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3.6864 MHz

Discussion in 'Electronic Basics' started by pawihte, Sep 15, 2010.

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  1. pawihte

    pawihte Guest

    The answer to this question will probably make me feel stupid,
    but here goes.......

    I know of common applications for crystals and oscillators
    operating at 32.768 kHz, 2.048 MHz, 3.579 MHz, 4.194 MHz, 4.433
    MHz, 27 MHz, etc etc.(and in general why those figures are
    chosen), but I don't know the significance of 3.6864 MHz which,
    judging from its ubiquitousness on shop inventories, must have a
    common application. There must be many more standard frequencies
    that I don't know about, but I keep coming across this one in
    recent times. Is it in audio? Data transmission? Please enlighten
  2. Rich Webb

    Rich Webb Guest

    One reason (there may be others) is that this freq and its close
    relatives 7.3728 and 14.7456 MHz divide-down nicely to standard serial
    baud rates with integer divisors.
  3. pawihte

    pawihte Guest

    Thanks to all those who took the time to reply.

    So that number is based on an integral multiple of standard baud
    rates like 19.2k. Still, I can't help wondering why they simply
    didn't standardize on binary multiples like 2.4576 MHz, etc.
  4. Rich Grise

    Rich Grise Guest

    There is no "stupid" on sci.electronics.basics, the operative word
    being "basics," which start at zero.

    There are some stupid people who troll the group, but just ignore
    them - it won't take long to figure out which they are. :)
    The only one that jumps off the tip of my frontal lobe is 3.579545,
    the color burst for NTSC color. Could it be PAL?

    It could also be in the 80 meter ham band, which last time I
    looked was something like 3.5 - 4.0 MHz.

    Good Luck!
  5. pawihte

    pawihte Guest

    Nope. I'm familiar with the chroma sub-carrier freqs of 3.58 and
    4.433 MHz (rounded off here) for NTSC and PAL respectively. I
    accept the other replies saying that it's an integral multiple of
    standard baud rates.
  6. Rich Grise

    Rich Grise Guest

    Yeah, after I blurted out my answer, I read more of the thread,
    and that seems to be the best answer. :)

  7. Nobody

    Nobody Guest

    Binary multiples of *what*, though? The ratios of the standard baud rates
    aren't all powers of two; some involve a factor of 3 (14400 is 4800*3).

    Using 300 multiplied by a power of two wouldn't give you any easy way to
    obtain 14400, 28800, 57600 or 115200 (you would need to multiply
    by 3, which is much harder than dividing by 3). Including the factor of 3
    means that you can get all of the standard rates by dividing by
    (2^N * 3^M) where M is 0 or 1:

    3686400 / 16 = 230400
    3686400 / 32 = 115200
    3686400 / 64 = 57600
    3686400 / 32 / 3 = 38400
    3686400 / 128 = 28800
    3686400 / 64 / 3 = 19200
    3686400 / 256 = 14400
    3686400 / 128 / 3 = 9600
    3686400 / 256 / 3 = 4800
    3686400 / 512 / 3 = 2400
    3686400 / 1024 / 3 = 1200
    3686400 / 2048 / 3 = 600
    3686400 / 4096 / 3 = 300
    3686400 / 8192 / 3 = 150
    3686400 / 16384 / 3 = 75
  8. pawihte

    pawihte Guest

    I'm not sure I get it. Is there something special about operating
    close to 4 MHz but not exceeding it?
  9. pawihte

    pawihte Guest

    Now that you've listed out those numbers, they're not unfamiliar
    to me, but they're not something I'm regularly involved with

    Out of the 15 you listed, two-thirds involve dividing by 3 and
    one-third can be obtained by dividing with a binary integral
    which IMHO is technically easier. The reverse is true with a
    starting frequency of 2.4576, 4.9152 MHz, etc.
  10. pawihte

    pawihte Guest

    I understand the principle. To put my earlier question another
    way: Is 4 MHz a common limit of processors for this type of work,
    somewhat like the 4 GHz barrier faced by desktop processors?
  11. Jasen Betts

    Jasen Betts Guest

    no it's not
    3.6864 MHz get you all the standard baud rates with an integer divisor
    4.9152 MHz only gets you most of them but with a power 2 divisor,
  12. Bob Masta

    Bob Masta Guest

    No, there is no particular bleeding edge technology barrier
    like that, but even at lower speeds the chip cost may rise
    with higher speed ratings.

    The original IBM PC/XT used a 4.77 MHz CPU clock to stay
    under the 5 MHz limit of the chips that were readily (and
    cheaply) available at the time. It derived that from a
    14.31818 MHz crystal that it divided by 3. It then further
    divided by 4 to get 1.1931817 MHz to use for the system
    timer. The system timer counted up 65536 cycles of that
    clock to get the famous 18.2 Hz timer interrupts. These
    values all seem to be rather arbitrary, until you note that
    counting up 65536 of those timer interrupts gives 3600
    seconds... one hour... almost exactly. (The PC/XT only had
    16-bit internal registers, so the count went from 0-65535
    and then overflowed into the hours counter.)

    Best regards,

    Bob Masta

    DAQARTA v5.10
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Sound Level Meter
    Frequency Counter, FREE Signal Generator
    Pitch Track, Pitch-to-MIDI
    DaqMusic - FREE MUSIC, Forever!
    (Some assembly required)
    Science (and fun!) with your sound card!
  13. CPUs rarely need an exact clock frequency, either because it's not doing
    something that requires it, or because one can redo the software to
    accomodate a different clock frequency.

    You asked for reasons why there'd be more than one crystal frequency
    to do the same thing. The answer is because it's better, design wise,
    to use the same crystal for clocking the CPU and for generating the baud

    So if you had a CPU that had a maximum clocking rate of 1MHz (and
    that was certainly common in the old days, when this particularly
    crystal frequency was chosen), you'd have to have a divider before
    the CPU on either crystal frequency. But you'd get closer to the maximum
    1MHz with the 3.6864/4 (.9216MHz) than with 2.4576/2 (1.2288), well,
    the latter would be too high, though of course one could often get
    away with a higher clocking frequency. It became more straightforward
    with some CPUs, such as the Z80, that used a crystal frequency four
    times the actual clock frequency. The latter crystal would have
    the CPU running at .6144MHz, significantly less than the maximum
    1MHz clocking frequency. Whether or not one really needed to
    run the CPUs at the maximum clock frequency, it wsa as much
    a speed thing as anything else. The exact frequency meant little
    in most cases, running it as fast as possible was.

    At that time, small increments were important. I remember when I
    changed my Ohio Superboard from the usual 1MHz CPU clock to 2MHz,
    it suddenly showed a snappy improvement, yet nobody would bother
    at this point to make a 1MHz change in clock frequency.

    It matters less these days, since most CPUs run at a much faster
    frequency. But it mattered back then, enough to warrant adding
    yet another commodity crystal frequency to the parts bins.

  14. pawihte

    pawihte Guest

    Uh, not exactly. What I meant was, why involve division by a
    non-binary integral? Dividing by multiples of 2 would have made
    things simpler, if not by very much. This was clarified to some
    extent by someone else who pointed out that standard baud rates
    do not go up by factors of 2 throughout the whole range.
    The operative word for me is (or rather was) the 'if' at the
    beginning of the para. Why was it felt necessary to stay at or
    near 1, 2 or 4 MHz clock frequency? But I'm beginning to see the
    light. Early CPUs were usually rated for maximum clock freqs in
    multiples of 1 MHz, so their utilisation would be optimum at or
    just below those frequencies, right? This was not immediately
    obvious from the first mention of the 4 MHz figure.
    My first computer was a 7 MHz Amiga A500, but I was only an end
    user. I've dabbled in electronics for a long time, but was very
    late in getting directly into the digital world. Hence my
    ignorance of how such standards came to be adopted.
  15. Nobody

    Nobody Guest

    No it isn't. As I said before:
    E.g. 2457600 / 14400 = 170.6666...

    If you start with 2.4576MHz, two thirds can be obtained by dividing by a
    power of two and the other third can't be obtained without a frequency
    multiplier (VCO + divide-by-3 + PLL).

    Divide-by-3 isn't exactly hard (if you want 50% duty cycle, divide-by-6 is
    easier, and just as useful in this context).
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