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LPT square wave

Discussion in 'Electronic Design' started by [email protected], Oct 3, 2006.

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  1. Guest


    I'm trying to write a program in C++ to communicate with an AVR
    microcontroller's SPI via the LPT port.

    I have to generate a clock (SCK) signal on one of the pins of the LPT.
    I am using the WinIO library. I'd like to have way to create a square
    wave of a given frequency (within some limit of course).

    My question is: what is the best way to generate such signal and what
    is the maximum frequency one can obtain? Excluding of course such code:

    for (;;) {
    pin = 0; pin = 1;
  2. Some things cross my mind. Why do you need to generate a clock? Are
    you sure the LPT signals on your computer are electrically compatible
    with the circuit? Does it have a separate power supply? How precise
    do you need this clock? What do you mean by "best way?"

    A perfectly good way of generating a clock might very well be
    something similar to what you wrote. Why do you exclude it?
    (Especially since it may very well be the maximum frequency you can
    achieve, in this case.)

  3. Guest

    I am using SPI protocol (which is synchronous protocol) where PC is
    Master, so it has to provide a clock signal to the circuit.

    It is 100% compatible. The LPT output level is TTL so no problems here.
    Also i have programmer which is interfaced via LPT and also uses SPI,
    The example will not work, because the microcontroller is running at
    16MHZ. The AVR documentation says that the clock signal must stay
    unchanged for at least 3 CPU (AVR) cycles because only then the uC will
    be able to sample it properly.
    The conclusion is that I need to generate a clock having 15MHZ or
    On the other hand, with the 1KHZ frequency I get only 125 bytes / sec
    which is pretty low.
  4. Guest

    What's wrong with:

    pin = 0;
    pin = 1;
  5. Guest

    FIrst of all, a very precise amount of time is usually not easy to
    achieve. Take into consideration
    - Threading (in Wingroze)
    - The program has to do something else in the meantime
  6. PeteS

    PeteS Guest

    SPI does not require precise amounts of time. Unless you are trying to
    communicate at some specific rate, then live with the inaccuracies
    added by Winblows and simply run the clock at whatever you can get.
    Just make sure you transition the data bits well clear of the clock
    transitions (which is not easy when the OS gets in the way - one of the
    reasons I detest OSs which do not permit me full access to the hardware
    in situations where I actually need it).


  7. Leon

    Leon Guest

    Generating SPI with software using the PC printer port is quite easy,
    just follow the timing diagram. The clock doesn't have to be a
    conventional square wave, just drive it high and low at the correct
    time with the data line held high or low. Connecting a 74HC595 with
    LEDs on the outputs to the printer port can be useful when debugging
    the software, the 74HC595 is SPI-compatible and makes it easy to see
    when you have got it right. I have one mounted on a small PCB with the

  8. Guest

    Generating SPI with software using the PC printer port is quite easy,
    Thanks for the answer. Can you tell me is it better to use some kind of
    timer or just plain delays?
  9. GM

    GM Guest

    Plain delays (for example 'for' loops) will be depended on the system speed.
    In other words, as an example, the same loop would be faster in a Pentium
    than in a 486. In a Windows program that I've coded for programming AVR
    chips, I used the High-Performance Timer instead of the plain old Timer that
    gives no less than 10ms interval, thus achiving tolerant (from the user's
    view) programming speeds
    Here is a link on how to use it

  10. Guest

    Thanks a lot. I'll try it.
  11. Printer port I/O is handled via the legacy ISA bus transaction
    mechanism. There is a precisely defined I/O cycle there, so on modern
    computers which are way way faster the limiting time will be this
    cycle time. For an 8MHz bus (I think the BIOS can allow this setting
    on many systems), this is 6 cycles or 750ns per I/O. I don't think it
    can be affected by read-around-writes, caches, etc. It's supposed to
    operate like an ISA bus, even if there isn't a physical ISA bus
    present (it's just internal to the chip set in those cases.)

    My preference for things like this is to actually use DOS on an older
    slow Pentium (one of the early ones running at 66MHz or so) or else to
    use a 80486 or a system actually sporting a true ISA bus on-board. I
    puchased a lot of copies of DOS 5.0 just for the purpose of legally
    installing them, as needed. I also keep a bevy of old DOS compilers
    that I own, from Microsoft C 4.0 up through their last 16-bit version,
    that is VC++ 1.52C. Plus their QuickC 2.0, various QB's and VB-DOS
    1.0, etc. Borland tools are also available, even today I think, for
    free. But under DOS you don't have WinOldAP (Win95, Win98) or else
    something works (WinXP) blocking all access to I/O addresses and
    double-checking, with all the attendant delays to it. You do the I/O
    and the I/O transactions happen. You can grab timers and the
    interrupt vector will take you to your code right away, rather than
    through some Windows mapping that eventually, someday, gets you there
    after who knows how much internal hand-wringing.

  12. Guest

    Borland... sure is. Turbo C++ 1.01 for DOS was released to Borland's

    The download link is kinda "hidden" (took me awhile to find it, I
    reproduce it here for others' convenience):

    The newer Borland C++ 5.5 command line tools don't support delay() and
    outportb() anymore, from what I understand. (I could be wrong.)

  13. Write the routines in assembly, if needed. I do it all the time.

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