FlashROM cartridge

Click on the image to see it without scaling

This is the first (of many?) FlashROM cartridge designs. The reason being is that I don't really like this one! :)

Note that the above schematic isn't complete (and it may not "look" totally correct (the schematic IS correct - to avoid confusion) as it was drawn rather quickly! e.g. "22 bit address bus" which is connected to 20 addresses... the other two addresses are on the 74HC139). To use it with the Super Nintendo, you must also connect the "Lockout Chip" (or "CIC") to the SNES connector. To use the above with the MegaDrive/Genesis, you need 16 bit data lines, which can be accommodated by simply modifying the above schematic (not shown). This schematic also doesn't include battery backup RAM support (but can be modified to support it at a later stage).

Advantages of the design

• It's a FlashROM cartridge!#@$ This means no messy power supply to keep the data alive, and it is portable.

• The FlashROM is contained on a SIMM (80pin). Therefore it is easy(er) to solder using a socket.

• The FlashROM ICs used on the SIMM are manufactured by AMD and are said to be the most reliable on the market.

• The FlashROM's speed is 90ns. If you include the delays of the demultiplexer and bi-directional buffers, it should be under the 120ns access time required by the Super Nintendo and Genesis however I cannot say for sure. (I got the data book for the 74HC series of ICs from RMIT University however it had no timing information - the tables were there, but no values!)

• The way White Semiconductors designed their SIMM, they have allocated 32 lines for data! Since I will be using a maximum of 16 at a time, there is a waste of connector space, hence the SIMM is a large one - 80pins. This means the connector is LONG which might (?) not fit into existing plastic cartridges!

• 80pin SIMM connectors are not too common as opposed to the 72pin versions. Hence it might be harder to obtain 80pin connectors, and the price may be more than the 72pin version.

• It uses four too many parts! Due to the way White Semiconductors connected the four FlashROM ICs on the SIMM, they have forced me to use four Bi-Directional buffers to "convert" the 32bit wide SIMM to 8 or 16 bits. They have all the /WR lines of the four FlashROM ICs separate (see above schematic) however the /OE and /CS lines are all connected together.

  It would have made more sense to have the /CS lines of the four ICs separate, and all other control lines (/OE and /WR) together. They even have enough pins left over on the 80pin SIMM to have all the /OE lines separate but they didn't! In my opinion, they didn't do a good job on the SIMM design (especially when there are 3 left over pins all next to each other [which could have been used for separate /OE lines for each IC on the SIMM] and next to the /OE pin!)

  If they had designed the SIMM better, the above schematic would only use 1 part (74HC139), instead of five as it does now. If that was the case, I would definitely use this schematic as the basis for a rewritable cartridge for CartDisk.

PCMCIA FlashROM cards contain circuitry to decode the PCMCIA "stuff" (hehe). To use it as a big chunk of memory (which is what I want to do), I'd need to have PCMCIA control circuitry on the rewritable cartridge, which increases the part count, etc etc... Therefore I'm forgetting about PCMCIA cards.

For the Miniature FlashROM cards, the maximum speed available is 150ns which is much slower than the 120ns speed requirement of the SNES. Everything about the FlashROM card is perfect except perhaps for one thing. It uses a new type of connector, and since the actual card is indeed miniature, the connector will be very small. I am not sure if this new type of connector will be solderable by "normal" people. If AMD releases a 120ns or quicker card, and the connector can be soldered, then this will be the basis for a CartDisk rewritable cartridge.

Now what?

I'll be waiting and watching for a better SIMM or miniature FalshROM card to hit the market, before I commit myself to a FalshROM cartridge design. In the meantime, if The Digital Fantasy comes up with a good/working SRAM emulator schematic to use SIMM DRAM, then I will commit myself to that.

John Pappas (DiskDude) is not connected or affiliated with any mentioned company in any way. The opinions of the Turtle Group Inc. do not reflect the views of the various companies mentioned here. Companies and all products pertaining to that company are trademarks of that company. Please contact that company for trademark and copyright information.