Modifying the SEGA SATURN to use standard SRAM backup/disable RTC.

The purpose of this mod is disconnect the SMPC from the battery, allowing for it to last much longer than what it does with SMPC RTC module drawing power from the battery.

For backup memory, the saturn uses an special chip from Epson, which is an 2 chip enable SRAM developed specifically for backup purposes. It's not the cause of the severe battery draw the SATURN has but in order to get an reliable solution I had to get rid of it.

So after I removed the SRM20257 chip, I replaced it with a very low power version of 62256 from Hyundai.

To make it work I added the following circuit:



Another circuit was added to control the /CE pin from the SRAM, using an mitsumi MM1026 SRAM watchdog circuit to protect the data during power cycles. After this was installed, all power going to the SMPC chip from the backup battery was cut.

The reason this circuit was added and the SRAM chip was changed is because the SMPC has two roles: It does part of the chip mapping on the Saturn address space and it serves as SRAM watchdog when the power is disconnected.

So the circuit with the inverter and OR ports make sure that the RAM still appears on the expected address space but makes the watchdog part of the SMPC redundant as the MM1026 is what is really controlling the SRAM enable as well as powering the SRAM chip.

The way the Saturn is wired right now causes the language selection screen to show up every time it's powered up, but the backup memory space is aways intact. :nod:

Now, all I need to do is wait and see how many years this battery cell will last. :shrug:

 

Nice mod maybe I can get this done to my Saturn as I always forget to remove the 4m cart to transfer my saves to my cart and lose data more often then I care.

 

In all of the time I've owned a Saturn I don't really recall losing any saves to lost power, though I had the tendency to keep mine plugged in.

What I'd kill for is a replacement for the capacitor Microsoft used in the XBox. Damn thing dies in 14 hours or so. Bunny investigated replacing it with anything remotely looking like a battery found it lasted about as long as any other console but didn't bother disclosing where the capacitor is. Probably could find it myself if I bothered.

Assembler posted a thread here a few years back detailing different batteries for which to replace the CR2032 with. My personal favorite was the uber military grade battery with a hypothetical life span of years. Though I'd imagine anyone leaving a Saturn unplugged for more than a few months is not likely to be worried over their saves.

 

x

 

Very neat mod as I remember it seems like the Saturn loses saves pretty regular for me. Batteries seem to die out quickly. It's a shame they designed it this way. They must have wanted to sell alot of memory carts.

 

I thought it was a nice function to have. Saved cash and there was enough for a few game saves. Saved on parents buying a new console only to realise that as soon as you started playing it you needed to go back out and buy a memOry card straight sway!

 

I agree it's nice, but as his mod describes they did some silly things that cause tremendous draw on the poor little battery when really it should only have powered the bare minimum of transistors to get the optimal battery life.

 

Question(s):

Popping in a 64kb sram chip won't do anything would it? IIRC earlier I had this discussion with someone and basically the thought was that without hacking the BIOS (gee, that hasn't happened yet right?</sarcasm>) the Saturn wouldn't be able to enumerate the extra space or maybe even cause unintended malfunctions.

This also might be a stupid question, but is there a reference circuit for the watchdog circuit somewhere?

 

The Sega CD apparently supports Backup RAM sizes in the BIOS that are not standard. I thought I read you could change out the internal 64kbit sram for a 256kbit sram. And in the cartridges I've heard you can put way more than the US standard of 1 megabit of sram. The carts themselves have a spot for another 1 megabit chip. Someone on another forum said they made their own cart with an insane amount of ram for it.

So the Saturn made indeed be able to figure out there is more internal space available to use and be able to use it.

 

I believe l_oliveira has made his own SegaCD backup cart with a 8mbit chip onboard, problem with wanting to use onboard is that the SegaCD tends to wipe saves.

I'd wager this circuit could be adapted for the SegaCD.

 

Hm I have an 20 years old MEGA-CD unit and It has never lost any saves... (Well I never left it alone for anything longer than six months).

It's internal battery is one of those Sanyo MNO2 rechargeable batteries. I suspect it could retain data for at least 1.5 year if fully charged. :thumbsup:

Since my last post on this thread I didn't touch my Saturn and finally I turned it on before posting.

This is what I found:







Of course it didn't keep the calendar since I disconnected the SMPC from the battery, but it didn't lose any data ! :clap:

 

This:



(Screenshot of actual real hardware. Very 1st revision of the JPN BIOS is incompatible with this but it works on all other versions regardless of region or console type)

512KB SRAM chip in a Backup cartridge. I don't think you can put more than 8KB internally as one can't tinker with the size register (it's inside the 208 pin SEGA VLSI, no ?)

Anyone willing to reverse engineer the CD BIOS to check if it would be worth the trouble hacking in more backup ram ? :shrug:

The original hint about the MEGA-CD ram size came from either Steve Snake or Charles McDonald. Too bad it was so long ago I can't even remember ... :\

 

Since someone mentioned the Sega CD, the battery in there WILL last >1 year if charged fully.

 

Now that I take another look at this, what exactly is the 7404 in the diagram? SN7404 hex inverter?

Digital logic is not my forte...for now.

 

Anyway you could re-upload the images?

 

Just did fix the links which Google broke for whatever reason only God knows...

 

Awesome.

 

Thanks, will using larger SRAM actually show in the memory manager? or did you use the same size in the replacement chip?

 

I think that was being said about the SEGA/MEGA-CD ...

Would you want to try ? Now that official schematics for the Saturn are out you might succeed at it... (If the BIOS do support it, though)

 

If you can find me a retailer of the SRAM with maybe double the current size of the installed SRAM I will try it, but if the BIOS doesn't support it it might just read what the original size was.