Reusable memory card

[jzaun]

I'm looking to build a reusable memory card for my projects seeing as SRAM costs can add up. I've attached the circuit diagram and this is the link to the Upverter Project.

Are there any suggestions or issues or comments? I'm still very much learning so the more the better. Really the most I've done in the past has been minor AVR based projects. I'm planning on making 2 for use with a W65C816S but I figure I could use one in a W65C02S based project as well, just don't use all the address lines.

Code: Select all

AS6C8008-55ZIN    - $7.24 x 8  = $57.92
CC0603ZRY5V9BB104 - $0.10 x 21 = $2.10
CD74HC238PW       - $0.62 x 1  = $0.62
SN74ABT2827DW     - $2.06 x 3  = $6.18
SN74AC245DW       - $1.01 x 1  = $1.01

So about $67.83 in parts, plus PCB costs and time.

[BigDumbDinosaur]

jzaun wrote:

I'm looking to build a reusable memory card for my projects seeing as SRAM costs can add up. At the moment I've got this:

6502.org wrote:

Image no longer available: https://upverter.com/Hooka/61cdeb8a9dc2975b/Memory-Card/embed_img/14508261850000/

Are there any suggestions or issues or comments? I'm still very much learning so the more the better. Really the most I've done in the past has been minor AVR based projects. I'm planning on making 2 for use with a W65C816S but I figure I could use one in a W65C02S based project as well, just don't use all the address lines.

Code: Select all

AS6C8008-55ZIN    - $7.24 x 8  = $57.92
CC0603ZRY5V9BB104 - $0.10 x 21 = $2.10
CD74HC238PW       - $0.62 x 1  = $0.62
SN74ABT2827DW     - $2.06 x 3  = $6.18
SN74AC245DW       - $1.01 x 1  = $1.01

So about $67.83 in parts, plus PCB costs and time.

The image to which you linked doesn't seem to want to load. Are you aware that you can attach files to your messages?

[BigEd]

(It's good to attach images and files, because in the past we've found things have vanished from the wider world, after a year or more, and yet in this world of 6502 enthusiasts, going back to threads several years old is a common thing to do.)

[jzaun]

@BigDumbDinosaur and @BigEd,

Sorry about that I've updated the original post with the attachment to the circuit.

[GARTHWILSON]

I was not aware of any 5V SRAM ICs that had over 512KB, which is what the 4MB SRAM module I make and sell has eight of. It's 2.300" long x 1.234" wide, not including connector. There are four half-megabyte SRAM ICs on each side. It's 10ns though, not 55ns, so over five times as fast, and still 5V. The data sheet is at http://wilsonminesco.com/WM-1_4Mx8SRAMm ... -12-15.pdf, and there are a couple more pictures on the front page of my website linked in my signature line. Here's a bare board:

WM-1PCBinHand.jpg (29.15 KiB) Viewed 2472 times

Although I have to make a little money on the manual assembly and testing, the fact that I bought boards and ICs in much higher quantity than you will may mean the whole thing turns out to not cost any additional money, and you can have them a lot sooner. I'll keep making them as long as the same (or compatible) parts are available, which I expect will be a long, long time. Most customers only order one at a time, but a couple of customers ordered four at a time, to fill up the 816's 16MB space. I can supply the sockets too, either soldertail or wire-wrap, and the module's pins can be either straight or right-angle. They're on .100" centers, so they're very hobbyist-friendly, as that's the hole spacing on perfboard.

[jzaun]

@ GARTHWILSON,

I ran across your memory card in another thread after I started on this. I was planning on contacting you to see about the cost of one in the future, but I'm still looking to finish this up myself. I know 55ns is a lot slower than other chips out there but I'm not out for speed as much as trying to learn as I go. I know I'll get it wrong a few time but I'm hoping to have some fun too

It's nice to know I wasn't completely in left wing with a memory card idea though and yours looks just about what I would want, though most likely mine will have all the chips on one side. I'm new to surface mount soldering. Heh, new to a lot of this really.

- Justin

[GARTHWILSON]

BDD did a bigger one years ago with all 8 ICs on the same side, like this:

[Dr Jefyll]

jzaun wrote:

Are there any suggestions or issues or comments?

Welcome, Justin. The decoder you've chosen ('hc238) may be a speed bottleneck -- it could be upgraded by switching to 'ac238.

The '238 style has active-high outputs. Even faster decoders are available -- and I expect the fastest of all will be in the more popular '138 style, with active-low outputs (such as CY74FCT138CT, rated at 5 ns max). It looks as though your circuit could easily be rearranged to use a '138 style decoder -- just swap the two CS inputs on the RAM's. One side effect is, the signal you call CE# (from buffer 3) would then become active-high, so at least a minor change is involved re the decoder enable inputs. Dunno if that matters.

The pinout you've chosen for the bus connector needs more ground and Vcc connections, partly for redundancy and also because their physical location makes a difference -- they should be interspersed with signal connections so that none of the latter is too far from a Gnd (or Vcc) connection. Garth has attended to this in the connector pinout for his RAM board, and so should you with yours.

In the same vein, perhaps you can find a more modern equivalent to the 74abt2827 bus drivers. I expect there's an equivalent chip that places its Gnd and Vcc pins at the center of the package, rather than the old way (diagonally at the corners). I wouldn't lose any sleep over this detail, but if you're eager to do the best job possible then see what's out there.

For researching and selecting IC's I have found Digikey's "Product Search" to be extremely effective. It has led me to chips I didn't know existed, and which perfectly fit the application. Recommended! -- well worth becoming familiar with.

cheers,
Jeff

[BigDumbDinosaur]

GARTHWILSON wrote:

BDD did a bigger one years ago with all 8 ICs on the same side...

I also did a four layer version that was slightly more compact. Both designs plug into a standard PCI slot connector.

4MB SIMM, Four-Layer

[GARTHWILSON]

Quote:

such as CY74FCT138CT, rated at 5 ns max

I was disappointed today to find out that this part's outputs only pull up to 3.3V, and 74ABT is similar. Is there anything faster than 74AC that pulls up to 5V??

[Dr Jefyll]

Some of the 74LVC stuff is really fast. But, disappointingly, not all of it is rated to run on 5 volts. The '138 style decoder is one of the parts that's 3 volt only. For 5V operation you'd have to go with the 74LV device (not 74LVC). Datasheet attached. The speed is probably less than what you're hoping for.

sn74lv138a.pdf

(1.05 MiB) Downloaded 152 times

[GARTHWILSON]

Fortunately the only things that really need the high logic 1 voltages are the WDC parts. Memory usually doesn't (including my 4MB module), and 74xxTxx doesn't, and that CY74FCT___C is indeed pretty doggone fast. At least it's available in the '573 and '138 (SOIC only). Neither of these two have any timings over 6ns max. I just printed out their data sheets. Thanks, Jeff. I'll be ordering some.