Genesis:FRAM Mod

The Sega CD holds saved games in a Static RAM (SRAM) chip on the Sega CD motherboard. The SRAM chip requires constant power in order to retain the game saves, which an on-board coin cell battery supplies. In the event that the battery dies, all game saves will be lost. However, it was discovered that the SRAM chip can be replaced with a Ferroelectric RAM (FRAM) chip which retains information regardless if power is available or not — and optionally, you can use a larger capacity FRAM chip in combination with physical switches to allow access to four separate data banks.

This mod applies to both the Model 1 Sega CD, Model 2 Sega CD, and the Sega CDX (and all "Mega" branded equivalents). It is unknown whether it is possible on a JVC X'Eye.

Choosing Chip
First, choose a FRAM chip to replace the stock one with. Replacement chip candidates include:


 * FM16W08-SG - 64kb (matches stock SRAM size, go with this unless you plan on bankswitching)
 * FM18W08-SG - 256kb (extra space not seen by BIOS, but can be bankswitched)
 * Some chips are sold without "-SG" on the end. -SG just means it's a SOIC package type (instead of -PG which is thru hole pins, which is not what we want).

Required Materials

 * Tools
 * Soldering iron
 * Hot air rework station
 * Multimeter with continuity mode
 * Tweezers
 * Philips head screwdriver
 * Components
 * Your chosen FRAM chip
 * Two SPDT switches (or one ??? switch)
 * Insulated wire
 * Solder
 * Flux
 * Desoldering braid
 * Isopropyl alcohol (at least 90%)
 * Swabs for cleaning (microfiber/foam are better than cotton since they don't leave strands behind)

Removing SRAM Chip

 * 1) Disassemble the console and pull out the motherboard.
 * 2) Locate the SRAM chip. It should be marked either MB8464A-10L (Model 1 or Funai produced Model 2) or MB8464A-80L (Sony produced Model 2) or LC3664BML-10 (Sega CDX).
 * 3) Optionally, cover nearby components with kapton tape or foil to protect them from heat.
 * 4) Turn on the hot air rework station and set it to 360 degrees Celsius with medium air flow.
 * 5) Start by preheating the chip from a distance of 4-5 inches with the hot air station and very evenly distribute the heat around the chip for 30 seconds or so.
 * 6) Next, circle the perimeter of the chip with your hot air nozzle, making sure not to apply heat to any one location for too long.
 * 7) * Have patience. This will take time, especially if the chip has adhesive underneath it. The hot air at this temperature should not hurt the board as long as it is evenly applied.
 * 8) While continuing the previous step, check if the chip has loosened with tweezers or another small implement by grabbing the side of the chip and gently try to lift without applying force. Once the chip is ready, it will easily come off.
 * 9) * Warning: If force is applied here, the solder and pads may pull up along with the chip!
 * 10) Once the chip eventually separates from the board, turn off the hot air rework station.
 * 11) Turn on the soldering iron and wick up any remaining solder on the pads with solder wick to make an even surface.

Installing FRAM Chip

 * 1) Special considerations:
 * 2) * If using a 256kb module and are not planning on bankswitching, lift pin 26 and attach it to GND since it is used as an address line and would be connected to the battery backup and produce undesired behavior.
 * 3) * If using a 256kb module and are planning on bankswitching, lift both pin 1 and pin 26 as these are the address pins that will be attached to switches.
 * 4) Place the chip on the board, noting the orientation of the chip should match the marking on the board.
 * 5) Line up the legs with pads on the board, and apply some flux to one corner.
 * 6) Once satisfied, tack one leg down with the soldering iron to fix the chip in place.
 * 7) * It may help to gently rest a finger or tape on top of the chip to hold it in place while you tack the first leg down.
 * 8) Repeat on the diagonally opposite corner of the chip to anchor the chip.
 * 9) Apply a generous amount of flux, and then solder the rest of the legs down with a dragging technique.
 * 10) * Ensure that all legs aside from pin 26 are soldered to the pads on the board with good solder joints.
 * 11) Use the multimeter to double check that no legs are bridged with their neighbor pins, and that all legs have good connections to the board.
 * 12) Clean up any excess flux with some isopropyl alcohol and swabs.
 * 13) Put the Sega CD console back together enough to connect to the Sega Genesis and test.
 * 14) Power on the console, format the system memory from the BIOS (dependent on Sega CD model and region).
 * 15) Play a game and save some data to the memory.
 * 16) Power off the console and disconnect the power for at least 30 seconds.
 * 17) Power it back on and verify that the memory is still saved. If so, the installation is complete.

Enabling Bankswitching

 * 1) Identify nearby solder points to get ground and 5V. Depending on the location of the chip, a good ground point might be one of the various exposed points on the edge of the board. Use a multimeter in continuity mode to identify a 5V point. The power supply connector in the corner of the motherboard is a great place to start with testing for this.
 * 2) * Touch one probe of the multimeter to the 5V pin on the connector, and then use the other probe to test other nearby solder points until the multimeter produces a long tone. This could be a leg on a chip, capacitor, test pad, etc.
 * 3) Solder a wire to the already-lifted pin 1 to switch 1, and two more wires from the switch to GND and 5V to allow pin 1 to be switched between GND and 5V.
 * 4) Solder a wire to the already-lifted pin 26 to switch 2, and two more wires from the switch to GND and 5V to allow pin 1 to be switched between GND and 5V.
 * 5) Test each bank by powering it on, saving data to it, powering it off, changing flipping switches to a different position, and repeating for each bank; verifying that no data is lost when you revisit a bank.

Troubleshooting
Sometimes installations go wrong. It happens. Here's a few tips to try and salvage things if it doesn't work the first time.
 * Double check that no adjacent pins are shorted together.
 * Double check that all pins are connected to the solder pads.
 * Follow the trace of each leg of the chip and verify continuity if possible.
 * Some traces might go to a via that's underneath another chip. It should be accessible on the other side of the board.
 * If there isn't continuity, the solder pad might have been lifted, and a jumper wire may be necessary.
 * Verify that the lifted legs / conducting wires aren't shorting on the RF shielding of the case.