Adding RAM retention to Game Doctor (Attn. Drakon, 133Mhz,)

[petik1]

One pesky problem the Game Doctor has is that every time you unplug the unit, all the files loaded are wiped out. I got to thinking, and I think it would be possible to add a button cell battery and have it function just as it would inside a game cartridge, or a CMOS battery in a PC. My problem is my lack of expertise. Would it be as simple as putting a diode and voltage regulator from the battery's positive terminal to where the RAM takes its usual +5V, and connecting negative to ground?

The RAM is four NEC 424800-10. Datasheet: http://www.dip8.ru/pdf/mpd424s4800g5.pdf

I'll try to dig up as much info as I can.

[famiac]

What an amazing idea. This definitely deserves input from a mind with more knowledge in this field. Game doctors are a pain in the bum.

[petik1]

Not gonna work that way - you have to organize refresh

English pl0x

[133MHz]

Unfortunately your device uses Dynamic RAM (DRAM) which needs to be constantly refreshed by the computer in order to retain its data - even if you keep it powered up the bits will fade in a short time without refresh, and that's not counting its relatively high power consumption (for battery powered applications). Game cartridges use Static RAM (SRAM) which doesn't need refresh and has a very low idle power consumption so the data can be retained for years under battery power.

For your idea to work you'd need to replace the DRAM chip(s) in your Game Doctor with SRAM (which isn't straightforward because of pinouts and the multiplexed address bus) and then implement the usual battery backup scheme, either two low dropout diodes or a dedicated watchdog chip and a battery. Depending on how the Game Doctor drives the DRAM you'll need to implement a latch to demultiplex the address lines and adapt the control lines accordingly.

[133MHz]

Back in my high school days I used to have a Macintosh PowerBook 145 laptop with a whole six megabytes of pseudo-static RAM (2 built in and 4 in an expansion board I think). Hours of fun playing Dome Wars with my mates and when the original 6V NiCd battery died, I hollowed it out and installed a Tyco R/C battery pack inside. Good times, good times.

[petik1]

Thanks for the info 133mhz. Thats a little disheartenimg. I'll assume theres circuitry in the GD that refreshes the RAM. Would that have any adverse effects with SRAM installed?

[petik1]

Will do.

Say, what exactly is refreshing the DRAM? Its obviously not always accessing a floppy. Something's storing that data.
Post Merge: June 17, 2012, 08:35:33 am
Here ya go 80sFreak: http://flic.kr/ps/NTZfw

Note that this is a Game Doctor SFIII. I'm not familiar with any other version, or how their internals differ.

[famifan]

Will do.

Say, what exactly is refreshing the DRAM? Its obviously not always accessing a floppy. Something's storing that data.

zomg! read some from the wiki http://en.wikipedia.org/wiki/Dynamic_random-access_memory if you can't into the datasheets reading.

usually CPU is refreshing the DRAM.

[petik1]

Yeah because, you know, I understand all of this.

/sarcasm

It's easier to understand when someone can describe process into simple terms for a person who has little knowledge on the subject. So if I'm not mistaken, the data is stored into the RAM, is read by the CPU, and rewritten before it has the ability to leak. Right?

I'll link to each of the IC's datasheets to make it a little easier.

• Goldstar GM82C765B: http://tinyurl.com/7hq4qwu


• Asahi Kasei AKM62256: http://tinyurl.com/7llbrad


• Goldstar GM76C28A: http://tinyurl.com/883wtak


• GD 3.3C BIOS (No datasheet needed, standard EPROM)


• Fairchild 9338: https://viewer.zoho.com/docs/xbdcUag


• HD74LS00P: http://tinyurl.com/7nh6qhe


• NEC 424800-10: http://tinyurl.com/cnstbx7


• NEC 424400-80: http://tinyurl.com/bqsyhut

Besides that, there is what looks to be a custom chip which has on it "DSF301 BUNG GF037 9406PX700"

I suppose I couldn't have the DRAM and the CPU both with a backup battery, could I?

[famifan]

So if I'm not mistaken, the data is stored into the RAM, is read by the CPU, and rewritten before it has the ability to leak. Right?

yes. But typical refreshing is not rereading and then rewriting, it is just the accessing to the data rows without doing anything else (such as reading or modifying).

Post Merge: June 17, 2012, 12:59:37 pm

• GD 3.3C BIOS (No Datasheet)


• HD74LS00P: (No Datasheet Found)

the first seems to be the typical 8bit parallel ERPOM (27Cxx)
the second is just well-known 74LS00

[petik1]

I figured it would be and EPROM, and I found a lot of pages about the 74LS00, but no datasheet. Hmph.

@133MHZ. Lines for A0-A9 and I/O1-I/O8 on the 424800 would need a demultiplexer when going to SRAM, right?

Can anyone point me in the direction of how to replace DRAM with SRAM, or am I getting way over my head?  

I found that the custom chip is indeed an ASIC.

[petik1]

Hell, I've got a job this summer, so I can actually fund this project. And honestly, it's more of a learning experience. Whether it actually works or not, I don't care. In the last few days of researching the topic, I now have a good understanding of some basic circuitry. I'm all for seeing this through.

[petik1]

Errm, not sure what Snowy Park or Midlands is, but ok!

If I can put together a parts list and a schematic, I'll give this a go.

Temporary Parts List:

• x8 512kB SRAM chip


• x? Demultiplexer


• x? Diode


• x2 2032 button cell


• x2 CR2032 cell holder


• x1 7805 Regulator (Or could I make use of the one on board?)