36.7 Mhz point

Discussion in 'Sega Dreamcast Development and Research' started by MastaG, Jun 4, 2019.

  1. MastaG

    MastaG Rapidly Rising Member

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    Hi,

    I'm trying to overclock my VA1 dreamcast without luck.
    My Dreamcast is equipped with a DCHDMI though.
    I pretty much followed this guide: http://www.racketboy.com/retro/overclock-your-dreamcast
    So I lifted the 33.3 MHz pin and soldered a wire which goes to the left side of the switch.
    Then I soldered a wire from the 33.3MHz pad to the center pin of the switch.

    I'm using this clock generator: https://uk.farnell.com/ael-crystals/o40m000000l126/crystal-oscillator-40mhz/dp/9510168
    Datasheet: http://www.farnell.com/datasheets/1806758.pdf

    So it's basically a square thing with 4 pins.
    Pin 4 is connected to a solid ground point.
    Pin 8 is connected to the 5v rail.
    Pin 5 is connected to the right side of the switch.

    So when I put the switch to the left side, which basically connects reconnects the 33.3MHz pin back to the mainboard, it will boot just fine, no problems.

    However when I put the switch to the right side, it should get the 40MHz signal from the crystal but it fails to boot.

    So there could be 3 causes:
    1. The 40MHz signal from the chip is not compatible? it's frequency stability is 100ppm, perhaps too much?
    2. My Dreamcast cannot be overclocked?
    3. DCHDMI is not compatible with a overclocked Dreamcast? according to chriz2600 it should work: https://www.reddit.com/r/dreamcast/...chdmi_be_installed_in_an_overclocked/ectm35a/

    Now I read about this page: http://dansprojects.com/dc_overclock.html

    It basically tells me to remove three jumper wires and put 2 10k resistors in the blue spots.
    But since I already have the pin lifted and the switch glued to the mainboard, I'd like to know which point supplies the 36MHz signal so I can reroute the wire from the crystal to that specific point :)
     
  2. MastaG

    MastaG Rapidly Rising Member

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    A picture says more than a 1000 words..
     

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  3. MastaG

    MastaG Rapidly Rising Member

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    Nevermind.. overclocking works fine here.
    However DreamShell does not show a picture on the VMU when running in 240MHz.
    So I thought it didn't boot, but actually it did.

    Also saving onto the VMU seems to crash.. so the VMU definitely requires to be overclocked as well.
    Is anyone willing to sell a couple of pre-overclocked VMU's for a nice price?
     
  4. Teddy Rogers

    Teddy Rogers Spirited Member

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    Maybe easier to replace the crystal in one of your own VMU's?

    Ted.
     
  5. MastaG

    MastaG Rapidly Rising Member

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    But I need to know which part to get then.
     
  6. Gizmo613

    Gizmo613 Active Member

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    Do a search on the Dreamcast section and there will be a post that I have listed a crystal that works.
     
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  7. fafadou

    fafadou Gutsy Member

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    I've heard the problem may come from the lenght of 40MHz wire if it's too long compare to the"bandwich" at 40MHz it doesn't work.
     
  8. MastaG

    MastaG Rapidly Rising Member

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    I used thicker single core kynar wire and a bigger switch for the clock signals.
    Like I said.. 240MHz seems to work great.. I've been playing Sonic Adventure International for quite some time.
    Only the stock VMU doesn't seem to like it.. it sometimes crashes when trying to write data (e.g. save my progress).

    Other than that it seems to work great.. 240MHz + Dreamshell + CF card = really fast loading times.
    I only need to carve out a slot for my CF reader right above the modem.. which is going to take a while..
     
    fafadou likes this.

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