DRAM vs. SRAM Memory upgrades?

[+Larry]

Any particular advantages to either of these types of upgrades when applied to the A8? Such as wiring complexity, compatibility, etc.

 

-Larry

[Rybags]

Not that I'm an expert on the subject, but:

 

SRAM is more expensive. But, Atari upgrades tend to be in 64K multiples up to half or 1 meg additional, so it's not too much of an issue.

 

SRAM doesn't need to be refreshed - so, an upgrade can be made simpler.

 

Many modern DRAMs have just one refresh line to worry about, rather than the older system of strobing rows - therefore making some upgrades simpler (AFAIK, some upgrades to >64K need an XE Antic since it provides 9 bits of row info for refresh).

 

SRAM is much easier to be made non-volatile (ie run on battery with the machine turned off), since no refresh support cirtuity is needed.

 

Compatibility - well, a memory upgrade either works or it doesn't. The Atari itself isn't really aware of what type of memory is there.

 

One possible problem is that you don't always want the lowest 1K of RAM to be non-volatile. XLs and later use 3 "Magic Numbers" in low RAM to determine if a Reset was caused by power up or pressing the Reset key. So, if those numbers stay set, the Atari won't automatically coldstart.

 

Problem? Not too much - the solution would be to have a battery decoupling mechanism so you could clear your RAM when powering down if you wanted to.

 

There is at least 1 SRAM battery backed cartridge project around - 512K bank switched. Probably not as attractive now since you can just buy a flashcart ready to go at a reasonable price.

Edited December 20, 2007 by Rybags

[Tickled_Pink]

Problem? Not too much - the solution would be to have a battery decoupling mechanism so you could clear your RAM when powering down if you wanted to.

SRAM doesn't necessarily mean battery backed RAM. Static RAM is just as volatile as DRAM. It just means that it retains the memory contents within its cells when the device it's attached to is on, without the need for constant refresh of its internal capacitors like DRAM does. So it's still volatile memory, just with a different (more expensive) technology used to store the data.

 

The fact that it's much simpler than DRAM to install into a system is why SRAM is used for non-volatile battery-backed RAM.

[DamageX]

Which Atari are you working with? I know that in the 800XL, if you're just going for 256KB you can take out the 64K*1 DRAMs and plop some 256K*1 DRAMs in there and, in a way, most of the "wiring" is already done. There is some additional logic needed (schematics should be on the web) but I think it works out to less soldering than what you'd need for a connecting a single 32-pin SRAM chip.

 

In general SRAMs are easier to connect though because you don't have to worry about refresh, and timing and addressing are simplified.

[HiassofT]

There is some additional logic needed (schematics should be on the web) but I think it works out to less soldering than what you'd need for a connecting a single 32-pin SRAM chip.

You can also piggy-back the SRAM on top of the OS-ROM (maybe the Basic ROM could also work, haven't checked the pinout yet) which saves you a lot of time.

 

Last year I created a very easy to build 512k SRAM upgrade (it only requires a GAL, an SRAM plus a bunch of resistors). It's quite similar to the upgrade made by Bernd and Mega-Hz, mainly because we were working in parallel and often exchanged ideas :-) Like Bernd's upgrade, this one also supports separate Antic access (256k Rambo compatible).

 

I packed the build instructions (only a text-file - sorry, no schematics) plus the GAL source code and JEDEC file into a ZIP and uploaded it to my website:

 

http://www.horus.com/~hias/tmp/sram11.zip

 

so long,

 

Hias