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Receiving low frequency square wave fm question

Discussion in 'Electronic Design' started by Lukos, Mar 7, 2007.

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  1. Lukos

    Lukos Guest

    I am looking at circuit design to use for model railway DCC design,
    specifically how to design a circuit to receive and decode it.
    The data is transmitted serially as FM data at approx 9600bps on a
    differential (RS485 I think) line where a 1 bit is a positive and
    negative half cycle of no more than 64uS and a 0 bit has half cycles
    in excess of 70uS.
    The current designs I have seen use a PIC to decode the data by
    sampling 22uS time slices and using the number of time slices to
    calculate whether the current data is a 1 or a 0 but this appears
    overly complicated since the data cycles are of a defined duration.
    Does anyone know if there is an IC (or combination of ICs) that can be
    programmed simply to receive this type of FM data and decode the bytes
    which could then be passed in parallel to a PIC for the functional
    decoding? You can assume that the electrical line is already received
    to produce a TTL signal of the incoming data (i.e. between 0 and 5v
    instead of differential). I have seen many flavours of UARTs but these
    don't seem to work with modulated signals, only with unmodulated
    serial data.

    Thanks in advance

  2. J.A. Legris

    J.A. Legris Guest

    This is exactly the sort of application for which a PIC (or other
    simple uC) is suited. It couldn't be much simpler.
  3. Well, it's a little more complicated than that. 0 bits can be stretched
    into extremely ugly shapes and 1 bits aren't whole lot better. It'd be
    pretty tough to do this without a microcontroller. I'm not saying it
    couldn't be done, but I'd bet that any single chip solution you find that
    can "just do this" is going to be a pre-programmed micro of some sort. All
    in all it's a pretty ugly way to do things IMO, seems like they could have
    come up with something a little more backwards compatible.
    Yes, a micro can be programmed to do this, but you already know that.
    I'm curious as to why that is. Are you hacking something? ;-)
  4. Lukos

    Lukos Guest

    I'm curious as to why that is. Are you hacking something? ;-)

    No, it was just I didn't want someone to waste their time explaining
    about the decoding of the bus into a TTL digital system since I
    already know how to do that with RS485 receivers etc.

    I want to build my own accessory decoders since the pre-manufactured
    ones are quite pricey for what they are and you need loads of them for
    a reasonable sized layout. Also I am trying to build some memory wire
    point motors which cannot be driven directly from a standard accessory
    decoder - I will use a regulator configured as a constant current
    source for each wire.

    Thanks for everyone's help - a PIC it is then : )

  5. That's what I would likely do. Aren't there any cheap pre-programmed PICs
    out there? I looked a little, but didn't find anything. Heck nowadays you
    can even find OBDII interface chips (PICs) for under $20, but I guess even
    that might be pretty steep for you if you need a bunch.

    From what I've been able to find, you won't need any specialized level
    converters just to receive data. It looks like four diodes to recitify the
    voltage from the tracks for a power supply and you can pick the DCC straigh
    off one of the rails. You might like this page:
  6. Paul Burke

    Paul Burke Guest

    You might want to look at the UK Model Electronic Railway Group:

    They produce kits for various DCC functions, and I'm sure if you join
    you can participate in the designs.
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