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  • Joined: 01 Jul 2016
  • Posts: 55
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Basic 32K cart
Post Posted: Mon Feb 13, 2017 12:31 pm
Last edited by KanedaFr on Wed Feb 15, 2017 9:43 am; edited 1 time in total
Hi,

Since I, at last, finished fixing my game, I'd like to make a physical version of it ;)

So I browsed littlescale blog and Maxim's pinouts page, plus some others related related posts.
While I was pretty confident at first, I started to get some trouble when I discovered the bus was shared with RAM (?)

I'd like to be sure I didn't miss a thing

Let's say I want to use 28C256

ROM <= CARTSLOT
- A0-14 <= A0-14
- D0-7 <= D0-7
- +5V <= +5V
- GND <= GND
- !CE <=  !CE or (!CE OR !M0-7) ?
- !OE <= !Rd or !M0-7 or (!Rd OR !M0-7) ?
- !WE <=  +5V


As you can see, I'm not sure of !CE and !OE
I saw a lot of home made card used the basic way (!CE / !M0-7) but is it really safe ?
I had the feeling it's safe on this basic 32K ROM only cart but it won't if I add a mapper and, even worst, a RAM onboard...

Bonus question, if I want to use a 29F256 flash without remove it from the cart, with a SMS slot to eeprom-burner adapter
I'll need to use the !WE and so is it correct to use
!WE <=  (!Wr OR !M0-7)

to avoid problem while writing to System RAM ?

Thanks

EDIT: fixed an very awful error, using AND while we need OR
29f256.gif (4.04 KB)
29F256 pinout
29f256.gif

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  • Joined: 14 Aug 2000
  • Posts: 740
  • Location: Adelaide, Australia
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Post Posted: Tue Feb 14, 2017 2:11 am
Yes you can use !CE / !M0-7.

The easiest way to do this is to get a 32kB cart like Super Tennis, remove the PROM and insert your EPROM/Flash on to the PCB.
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  • Joined: 24 Sep 2013
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Post Posted: Wed Feb 15, 2017 7:46 pm
Simplest cart I made was using an OR gate with !CE and !M0-7 (Or A15, which is the same signal)


          _____
 !M0-7 --|     |
         | >=1 |---- !ROM_CE
 !CE ----|_____|
.


I'd recommend a 74LS32 (not HC) since you can leave the unused inputs floating. This design is safe as far as I have tested.
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  • Joined: 31 Oct 2007
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  • Location: Estonia, Rapla city
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Post Posted: Wed Feb 15, 2017 9:43 pm
Floating inputs are a bad design practice regardless of logic family.
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