Dreamcast Triple Bios

What's better than one flash chip in a Dreamcast? Two, two flash chips, ah ha ha. Or at least that's the thought I had after trying the piggyback BIOS install on http://www.mmmonkey.co.uk/sega-dreamcast-region-free-bios/. Like others before me I went to my trusty search engine to see if there was any information about doing a triple BIOS install. I was a little surprised that while there were posts saying it could be done, there weren't many pictures or a nice guide. I pressed on and when I picked up another flash chip and did some test fitting in my Dreamcast, I came to a small issue. The triple stack seemed to be too tall and the only way to make it fit would be to cut into the shield(something I didn't want to do if it could be avoided). Maybe others managed it without cutting, but at least with the chips I had it didn't look very feasible. I thought of trying a double stack and then wiring the third to a breadboard but with all the wires that would be required it seemed like it would be very prone to failure. So I decided to try learning PCB design and after some revisions I got it to work and I now think it's ready for feedback from others so here's my board.

https://oshpark.com/shared_projects/qcGjSz4b

 

You know you could have just used 1 chip and switch the banks with the latter address lines.

Mx29l3211 will fit 2 bioses on it. There are even larger chips too.

Can't knock the learning experience and knowledge gained. But there wasn't much point

 

I read some posts about fitting two bioses on one chip but never very clear designs or pictures of a PoC so I decided to stick with the chip and design I knew worked. Could always check that out later. Thanks

 

Merge both bioses into one file.
Burn to chip.
Lift highest address line.

Switch this pin between ground and vcc to pick which half of the chip is in use.

 

Yeah, as Bad said, there was no real need for the custom PCB, but the project in itself, and what you learn through it, can be its own reward!

On the positive side, with a minor modification to your PCB, you could support 4 bioses!

 

Why do you have to merge the bioses? Can you not just toggle the address line and write to half the chip using the already built bioses?

Already thinking about that

 

Oh, if you are programming via dreamshell - sure.

I was talking about preprogramming the chips.

MX26L12811 will take 8 bioses iirc, but I don't think it's supported in dreamshell (but would work if preprogrammed). The Mx29l3211 is supported in dreamshell though.

MX29LV160 = 2MB
MX29L3211 = 4MB
MX26L6420 = 8MB
MX26L12811 = 16MB

Edit:

Supported chips via dreamshell flashing app:

 

Wow thanks for the info. Yeah I would prefer for it to be programmable via dreamshell so that it can always be easily reprogrammed. So 4MB is the largest supported within dreamshell? So can either go with a 1 chip 2x or a 2 chip 4x?

 

Correct. SWAT or someone else could add support for the larger chips though. Probably just a lack of interest as there's no real reason to bother with that many bioses.

 

New Version 2, now based on the MX29L3211. I added a switch on the board so once assembled and installed it will default to the original BIOS unless you use an external switch to activate the flash chip. This also cuts down on the number of wires that are needed for the switch. I also included a GND point for easy install of the switch but you do not need to use that particular GND point. Most importantly support for MX29L3211 was added after DreamShell 4.0 RC 4. To properly detect the MX29L3211 you need to replace /DS/modules/bflash.klf with the latest build of bflash.klf from github


https://oshpark.com/shared_projects/OhXIa5Pw

 

Like it, good work.

 

Is there an alternate point for d501? I replaced my old piggyback installation with this and installed the pcb without paying attention to d501. I’m a bit afraid the lifted pin on the original bios may be to brittle from being reoriented multiple times and I don’t want to risk breaking it to remove the bios pcb to get to d501. I don’t have a spare pcb to test for an alternate point sadly.

 

Yes, there's test points for either side of d501

Ill have a look at finding it when at a pc

 

Yes on the bottom of the motherboard you can find R532 and an unmarked TP502. Here is a picture with the Alt points for D501 marked.

 

Nice work .
So to activate the MX29L3211 i have to connect the "Flash pin" with ground and to flash a second bios on MX29L3211 i have to connect "Flash pin" AND "Flash 2 pin" with ground ?!

 

Thank you. Yes, that is correct.

 

Doesn´t work for me

The MX29L3211 is connected as the MX29LV160 except, that pin 1 is connected to pin 9 of IC509 and pin 44 is connected with ground or to 3.3V via 10k resistor?!

When trying to flash MX29L3211 with Dreamshell I get the following message : Error Writing Fail. Status Code 4.
Dreamshell detects MX29L3211 and I can read from it, but not flash it?

Any idea?

 

Yes, that is correct.

Check the solder points with the original BIOS and reflow as needed. Use a multimeter to check the continuity from the legs of the MX29L3211 to the legs of the Sega BIOS, pins 2-11 and 13-43 should all connect. Make sure you are on the leg of the Sega BIOS and not touching the castellation on the edge of the board. If your multimeter probes are too large you can strip the end of a wire and wrap it around the probe end then use the other end of the wire to probe. If pin 1 or 12 weren't connected correctly it wouldn't detect the MX29L3211.

 

Nice work, But why would I need a forth bios? and which one was that you have selected as forth? thanks.

i have the 2 bios mod on my dreamcast having the useless hdd sata mod, but the console which I really would loved to have a triple bios for would be mega cd 1 , as every bios of the 3 regions looks totally different

 

Triple bios on mega cd is easy, just use a 27c4096 and toggle additional address lines.