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Thread: Overclocking the 68k

  1. #1
    Cham Cham's Banana
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    Overclocking the 68k

    Hey Guys.

    I now overclocked my test rig.. which is a heavily repaired MV-1C from China.

    So I ordered an adjustable oscillator from aliexpress.
    https://www.aliexpress.com/item/1KHz...443884225.html

    I first measured the stock clock which is pretty near 12.0mhz - then I cut the trace of the CLK Signal of the 68k (on a part that can easily be soldered back on).
    Then I ran VCC and GND to the Oscillator module nearby the 68k and connected the OUT of the module.
    I slowly raised from 12mhz up - the maximum without graphic glitches is 14.1mhz which is the setting I will be using for now.

    Quite happy with the results.

  2. #2
    Rugal's Thug

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    The occurence of graphics glitches may vary depending on the game.
    Some have more delay between VRAM access than others, so they're more tolerant about overclocking.

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    Quote Originally Posted by furrtek View Post
    The occurence of graphics glitches may vary depending on the game.
    Some have more delay between VRAM access than others, so they're more tolerant about overclocking.
    I see - I tested it with Soccer Brawl which should not be too demanding. I think I will overclock my other Neo Geo's with a fixed frequency oscillator of 14.0 or 14.3 mhz and I keep one machine with the adjustable OSC for testing purposes.

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    Cham Cham's Banana
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    Quote Originally Posted by Xian Xi View Post
    At 16mhz, all the Slug titles ran fine for me but older games like FF2 had glitches galore.
    That's strange - So I will better stay in the 14 mhz range to have all games working.
    Why is it that the Graphics will glitch at higher clock rates ? which part of the Neo Geo MVS is the trouble maker here ?
    Is the LSPC2-A2 not able to hold up or will faster RAM help ?

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    Quote Originally Posted by RyuX View Post
    That's strange - So I will better stay in the 14 mhz range to have all games working.
    Why is it that the Graphics will glitch at higher clock rates ? which part of the Neo Geo MVS is the trouble maker here ?
    Is the LSPC2-A2 not able to hold up or will faster RAM help ?

    Probably the LSPC, the VRAM might be part of the issue too - the 2K VRAM runs fast (and hot) as default. TBH, I would never risk overclocking a Neo Geo as it's just asking for a shorter life.
    My YouTube channel contains some MVS repairs and stuff:-
    http://www.youtube.com/user/GadgetUK164

  7. #7
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    Quote Originally Posted by GadgetUK View Post
    Probably the LSPC, the VRAM might be part of the issue too - the 2K VRAM runs fast (and hot) as default. TBH, I would never risk overclocking a Neo Geo as it's just asking for a shorter life.
    I see where you are coming from with that - however usually those machines run 24/7, most of my RAM etc. are replaced already - the overclock is only around 17 % and the MVS is running a couple of hours per week at best. I think the risk is negotiable for me. But I will monitor long term effects for sure. I never could trace any substantial heat on any of my MVS boards when letting them run for hours at a time and touching all the parts. Only had a hot VRAM one time and it was a faulty VRAM that was causing the heat issues only.

  8. #8
    Rugal's Thug

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    Overclocking makes the 68k run faster but LSPC still runs at 24MHz. If the 68k talks to LSPC to quickly, it starts skipping accesses and glitches appear.
    If you overclock LSPC along with the 68k to avoid that, you'll also increase the framerate so ultimately your monitor will lose sync, the snake bites its own tail.
    Also as GadgetUK wrote, you'll bump into the speed limit of the VRAM.

    You can't know if a game will handle overclocking better than another by its age or by the amount of action on screen, you have to look at the code. SNK specified minimum delays to respect to keep LSPC happy, but some games have more delay than needed, making the "safety" margin for overclocking larger.

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    Quote Originally Posted by furrtek View Post
    Overclocking makes the 68k run faster but LSPC still runs at 24MHz. If the 68k talks to LSPC to quickly, it starts skipping accesses and glitches appear.
    If you overclock LSPC along with the 68k to avoid that, you'll also increase the framerate so ultimately your monitor will lose sync, the snake bites its own tail.
    Also as GadgetUK wrote, you'll bump into the speed limit of the VRAM.

    You can't know if a game will handle overclocking better than another by its age or by the amount of action on screen, you have to look at the code. SNK specified minimum delays to respect to keep LSPC happy, but some games have more delay than needed, making the "safety" margin for overclocking larger.
    I see
    I could however use another adjustable oscillator and fine tune both "overclocks" to the maximum in a kind of trial and error process.
    But I am already happy with the 14 mhz overclock so I guess I won't bother..

  10. #10
    Kuroko's Training Dummy
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    I am reminded of this awkward Sega Genesis overlocking video, I guess because they both use the 68K.


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    Quote Originally Posted by Jumpingmanjim View Post
    I am reminded of this awkward Sega Genesis overlocking video, I guess because they both use the 68K.

    Thats a nice video in my opinion.. I will mod my Japanese Genesis soon now

  12. #12
    Cham Cham's Banana
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    Installed the adjustable clock into my other MV-1C now (main rig).
    I can clock a little bit higher on this one before it is glitching compared to my other machine.
    Also installed the UniBios with the NeoBiosMasta.

    Will make some more detailed measurements on when it will glitch on which game later on when I have my oscilloscope set up properly

  13. #13
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    Quote Originally Posted by RyuX View Post
    Thats a nice video in my opinion.. I will mod my Japanese Genesis soon now
    That adjustable oscillator is pretty cool, will use it for MVS and had I known it exists I would have used it for the Mega Drive.
    As for the Megadrive I used two additional oscillators and a switch with three positions for a selectable stock / 10MHz / 13MHz:

    2019-06-15 22.12.12.jpg

    It looks like 10Mhz is the sweet spot, can use no problem with Everdrive and there is less slowdown in some games.
    On 13 MHz with an Everdrive it will not boot - either garbled graphics, 'FAT not found' message, white background etc.
    Some carts boot fine though, and fitting a CPU halt switch it is possible to boot up at stock or 10Mhz and then halt and go up to 13Mhz. But it seems like a lot of hassle when 10Mhz works without any issues.
    Regards, MtothaJ

  14. #14
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    Quote Originally Posted by MtothaJ View Post
    That adjustable oscillator is pretty cool, will use it for MVS and had I known it exists I would have used it for the Mega Drive.
    As for the Megadrive I used two additional oscillators and a switch with three positions for a selectable stock / 10MHz / 13MHz:

    2019-06-15 22.12.12.jpg

    It looks like 10Mhz is the sweet spot, can use no problem with Everdrive and there is less slowdown in some games.
    On 13 MHz with an Everdrive it will not boot - either garbled graphics, 'FAT not found' message, white background etc.
    Some carts boot fine though, and fitting a CPU halt switch it is possible to boot up at stock or 10Mhz and then halt and go up to 13Mhz. But it seems like a lot of hassle when 10Mhz works without any issues.
    I never checked which rams the SEGA System C1 uses, but I could give a check.

    I remember to have successfully repaired one using a few parts from a Mega Drive model2, such as an oscillator.
    Being this arcade hardware heavily based on the Mega Drive/Genny, I wonder if swapping both rams and CPU directly would work fine without any kind of issue.

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