Success! First ever super capacitor and battery-less backup RAM on a Turbo Duo.
Well, ever since I discovered these new FRAM and nvSRAM devices available on the market that don't require power to keep their contents, I was curious if it was actually possible to make one of these work as a replacement for the 2 KB chip that came as the standard backup RAM in PCEngine and TurboGrafx CD-ROM accessories. The short answer is YES. I've done it and it works.
I performed this mod on a TurboDUO system that I recently recapped and performed a bunch of other mods as well. I figured what the heck, lets try batteryless RAM too. After looking at the spec sheets of both FRAM and nvSRAM devices, I settled on the nvSRAM. For two reasons, it supports the same access method for addresses as the original RAM in that you can change address lines without bringing the chip select line back high in-between reads and writes (which FRAM REQUIRES), and the second is because it is cheaper in price.
The new chip installed here is a nvSRAM from SIMTEK (Cypress). It is a 8 KB AutoStore SRAM hybrid chip consisting of standard SRAM cells with FLASH cells paralleled on top of the SRAM bits. The way these nvSRAM chips work is that they have a flash cell manufactured onto the top of a normal SRAM cell, when power fails, they program all of the flash cells in parallel from the bits stored in the SRAM cells using a charge from the "VCAP". This process takes about 10 mS to complete and is how the chip stores the RAM contents without a battery. On power up, the chip automatically parallel loads the flash bits back into the SRAM bits. This process is all done in the chip automatically without software intervention.
This mod should work on any PCEngine/TurboGrafx hardware that has the Sanyo backup RAM hardware in it. These instructions detail the steps required on the US Turbo DUO console that I own. I also plan to mod a pair of original Turbo Grafx 16 CD-ROM dock units that I own as well in the future.
If you are really paying attention you will note I am using a RAM chip 4 times bigger than the original because this is the smallest size they make for nvSRAMs. I also properly wire up the extra address lines A11 and A12 so the turbografx hardware can access all 8 KB. From a technical standpoint this should work but it is untested by me whether the extra 6 KB is usable by games. I am hoping the current games that use the backup ram can actually see the extra RAM, if not, the backup ram routines in the system card can certainly can be hacked to work with it.
I've noticed all the emulators out there seem to implement 8 KB backup RAM anyways, so this is all probably working already. Again I have not tested that yet. Certainly another option to to install some kind of bank switching to these two pins A11 and A12 to switch in 4 banks of 2 KB at a time instead of wiring in A11 and A12 as I have done. My own personal plan is to develop a backup ram manager, if needed, to simply swap a page into the first 2 KB of RAM if the system does not use the whole 8 KB on its own.
Please do not attempt this mod unless you have expert soldering skills, this is not a mod for the faint of heart.
The basic steps to perform this mod is to unsolder the original Sanyo RAM chip (IC512), capacitors C511 and C509,
and resistor R543. You may also want to temporarily remove C311 because it is in the way of easily removing IC512.
I was also performing a Full cap replacment at the time, so this was not in my way anyways.
Then you add two new 0.1u ceramic caps, a 100u cap, and a SIMTEK STK12C68-SF45 nvSRAM chip to finish.
STEP 1:
You need to remove some parts and scratch away some of the solder resist from some traces on the TurboDUO board.
Please see the following picture. You remove the Sanyo LC3517BML chip (IC512), the ceramic cap C511, the super capacitor C509, and the charge resistor R543. I have circled all the parts to remove in Red. Be very careful to not damage any of the pads for IC512 as you remove the old RAM chip. A hot air rework station will make quick work of this, I did one pin at a time using a fine grade solder wick and lifting one pin at a time very carefully.
You also need to scratch away the solder resist at the four lines I indicated in Orange. Once the copper is exposed, tin with solder and then use solder wick to clean the new pads. Two new 0.1u 0603 size cermaic capacitors will be installed on these new pads. See the Step 2 picture to see how they are placed.
STEP 2:
This shows the parts removed and two new 0.1u 0603 size caps installed on the new pads made in Step 1. I also
removed C311 in order to make it easy to remove IC512 without it being in the way of the soldering iron. I was also doing a full cap replacement at the time. The two circled orange spots show the new capacitors installed.
Now cut and place two pieces of electrical tape down to protect the traces that the new RAM chip will be on top of and also to protect accidentially getting any solder on pads we don't want to solder to. Pins 21 and 24 must be covered, we will not be soldering to these pins. I also covered up the three traces that run beside the chip to make sure I didn't damage them. These two pieces of tape will remain under the new RAM chip.
STEP 3:
Now prepare the new RAM chip, this will go alot easier if you pre-solder on the wires you need before you solder the bulk of the pins down on the original pads of the old RAM chip. Use Kynar wire to do this. Slightly bend up and solder a length of Kynar wire onto pins 1, 2, 23, 27, and 28. Also, slightly bend up pin 26 but do not solder a wire onto this pin, it will be left unconnected. The reason to slightly bend up the pins is so the entire RAM can sit flat on the old pads of the board and the bent pins sit on top of where there is electrical tape.
I have labeled the signal names of all the wires that are attached to the new RAM chip.
STEP 4:
Carefully line up the new RAM chip onto the pads of the board. Solder each pin down where you have a pad, and don't solder the pins down that are over the electrial tape we laid down earlier. These wires will be attached to the inidcated points onto the board instead, except for pin 26 which will remain unconnected.
For each wire, trim to length and solder to the points shown on the picture. Two wires A11 and A12 will not be soldered to the top of the board, instead route these wire through the two holes shown on the board and proceed to Step 5.
STEP 5:
Solder the remaining two wires to the points shown on the picture on the opposite side of the board.
Step 6:
Solder a 100u electrolytic capacitor into the holes of the old C509 super capacitor. This capacitor provides a charge to the new nvSRAM chip on power failure in order to save the RAM contents into the flash memory of the chip.
If you removed C311, you can now reinstall it.
Step 7:
Put it all back togethor, crack open a beer, and enjoy the fact your save games will no longer disappear on you if you leave your console off for over 2 weeks!
Feel free to ask any questions. However I will answer one right away, No I won't mod your console for you, it takes a bit of time to do and I'm quite busy with my other projects.
Topic: Success! First ever super capacitor and battery-less backup RAM on a Turbo Duo. (Read 403 times)