Understanding a 139 decoder to wire up 2 roms.

For hardware talk only (please avoid ROM dumper stuff)
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Understanding a 139 decoder to wire up 2 roms.

Post by darknezz19 » Sat May 09, 2015 5:18 am

So I'm playing around with this 139 decoder and a couple 27c322's to try and make an 8MB Genesis cart but so far have not got it to work. Some questions have come to mind about how this should be wired up. This is going off a setup for a 6MB Snes game as a reference which has the address input line connected to A19 on each of the two flash chips. Then it uses the OE lines, one output to each chips line, to control which rom to enable. Pin 1 on the 139, which I believe just enables that side of the chips decoder outputs, is tied to mad-1 pin which is OE. Couldn't this pin just be tied to any 5v source though? Also the decoder output are setup so the first output goes to the second rom, and the second output goes to rom 1, why is that? I tried swapping these just to see if it would work the other way but it wouldn't.

Furthermore after looking at a Genesis pcb with 2 rom chips that uses a 139 to decode it's using the CE lines on the outputs instead of the OE lines. I'm just going off of pictures from romlab so it's hard to tell exactly how it's setup though. I have tried using both the OE, and then the CE for outputs to the 27c322s but it's hasn't made a difference. For the address input I've tried A19, A18, A20, both A20 and A19 together, A21, A22, A23, A24 and AS. Gave it my best shot, any advice?

Pictures for reference:

Data Sheets:
http://www.nxp.com/documents/data_sheet ... 39_CNV.pdf

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Post by r57shell » Mon Jun 22, 2015 1:53 pm

post resulting schematics.
words is not enough, and we can find datasheets.
but scheme that you build, and that is not working.... you must post it, to speed up process.

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Post by bastien » Tue Jun 23, 2015 4:43 am

/CE is not enough for a 8Mb Cart.
You must use /ASEL.
Whats is your game ? mortal kombat ? bad apple ?.
A23 and A24 does not exist in the adress bus.

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Post by l_oliveira » Tue Sep 01, 2015 2:03 pm

bastien wrote:/CE is not enough for a 8Mb Cart.
Better wording for this ...

/CE gives you 4MB of memory window.

If you ignore /CE you will have bus conflict problems on systems with MEGA/SEGA-CD unit attached.

That's why very big games like Super Street Fighter have mapper chip in them. Pier Solar, too, for example. (Pier Solar actually need to be compatible with MEGA-CD for the additional features with the extended music CD)

Bus conflict is a bad thing, which can result on hardware damage.

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Post by HardWareMan » Thu Sep 03, 2015 5:41 am

bastien wrote:A23 and A24 does not exist in the adress bus.
M68K address bus contains A1 thru A23 (total 23 lines). A0 doesn't exists but M68K have LDS/UDS strobe signals for that. All of this signals are exists on cartridge slot.

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