This is a module that replaces the 3 mask ROMs in a Commodore 64 with a single parallel NOR flash ROM chip that's extremely reliable and still, as of October 2023, being manufactured. Why would you want to do this? Well, sometimes these old MOS mask ROMs fail, and when they do, you have to replace them with a newer part. And to do that, you will probably have to use some sort of adapter to fit a newer EPROM or EEPROM chip into the socket. Well, if you're going install an adapter anyway, why not just do it once?
In addition to improved reliability, this module draws less power and produces less heat than the original mask ROMs. Furthermore, because the flash ROM is a larger capacity (128KB) and reprogrammable, it can also hold 3 alternate ROM images for each ROM in addition to the standard ROM image (for a total of 4 each). 3 sets of DIP switches are provided to select between the ROM images.
While originally designed for my 326298 Rev A board, this will work in the other Commodore 64 longboards that have 3 24-pin ROM sockets.
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The theory of operation of the module is quite simple. Each of the 3 mask ROMs are connected to the system address bus (A0-A12 for the BASIC and KERNAL ROMs, A0-A11 for the Character ROM), the system data bus, and their own individual chip select, which are controlled by the PLA. The module intercepts all of these signals at the ROM sockets.
The 128KB flash ROM is divided into 3 segments:
- A 32KB segment to hold up to 4 8KB KERNAL ROM images.
- A 32KB segment to hold up to 4 8KB BASIC ROM images.
- A 16KB segment to hold up to 4 4KB Character ROM images.
These segments are selected by decoding the mask ROM chip selects and mapping them to the A15-A16 address inputs to the flash ROM:
- 00 - KERNAL
- 01 - BASIC
- 10 - Character
- 11 - (not used)
Within each segment, 2 DIP switches control the upper 2 bits of the offset within the segment:
- A13-A14 for the KERNAL and BASIC segments.
- A12-A13 for the Character segment (A14 is always 0 in this case).
Because A12's behavior is dependent on which ROM segment is selected, the decoding logic intercepts it and adjusts it as necessary.
The decoding, segment selection, and image selection logic is implemented in a GAL22V10. The logic equations can be found in gal-files/c64-128k-rom-mux.gal. You can read the comments in that file for a more detailed explanation of what's going on, but here are the basic rules:
- The flash ROM CE# and OE# signals are asserted when exactly one of the 3 mask ROM chip selects are asserted.
- Because the value of the A12 input to the flash ROM is dependent on which mask ROM is being selected, the assertion of CE# and OE# is delayed by one propagation delay through the GAL, ensuring that A12 will be stable before the output is enabled.
- The individual image selection within the selected segment follows the DIP switch setting corresponding to which of the 3 mask ROMs is being selected.
The module has pins that make the following connections to the mask ROM sockets on the C64 board:
- U3 - 1-24 (all pins)
- U4 - 20, 21
- U5 - 9-24
Building the module is pretty simple. I've used all through-hole parts to make it easier to assemble for the casual hobbyist. You only need a few parts. Most of the parts can be purchased from Jameco Electronics, a small business located in the San Francisco Bay Area that's been serving the elctronics hobbyist community for decades. Alas, there are a few items on the list that Jameco doesn't have, so I've provided links to the parts on Mouser, as well, at least for one parts that Mouser carries (alas, Mouser does not carry all of these parts, either).
- One of the c64-rom-326298 PCBs. The Gerber files are provided here and I have set up a shared project on PCBWay here.
- A standard DIP-32 0.6 inch socket for the flash chip. I prefer the machined type (Jameco part number 105381).
- A standard DIP-24 0.3 inch socket for the GAL22V10. Again, I prefer the machined type (Jameco part number 39386).
- Only if your ROM are not already in sockets, 3 standard DIP-24 0.6 inch sockets to receive the ROM replacer board. For these, the dual-leaf type sockets are actually better because they're a little more tolerant of pins that are not perfectly-aligned (Jameco part number 112264).
- 2 100nF / 0.1uF ceramic capacitors with 2.54mm lead spacing that fits into a 4mm x 2.6mm footprint (Jameco part number 25523 or Mouser part number 594-K104K15X7RF5UL2 are good candidates).
- 1 7-pin 4.7K ohm bussed 6-resistor array in a SIP package (Jameco part number 24660 or Mouser part numbers 279-SIL07E472J or 652-4607X-1LF-4.7K).
- A Microchip SST39SF010A 128k x 8 flash ROM chip, DIP-32 package (Mouser part number 804-39SF010A7CPHE).
- A Lattice GAL22V10 (-15 or -25) or Atmel / Microchip ATF22V10C-15PU. Lattice GAL22V10s can be acquired on the secondary electronics market (eBay, AliExpress, etc.). ATF22V10Cs can be acquired from the usual electronics suppliers (Mouser part number 556-AF22V10C15PU).
- 3 2-position SPST DIP switches (Jameco part number 109059 or Mouser part number 653-A6E2104N).
- Round (machine tooled) pin headers to populate 42 pins. I usually buy these on AliExpress in bulk and have them on-hand, here, for example. I have been told that these pin headers can be found on Mouser with Mouser part number 200-MTSW15007TS180, although I have not verified this myself! Anyway, you must use this type because the usual square pin headers will damage the sockets on the C64 main board.
You must use sockets for the ICs for two reasons:
- There must be clearance for the pins that face downward towards the C64 main board, some of which are below the ICs on the board.
- You probably want to be able to remove the flash chip at some point to reprogram it with additional ROM images.
(I suppose you could solder the GAL to the board after programming it if you really wanted to, but really it's best practice to put it in a socket, in my opinion.)
You will also need a programmer to program the GAL and the flash ROM chip. I use a TL866II Plus, which can program both the Lattice and Atmel / Microchip versions of the 22V10. You want to program the 22V10 with the file gal-files/c64-128k-rom-mux.jed. I do not supply ROM images for the flash chip. You'll need to provide those yourself, either by dumping your existing ROMs or by getting ROM images another way (e.g. from zimmers.net).
If the mask ROMs are not socketed in your Commodore 64, then you need to de-solder the orignal ROMs and install the DIP-24 sockets mentioned in the parts list above. Do this first!
When assembling the board, you want to get the downward-facing pin headers that insert into the ROM sockets as vertically aligned as possible. One way to doing this is to insert the pins into a breadboard, set the board down on top of the pins, and hold the board flat as you tack a couple of the pins to the board to keep it still while you finish the rest. Other option is to insert the header into the board, tack one pin to the board and then, while heating that one solder joint, carefully moving the header around until it is perfectly perpindicular. Just be careful not to burn your fingers!
Next, place the chip sockets on the PCB, flip the PCB over, and solder the sockets to the PCB from the bottom side.
Next, populate the ceramic capacitors and resistor array and solder them into place.
Finally, populate the 3 2-position DIP switches with the "ON" position being towards the flash ROM socket and solder them into place.
I've provided a small command line tool, mkflashimg.sh, to make it easy to assemble a flash ROM image. It's a shell script that should run just fine on Linux, NetBSD/FreeBSD, macOS, and maybe even Cygwin. Here is an example of how to run the tool:
dhcp-194:thorpej$ ./mkflashimg.sh -k kernal.901227-02.bin -k kernal.901227-03.bin -k JiffyDOS_C64_6.01.bin -k JiffyDOS_C64_6.01.bin -b basic.901226-01.bin -c characters.901225-01.bin c64-rom-326298.bin
$KERNAL_0='kernal.901227-02.bin'
$KERNAL_1='kernal.901227-03.bin'
$KERNAL_2='JiffyDOS_C64_6.01.bin'
$KERNAL_3='JiffyDOS_C64_6.01.bin'
$BASIC_0='basic.901226-01.bin'
$BASIC_1='empty'
$BASIC_2='empty'
$BASIC_3='empty'
$Character_0='characters.901225-01.bin'
$Character_1='empty'
$Character_2='empty'
$Character_3='empty'
Write flash image to 'c64-rom-326298.bin' (y/n)? y
-rw-r--r-- 1 thorpej staff 81920 Dec 14 16:13 c64-rom-326298.bin
dhcp-194:thorpej$
In this example, I've put the original rev 2 KERNAL for my machine in KERNAL slot 0, the rev 3 KERNAL in KERNAL slot 1, and JiffyDOS in slots 2 and 3 (so that no matter the setting of the "0" bit of the KERNAL selector switch, if the "1" bit is set, I will always get JiffyDOS). Once you've assembled the flash ROM image, use the programmer of your choice to write it to the flash ROM chip.
And that's it! Just pop the programmed flash chip and GAL into the sockets, set the DIP switches to select the ROM images of your choice, insert the module into the C64, and enjoy!
There are other ways you can use this board, too! For example, if you buy a JiffyDOS ROM chip, a switch is provided to let you select JiffyDOS or the stock KERNAL. If you want to install such a switch, all you need to do is tack the wires from the switch to the appropriate DIP switch solder pads on the board; if either the DIP switch or external switch for that position is set to "on", then that switch position will be "on". It's up to you to decide if you want to drill a hole in your precious Commodore 64 to do this!
If you have any questions about the board, you can reach out to me on Twitter (@thorpej) or Mastodon (@thorpej@mastodon.sdf.org). You can also check out my YouTube channel, which has this and other retrocomputing related content.