oompa loompa
I AM THE GOVERNATOR 
"Git 'Er Dun!"
Posts: 1,301
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Post by oompa loompa on Jun 27, 2007 0:10:31 GMT -5
Okay, someone e-mailed me asking if this guy is for real: benheck.com/phpBB/viewtopic.php?t=19486the link is to a project on a forum, which is to portabalize a sega genesis system by taking genesis ic's, building a pcb, and putting everything together. the joypad buttons, cpu, vdp, the cartridge connector, everything will be on one board. there's another link within that page that links to the actual project page (in spanish or something, tomman can maybe understand  ) what i see wrong with this project: - the 74hc15 is not the correct joypad multiplexer. the multiplexer for a 6 button joypad is custom, and isn't in the 74 logic series - the guy uses two 16 bit wide rams for the genesis work ram. the guy can just use only one chip, he doesn't need to use two - the vram he's using will not work at all. you can never use static ram for the genesis vram  - he includes the schematics for the genesis model 1 which are available on charles' site. it looks like he's using an asic chip from the model 2 genesis though - i haven't looked at the actual cad pcb layout, but i know there are a lot more discrete components that need to be included (just by looking at the pcb gifs) it looks to me like either this is a phoney, or the guy doesn't know what he's doing (until i actually look at the cad files, we'll see ) |
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Post by jlf65 on Jun 27, 2007 1:06:10 GMT -5
The joypad muxer doesn't matter. You can use the "old" muxer with 6 button just fine. The 6-button communicates via bits that were previously undefined as you toggle the joypad successively. Here's a brief hand-waving idea of how SEGA did controllers:
3-button:
wait for VBLANK
read reg
toggle
read reg
toggle
6-button:
wait for VBLANK
read reg
toggle
read reg
toggle
read reg
toggle
read reg
toggle
6-button controller notice the extra toggle/reads and return the extended data in the extra reads. The wait for VBLANK is needed to allow the controller to reset for the next read. If you poll the controller too often (more than once each VBLANK), it returns indeterminate data. You can use the extra toggle/reads on 3-button controllers, but you get the same data on the extra reads. The undefined bits are defined to tell you what kind of controller is attached. I'd have to consult my notes again to tell you what bits mean what. The point is that the muxer has NOTHING to do with the way data is returned from SEGA controllers. It's merely the number of toggle/reads done each cycle.
As to the other points... if he's using multiplexed addressed ram, there are no 64K rams. When using a multiplexed bus, the size goes up by 4X, not 2X because each line represents two address bits. That's the whole point of multiplexing after all. ;D
I also don't see why static ram couldn't be used for the VRAM. Since he's making the circuits himself in the ASIC, there's no reason he shouldn't be able to use ANY kind of ram. It's just a matter of doing the circuits to use that type of ram instead of another. Personally, I'd have simply gone with one single big FAST static ram.
It seems you think he's using an ASIC pulled from the actual Genesis... I think he's making his own.
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Post by Tom Maneiro on Jun 29, 2007 15:25:48 GMT -5
Seems like a nice idea (and it's in Spanish too). It's too expensive to build a custom ASIC, instead of burning a FPGA? Here is the list of materials: Notice the odd chinese name in the "subprocessor": it's the custom ASIC, although it sounds more like a "stock" part from a Chinese builder than a DIY-part. (and everybody knows from where the Radica is/was). If you look in Google for the TV16B, you will get your usual part-miner pages, but NO datasheets. The guy mentioned that he had to deal directly with that chinese IC manufacturer to get the datasheet. UPDATE: Hey, here is a (hi-rez) pic of the TV16B! It's a pirate clone of our beloved Genesis VDP/IO/whateverelse chipset! So this project is quite real, but using pirate parts  |
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Post by GiGaBiTe on Jun 29, 2007 19:14:36 GMT -5
The VDP in the genesis is just a heavily modified Texas Instruments video chip, so essentially its a ripoff of a ripoff.
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Post by jlf65 on Jun 30, 2007 13:50:05 GMT -5
Seems like a nice idea (and it's in Spanish too). It's too expensive to build a custom ASIC, instead of burning a FPGA? Notice the odd chinese name in the "subprocessor": it's the custom ASIC, although it sounds more like a "stock" part from a Chinese builder than a DIY-part. (and everybody knows from where the Radica is/was). If you look in Google for the TV16B, you will get your usual part-miner pages, but NO datasheets. The guy mentioned that he had to deal directly with that chinese IC manufacturer to get the datasheet. UPDATE: Hey, here is a (hi-rez) pic of the TV16B! It's a pirate clone of our beloved Genesis VDP/IO/whateverelse chipset! So this project is quite real, but using pirate parts I see - he's using that pirate part rather than the SEGA ASICs. Yeah, it'd be better if he just used an FPGA, but that would require more work on his part.  With an FPGA, if he ran into compatibility issues, he could correct them in the FPGA. With the ASIC, he's stuck with whatever it can handle. |
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