Building a TI-99/4a FlashROM 99

FlashROM 99, what is it? It is a rather cleaver cartridge board created by RalphB of the Atariage TI-99 forum (odd the best TI-99 forum is on a Atari site). With RalphB’s board, assorted chips, resistors and RalphB’s pre-programmed Atmel chip you can load any number of TI-99 ROM based cartridges from an SD card right into your TI-99/4a.

First, here are some important links.

RalphB GitHub build info, software and instructions.

The respiratory of FlashROM 99 compatible files:

Some info and links from <omega> of Atariage.

From GitHub, here is what you will need:

  • 1x FlashROM 99 board (can get from RalphB)
  • 1x ATmega 8515 (either 8515-16 or 8515L-8) (also can get from RalphB)
  • 1x 62256 32K SRAM (any speed rating)
  • 1x 74HCT377 (LS may also work, but untested)
  • 3x 74HCT541 (LS may also work, but untested)
  • 6x 100 nF capacitors (e.g., multi-layer ceramic X7R, no electrolytic!)
  • 1x 68 Ohm resistor
  • 1x 330-470 Ohm resistor (depending on LED used, smaller values will be brighter)
  • 1x 10 kOhm resistor
  • 1x LED (e.g., 5 mm red or yellow)
  • 1x micro push button
  • 1x SD card module

I ordered everything from RalphB. In reality, you could assemble everything yourself if you knew how to create the board and program the ATmega. I, personally, do not know how so that is why I ordered it all. RalphB will also gladly sell you a completed board but where’s the fun in that.

If you want to order the completed or do-it-yourself board and/or components from RalphB just drop him a line on Atariage.

Tools you will need:

A good surface to solder the board on or a helping hand device.

a magnifying glass helps to inspect the solders.

Tweezers to place the tiny components.

Soldering iron (station) and solder. Here is the soldering station I use.


A Weller WLC100. It’s a middle line solder station that tends to get the job done and isn’t too expensive. My was about $50.00.

Solder of choice was: sn60/pb40 with flux RA, 22 gauge. I’m not a real expert on types of solder but from my research this looked to be the best for the job of soldering circuit boards.

Best soldering tips, for me, to use is a needle point like this.


Now some soldering tips, pardon the expression.

get a variable heat soldering station. My old soldering iron seemed to get too hot in some cases and would damage the chips. With this variable temperature soldering station I am able to dial the temperature to ‘just right’ by starting low and pulling it up to where the solder melted quickly (within two seconds).

Get a needle tip. I was able to heat just the area I needed and no more.

Wipe off your tip often on the sponge. Too much solder will cause messy connections.

RTFM on the GitHub site closely. I manged to solder one of the chips in backwards and had to remove it and replace it which was a real hassle.

OK, to the build.

Here is the reference photo off the GitHub site I used for the build. Note: the light blue 1 nf capacitor on the right next to the blue resistor is no longer used.


The first thing I soldered on the board was the capacitors, resistors, the reset switch and the LED.


Simple enough since orientation of the capacitors doesn’t matter.

On the back side of the reset switch RalhpB recommends that you put a piece of tape over the contacts. This is need to be sure the switch won’t short on the SD card socket which will come later.


Now I soldered on the CD card socket. The SD Card socket is soldered over the reset switch in the appropriately supplied hole on the board. I made a slight mistake and have the SD card socket device at a slight downward angle. So, be sure to have the SD card socket straight with the board if this is an issue for you.


First, below is a really good YouTube tutorial on sockets, chips and soldering sockets and chips. If you are inexperienced (as I am) watch this first.

Next is solder in the chips and sockets. RalphB notes that you have to thru-hole mount the three 74HCT541 chips in the front or the board won’t fit in the slot. In other words, you have to slide the chips into the board with the pins sliding through the supplied holes. Then solder the chips into place (see notes on chip and socket soldering coming up). Be sure not to bend one of the pins as you slide them through the holes.

I did socket the ATmega 8515 (which RaplhB supplied). I also bought sockets for the 62256 32K SRAM and the 74HCT377. There is room to socket these three chips without a problem.

Now soldering in the three 74HCT541. The next photo is, again, the reference photo off GitHub and shows the orientation of all the chips.


Note that the orientation isn’t consistent. Some face left and others right. I did not look at that closely and soldered in one of the 74HCT541 surface mount chips wrong and had to remove and replace it. In the removal I damaged the chip and had to go purchase a new one @ $.66, bummer.

Here is a picture of me soldering in the ATmega 8515 socket.


I solder chips in from the back, always watching to not put on too much solder. Rule of thumb is to solder for only two seconds. Also, I would flip and cool the chip after every third solder to make sure I didn’t over heat the chip. Another good idea is to do an X pattern in soldering the chip pins. This helps in keeping any one part of the chip from getting too hot at once.


After I soldered in all the chip I tested the completed board in my TI-99/4a.I got a small 512k SD card I had laying around, formatted it with FAT and then loaded on the memory test program:

This program will check all four banks and give an OK if the banks are, well, OK.

Fortunately mine tested OK on the first try. Next I downloaded some games. All tested fine.

Using the raw board is a bit of a difficulty. I found it very hard align the board into the slot properly without using a small flashlight to see where the slot was in the TI-99.

So I made a cartridge for it.

I went down to a local retro computer store and bought an old TI Math cartridge for $3.00. Tore it apart and cut the back off. I inserted the board and put some double sided foam tape to hold it down. this is how it looks.


Some have 3-D printed a cartridge and other have fit the board in so that the slot sticks out the front so that the whole cartridge fits. I elected to do it this way.

And here is the finished project ready to play a game of Parsec.