Dreamcast [DEV] : MOD Dreamcast REV0 (Va0) with GDEMU

Xrider

Xrider

Well-known member
Registered
Joined
Apr 19, 2020
Messages
65
Hello,

Just a little message to let you know that the Dreamcast Rev0 curse (Va0 motherboard) with GDEMU has been lifted for the first time, and the mod is MADE IN FRANCE
Yes, this is a world first to my knowledge

dreamcast-motherboard-vao-modded-gdemu.jpg


The Techniques:

The Va0 (Rev0) motherboard has a logic level of 5V on the G1 bus

On this G1 bus of the Va0 we find:
  • The GDROM (logic level and 5v power supply)
  • The Bios (logic level and 5v power supply)
  • Flash memory (logic level and 5v power supply)
  • Holly, the Dreamcast GPU (5V and 3.3V tolerance)

The other motherboards of the following revisions (Rev1 and Rev2) work with a 3.3V logic level on the G1 bus
The GDEMU was designed for the Rev1, they represent the vast majority of Dreamcast

On this G1 bus of the Va1 we find:
  • The GDROM (logic level and 3.3v power supply)
  • The Bios (logic level and 3.3v power supply)
  • Flash memory (logic level and 3.3v power supply)
  • Holly, the Dreamcast GPU (+3.3V and +5V tolerance)

Here is an extract from the Dreamcast service manual representing the G1 bus:

logic-diagram-drecmast-holly-gdrom.png


dreamcast-motherboard-vao-modded-gdemu-flash.jpg


To do this, here are the components to use:

BIOS: MX29LV160
FLASH: MX29LV002

It will therefore be necessary to unsolder these two components and pre-program the two memories (MX29LV160 with a bios, I recommend Japanesecake 1.02 https://japanese-cake.livejournal.com and the MX29LV002 with the dump of your old flash memory).

Then you have to install the two components, lift the power pins, and connect them to a 3.3V point (and yes, otherwise they will take 5V and they will not like ... lol)

Then on the front of the CM, remove component D503 and make a strap:



Good news, region changer now works on Rev0 without adding a shunt or anything else!

WARNING :
The original 5V GDROM reader will be incompatible after modification!
Use only the GDEMU or a 3.3V reader found in the Rev1


Regards,

Xrider
 
Last edited:
Xrider said:
Good news, region changer now works on Rev0 without adding a shunt or anything else!
Click to expand...

This is not a good idea to be left like this, as software may write to the flash and brick the console. The Sega DC checker software for example clears the flash and will also clear (and brick the console) factory region coding if this mod has been performed. I don't know how widely used it is, but I myself do use it to check DC consoles I handle. Other software may do it too

MX29LV002 supports protecting specific sector ranges just like the original DC chip does, might be better to wire it up to protect the 8K partition 0 at $1A000 if it is possible
 
darcagn said:
The Sega DC checker software for example clears the flash and will also clear (and brick the console) factory region coding if this mod has been performed.
Click to expand...
My dead Dreamcast can confirm this.
 
Xrider said:
Holly, the Dreamcast GPU (5V and 3.3V tolerance)
Click to expand...

as a rule, such microcircuits have separate power inputs for the core and the bus
you just changed what is connected to the G1 bus, but I do not see any changes in HOLLY

if DVCC HOLLY remained connected to +5V, then you made it worse with your modification
it was much easier to do like all manufacturers of flash cartridges for MegaDrive, to replace all resistor assemblies from 33 ohm to 100 ohm
 
Last edited:
megavolt85 said:
as a rule, such microcircuits have separate power inputs for the core and the bus
you just changed what is connected to the G1 bus, but I do not see any changes in HOLLY

if DVCC HOLLY remained connected to +5V, then you made it worse with your modification
it was much easier to do like all manufacturers of flash cartridges for MegaDrive, to replace all resistor assemblies from 33 ohm to 100 ohm
Click to expand...
Hello megavolt.

i confirm, holly work at 3.3v on VA0
Check this (All VCC input is +3.3V):

File:Dreamcast-VA0---2---GPU,-VRAM.png - TechWiki

console5.com console5.com

ofcourse, holly accept +5v and +3.3v on g1 bus.

You can see the datasheet spec from sega "+5V Tolerance" on Holly G1 BUS
 
Last edited:
darcagn said:
This is not a good idea to be left like this, as software may write to the flash and brick the console. The Sega DC checker software for example clears the flash and will also clear (and brick the console) factory region coding if this mod has been performed. I don't know how widely used it is, but I myself do use it to check DC consoles I handle. Other software may do it too

MX29LV002 supports protecting specific sector ranges just like the original DC chip does, might be better to wire it up to protect the 8K partition 0 at $1A000 if it is possible
Click to expand...
Hello,

Don't worry about this :-D

Your console can boot if the flash is dead with Japanesecake 1.02 https://japanese-cake.livejournal.com
No black swirl of dead or other thing...

And good new if the content of your flash is out...
Our friend "Dragoncity" have software solution for fix this :
dragoncity17.wordpress.com

[Sega] Dreamcast – RetroDream

Requis : – Dreamcast Région « Europe » > Swirl Bleu pour la meilleure compatibilité !Ensuite que cela soit PAL/NTSC (Broadcast) cela ne change rien. – PCB GD-ID…
dragoncity17.wordpress.com dragoncity17.wordpress.com
Chapter 4.5.3 – Black Swirl of Death
 
Xrider said:
i confirm, holly work at 3.3v on VA0
Click to expand...

in this case, the BIOS can not be changed, it will work fine if it is powered from +3.3V
and the flashrom will have to be replaced, because when the power supply is 3.3V, there may be problems with writing

darcagn said:
MX29LV002 supports protecting specific sector ranges just like the original DC chip does, might be better to wire it up to protect the 8K partition 0 at $1A000 if it is possible
Click to expand...

it is possible, it can be done even by the dreamcast itself
 
i don't know about electronics but wouldn't be more easy to mod the gdemu? with voltage regulator/filter or something?
 
madsheep said:
i don't know about electronics but wouldn't be more easy to mod the gdemu? with voltage regulator/filter or something?
Click to expand...
I'm also curious about something on a similar route to this. One thing I was wondering was if it would be possible to implement a series of level shifters into a PCB that plugs in between the Dreamcast and GDEMU? Provided the logic of the 5V and 3.3V circuits are comparable I'd imagine this would be possible. I'm not sure if overheating would be an issue at these levels but I imagine that it'd be manageable?

Still I imagine if it was that simple we would've seen someone try something like that sooner. Anybody have any thoughts?

For reference: https://hackaday.com/2016/12/05/tak...and-5v-logic-communicate-with-level-shifters/
Bi-directional level shifter circuit: https://www.sparkfun.com/products/12009
 
EDMD_V2_2_Front.jpgEDMD_V2_2_Back.jpgEDMD_V2_3_Front.jpgEDMD_V2_3_Back.jpg

is china Everdrive MD
if you do not understand why I posted them here, then I will explain
FPGA and flash in these cartridges are rated for 3.3V, and the bus for MegaDrive is 5V
Now look at how the levels are matched, the entire bus is connected through 100 Ohm resistors, which limit the current and prevent the FPGA from burning

GDROM is also connected through resistors, but from a nominal of 33 Ohm, it is enough to replace the resistor assemblies by 100 Ohm and you can not be afraid that GDEMU will burn out, while maintaining compatibility with all GDROM boards
 
megavolt85 said:
View attachment 9881View attachment 9882View attachment 9883View attachment 9884

is china Everdrive MD
if you do not understand why I posted them here, then I will explain
FPGA and flash in these cartridges are rated for 3.3V, and the bus for MegaDrive is 5V
Now look at how the levels are matched, the entire bus is connected through 100 Ohm resistors, which limit the current and prevent the FPGA from burning

GDROM is also connected through resistors, but from a nominal of 33 Ohm, it is enough to replace the resistor assemblies by 100 Ohm and you can not be afraid that GDEMU will burn out, while maintaining compatibility with all GDROM boards
Click to expand...
So the logic voltage thresholds scale correctly then when lowered to 3.3V? I also wonder how tight of tolerances you would want to have for this to function right.
 
resistors do not lower voltage
the voltage drops on the protective diodes in the FPGA, but these diodes can burn out from the current flowing through them, so that this does not happen, you need to limit the current, this requires resistors, they will limit the strength of the current flow

on the motherboard there are 6 resistor assemblies of 33 Ohm each (one assembly is 4 resistors), they need to be replaced with 100 Ohm resistor assemblies
RA507, RA509, RA510, RA513, RA514, RA515, RA525
 
megavolt85 said:
resistors do not lower voltage
the voltage drops on the protective diodes in the FPGA, but these diodes can burn out from the current flowing through them, so that this does not happen, you need to limit the current, this requires resistors, they will limit the strength of the current flow

on the motherboard there are 6 resistor assemblies of 33 Ohm each (one assembly is 4 resistors), they need to be replaced with 100 Ohm resistor assemblies
RA507, RA509, RA510, RA513, RA514, RA515, RA525
Click to expand...
This looks like it mght be worth a try, located the resistors and found a suitable 100 ohm substitute on Mouser.
1607620825425.jpeg
100 ohm alternative from Mouser:
https://www.mouser.com/ProductDetail/Bourns/CAY16-1000F4LF?qs=2hAPiALdQ0nAuzgmnoRPdg==

Here's the initial search with some alternative components as well:
https://www.mouser.com/Passive-Comp...zimuzZ1yzrt5bZ1z0wo9lZ1z0wo9kZ1yzs1hoZ1z0wljo

Considering giving this one a shot, not sure yet if I wanna use one of my working DCs or find a VA0 with a broke GDROM drive for cheap instead. Haha
 
therobotx5 said:
Awesome. Thanks for verifying. I really didn't want to buy another console. I just got my gdemu and going try this on my VA0. I'll post my results when I'm done modding as well. More people should be aware that it is possible to get a gdemu working safely on a VA0 model.
Click to expand...
Hi, I recently bought a DC signature model from Japan. Would also like to do this mod. Are all Chinese Gdemu the same hardware?. Has anyone had any adverse affects from doing the mod? Much better than forking out £200 for a MODE. Cheers :)
 
You must log in or register to reply here.
Back
Top Bottom