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Generating pulse wave

Discussion in 'Electronic Design' started by Slater, Sep 27, 2010.

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  1. Slater

    Slater Guest

    I have this chip:
    http://www.gamesx.com/rgbadd/cxa1645.pdf
    As explained on page 2, pin 6 needs a "0.4 to 0.5 Vp-p sine wave or
    pulse". Also explained on page 12, paragraph #3. The duty cycle must
    be 50%.
    For PAL operation, the frequency is 4.433619 MHz. I already have a
    crystal of such frequency. How do I interface it to the chip?
    I'm trying to build this circuit:
    http://www.nexusuk.org/projects/rgb2svid/circuit
    But I can't find a 74HCT04 anywhere!
     
  2. Joerg

    Joerg Guest


    With that circuit pin 6 will get a lot more than 0.5Vpp.


    Wow, you must really live in the boonies :)

    Anyhow, it's just six inverters in there. You could also use a 74HC04,
    or a 74HCT00 or 74HC00 wired up a bit differently. If you can't get
    those I suggest to move closer to civilization :)
     
  3. Slater

    Slater Guest

    Wow, you must really live in the boonies :)

    Exactly!!
    I know where to get a 74HC04. Could you please tell me how to wire it,
    in such a way that specifications (0.4 to 0.5 Vp-p) are not exceeded?
    A little drawing would be wonderful!

    Thank you!
     
  4. Slater

    Slater Guest

    Yes!
     
  5. Andrew Holme

    Andrew Holme Guest

    Page 2 says "0.4 to 0.5" but note 3 on page 12 says "0.4 to 5.0 Vpp" and
    talks about it going outside the GND/Vcc supply rails if the duty cycle is
    not 50% (due to the internal 1/2 Vcc biasing and ac coupling) which confirms
    the 5.0 Vpp. Also, the 74HCT04 output is 5.0 Vpp. So that 0.5 on page 2 is
    a typo.
     
  6. Joerg

    Joerg Guest

    Must be east of Tromso then :)

    You just plop it in, has the same pin configuration. Looking at the
    datasheet it appears that the first part on page 2 may not be accurate.
    Note 3 on page 12 says, quote "The SC input (Pin 6) can be either a sine
    wave or a pulse in the range from 0.4 to 5.0Vp-p". 5Vpp is what will
    come out of your 74HC04, or ion reality a few hundred millivolts less
    but that's fine. The 74HC04 generates pulses, not sinewaves.

    So you shold be fine without attenuation. If you ever need to attenuate
    it would be easy: 4.7k resistor from U3 pin 4 to U1 pin 6, then 560ohms
    from U1 pin 6 to ground. But I don't think you need to do that here.


    You are welcome :)
     
  7. Slater

    Slater Guest

    How much attenuation will those values give?
     
  8. Slater

    Slater Guest

    Yes, it's for converting the RGB signal from a videotape to S-Video
    :)
     
  9. Joerg

    Joerg Guest


    It's a simple voltage divider, resulting in roughly a factor of 9.4, so
    a 5V signal would become 530mV. Realistically less because logic chips
    never do a full swing into any load. But according to page 12 of the
    datasheet I don't think you need to do this.

    --
    Beste hilsen, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
  10. Slater

    Slater Guest

    Thank you all! :)
     
  11. Joerg

    Joerg Guest

    Sure, but tough luck finding one of those in the woods of Norway ;-)

    It's not nice and harder on the crystal but a 74HC04 usually works ok
    for crystals in the low meggeehoitz range.
     
  12. Joerg

    Joerg Guest


    It seems the OP lives in Norway. In Europe many VCRs had (have?) RGB,
    via the SCART-Connector. We still have our old European VCR, to be able
    to watch some of the classic movie from there. Until a while ago we
    could occasionally find a foreign movie at the video rental place but
    that went belly-up.
     
  13. Rich Grise

    Rich Grise Guest

    Harder on the crystal? Use either a smaller cap, or a bigger resistor. ;-)

    Cheers!
    Rich
     
  14. Joerg

    Joerg Guest

    Then, eventually it oscillateth no more :)
     
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