few microseconds.

    In that case you need transmitters and receivers with a lot higher
    bandwidth, like the ones suitable for video. There are cheap 2.4 GHz
    transmitters/receivers. But false and missed triggers are probably
    not to be tolerated as well. So I would not recommend that, unless
    your environment is guarranteed to be free from of all possible noise
    sources.

    Is there really no way to use a cable, or perhaps something optical?
     
    Frank Bemelman, Apr 16, 2005
    #3
  4. As an alternative to RF, you might consider a high-power IR LED
    and detectors. Driving 10 pulses of 1 uSec should be no problem.
    Getting detectors that will reject noise and ambient light with
    reasonable (and matched) phase delays might require a bit of
    engineering.



    Mark Borgerson
     
    Mark Borgerson, Apr 16, 2005
    #4
  5. Probably, especially if you use a tiny uC to also help with the
    time-sync.
    Simplest design would use the fast RSSI output available from some
    devices.
    Smarter design would use a RF uC, for keyless entry/remote control
    ( eg Atmel do some RF+AVRs ).
    You would send a data frame, and use every edge to improve the
    time-precision.
    With a tiny uC, you could also calibrate system delays,
    on a CAL fixture once.
    -jg
     
    Jim Granville, Apr 16, 2005
    #5
  6. rogerphilips

    mackys Guest

    Some kind of CW or OOK system, probably in the low AM band for cost
    reasons, would seem ideal for quick response as well as inexpensive parts.
    For slightly more money, but slightly better detector sensitivity
    and noise rejection, an FM/FSK part also be possible.

    The theory with CW or OOK being that you simply have the slaves
    listening for presense of absence of the radio wave. When the master
    starts transmitting, the RSSI line of the receivers should go high
    fairly quick. How quick will depend on the low pass filter that
    the RSSI is subjected to inside the reciver. The radio wave itself
    travels at what is essentially the speed of light, so over 20m the
    delay from that will be as close to nothing as you can get.

    The only downside I see immediately to this kind of setup is
    that if there's sufficient stray radio noise in the same band
    as you're using, the receivers are going to fire falsely. Two
    ways around that I can think of off the top of my head. One, use
    a radio band that's rarely used so the chances of accidental
    interference will be small - and then cross your fingers. Two,
    modulate the carrier with some sort of unique signal. Downside
    to this is that then you have to wait not just to hear some signal,
    you actually have to decode the signal and compare with the correct
    signal. This takes more time - time that you perhaps do not have.
    I looked around and couldn't find a pre-built module that would
    bring its RSSI high in ~10uS. However, this Maxim receiver chip
    seems close:

    http://pdfserv.maxim-ic.com/en/ds/MAX1470.pdf

    (More info at http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3104 - they
    even have a dev kit: http://www.maxim-ic.com/quick_view2.cfm?qv_pk=3648 )

    Page 3, near the bottom, quotes the maximum data filter bandwidth
    as 100 kHz, which will means the fastest the data output could go from
    off to on would be 10 uS. Note, this is an ASK part - it uses how strong
    the incoming signal is, rather than whether it's there or not. But I didn't
    hunt very hard, so there may be an OOK/CW part similiar to this somwhere.
    ASK might be beneficial however - the output would only be high when there's
    A) signal in the air and B) a data "1" going into the transmitter.
    However, this option will eat more power. If you're running off
    batteries it could be enough to matter.


    The above nonwithstanding, I would recommend you dig deeper and try and
    find a prebuilt module that will do almost everything for you, instead of
    trying to roll your own RF circuitry. RF is tricky business, and all sorts
    of crazy shit happens that most people versed in only digital logic would
    never expect or be able to troubleshoot. Unless you have successfully built
    RF systems before, or have a lot of time on your hands, you're almost
    always better off letting someone else deal with the difficult parts
    of RF design.


    Like the other posters to this thread, I'm curious... how come
    you can't run a wire to each uC? Or bette, a twisted pair of wires,
    RS-485 style. Better noise immunity, lower cost, easier to build.
    Slightly more hassle to install, but if you're plugging this thing
    into the wall anyway it ought to be trivial.


    -Ben
     
    mackys, Apr 16, 2005
    #6
  7. rogerphilips

    mackys Guest

    http://www.laipac.com/easy_916f_eng.htm

    This doesn't seem to guarantee RSSI high in any particular amount of
    time, but it does do 100k data rates. You could send a byte of data
    consisting of the bits "00000001". The data line sould go high at the
    last bit, which in theory ought to be very close to the same time at
    all receivers.


    -Ben
     
    mackys, Apr 16, 2005
    #7
  8. rogerphilips

    larwe Guest

    In practice, it isn't though - either due to temperature variations, or
    simple ol' component tolerances in the slicers. I spend my time at my
    day job jumping through various hoops to make realtime protocols work
    properly over radio links. Nothing is simple.
     
    larwe, Apr 17, 2005
    #8
  9. rogerphilips

    mackys Guest

    Wish I could say I was surprised to hear that. But, do you think
    they'll all fall within 10 uS of each other?


    -Ben
     
    mackys, Apr 17, 2005
    #9
  10. rogerphilips

    Tauno Voipio Guest

    For microsecond timing, a wide-band signal is needed. The
    current practice to do it reliably without excessive pulse
    power, is to use a pseudo-noise signal and a correlator
    like GPS uses.

    However, for many users, this is not more simple.
     
    Tauno Voipio, Apr 17, 2005
    #10
  11. rogerphilips

    larwe Guest

    I've tried to translate a wired protocol with 25ms timing constraints
    in it, and failed to get it to work reliably systemwide. The system in
    question is a 10kHz ASK transceiver using a simple comparator+RC
    slicer. Part of the problem is that the absolute RSSI voltage out of
    the receiver IC is not guaranteed; only the relative value (vs. two
    input signals "n" dB apart) is characterized - this causes a noticeable
    batchwise variance. I haven't really analyzed what was going on in
    great detail, because I had to try to get it to work with existing
    designs and components - since that was impossible, other methods had
    to be chosen.
     
    larwe, Apr 17, 2005
    #11
  12. Thus, you would need a baseband signal with a rise time less than 20
    us. This would require about 15 kHz baseband bandwidth. If an RF
    carrier is gated with this kind of baseband signal, this is basically
    amplitude modulation, producing two sidebands and occupying a 30 kHz
    bandwidth. The receiver filters should be wider than this to allow for
    any frequency errors in the transmitter and each individual receiver.

    Getting a legal unused channel that wide might be a problem.

    Paul
     
    Paul Keinanen, Apr 18, 2005
    #12
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