Guide Nold's Alternative Dreamcast Pinouts & Workarounds

Discussion in 'Guide Submissions' started by Nold, Dec 4, 2017.

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  1. Nold C L U B C L A M S H E L L Staff Member . .

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    I took some time off and took a deeper look into the dreamcast. Here are the results!

    VGA- & Composite-Video, Audio & SD-Card wiring
    Nothing special here... The filters for R, G, B, composite (and luma/chroma) can even be removed. Got no problems with a long VGA-Cable. (H/V = Sync for VGA)
    Note: You'll still need some additional components for VGA! See f.e. mmmonkey's website. (100uF caps also work fine).

    dc_alternative_vga.png


    Controllers & RTC
    Every pair of filters is for one controller. They are connected to pin 1 & 5 of the controller. DON'T remove these filteres! (There are connections on both sides of them).
    The RTC voltage is connected to pin 2 of the connector, using a 10Ohms 1W resistor. Don't use a 3.3v Battery here! (The Dreamcast uses recharable batteries).
    Last we have the PWM-Signal comming from the FAN. It's required for the DC to boot, but we won't need it...
    dc_alternative_controller.png

    Faking the FANs PWM-Signal
    Why use Arduinos or 555's... damn... just use some solder!
    fake_pwm_hack.png
    Some background: The DC uses IC601 (a multivibrator IC) as some kind of ADC (Analog-Digital-Converter), to convert the PWM-Signal from the fan (up to 2.5v), to 3.3v (= logical "true"). So when there's no fan, it'll put out 0v (= logical "false"). So all we need to do is pull-up the output signal of the IC & done! How nice Sega prepared this mod for us & gave us some free solder-pads.. (It might be safer to use a resistor here, but it should even work without the IC, anyway; also there is a capacitor, protecting a possibly connected fan.)


    "Faking" the GD-ROM Clock / AIDA-Clock Signal
    Last I've found out that the Dreamcast has an internal PLL, that the workaround shown in this post uses and it actually works! (With SWAT's DreamShell no g1-ata BIOS flashed)

    dc_no_g1ata_clock.png
    Background: As told on the image, the AICA (Dreamcast's Audio-Chip) needs a clock signal, which it'll use for it's RAM-BUS-Clock (2xCD_CLK). The GD-ROM normally puts out a 33.8688MHz, so RAM-Clock is 67.7MHz; Without a GD-ROM audio will be funny sloooowww.... :D but it'll also slow down the DC.
    So I've found out (using the available Dreamcast schematics) that the DC has an internal PLL which generates a 33MHz clock. And the Test-Point in the image is the best connection point for it!
    So this will basically slow down the AICA RAM a little bit, but I played for a while & couldn't notice any difference.. otherwise just use a active crystal...



    Hope these images are of any use for someone! :)

    //Update 5.12.17: Cut trace for PWM-Hack to prevent current sinking (26mA) through IC601.
     
    Last edited: Apr 7, 2018
    cheese, YveltalGriffin, Shank and 5 others like this.
  2. Shank Certified Wiitard Staff Member . .

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    This is great! Dreamcast needs some love. If you aren't already, hop on the discord to the Dreamcast initiative channel and we can compare notes.
     
    mknrls likes this.
  3. Crazzyleprechaun Formerly known as GarageFactory .

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    This is an amazing guide great work dude I will be using this if I ever find some time to work on my Dreamcast portable
     
  4. ShockSlayer Ivan - the tyranny of evil men . .

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    are alternative pinouts the same as alternative facts
     
    BocuD, Stitches and GingerOfOz like this.
  5. Nold C L U B C L A M S H E L L Staff Member . .

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    [​IMG]
    ... and we're gonna build a HUGE portable with that! And I'm gonna make ShockSlayer pay for it! ;-)
     

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