ReSeed

ReSeed

ReSeed is an expansion card for the Commodore 16 and Plus/4 computers that allows interfacing the computer with a MOS 6581 or 8580 Sound Interface Device, the chip that is used to deliver audio on the Commodore 64.

It is based on a design by Solder of Synergy.

Summary

Sounds from the C64 now for the Plus/4! Just plug the card into the Expansion Port! You can adjust the volume on the card. You can hear the sound over the Plus/4 audio or take it over a 3.5mm-Stereo-Cinch to a mixer or HiFi-Device. A 9-pin D-SUB connector allows connecting a 3^rd (analog!) joystick or a proportional-mouse or (why not?) paddles!

(Slight rewording of Solder's original description of the card.)

Design

ReSeed is derived from Solder's SIDcard, with a few enhancements.

Modifications Introduced in V2

The focus for V2 was on reducing the output noise.

• New component values for the step-up voltage regulator: The values of some components were fine-tuned (again) in order to place the switching frequency of the step-up voltage converter way out of the audio band. These new values can also be retrofitted on a V1 board and will improve things dramatically (See V1.1).
• Alternative voltage step-up module: Cheap step-up voltage boost modules can now be used instead of the TL497-based circuit. Such modules, which can be easily sourced from Chinese portals, are much more modern and use a higher switching frequency, resulting in (near-)zero switching-induced noise. Using them also speeds up the assembly process, as many components can be skipped.
• Linear Regulator: If you are really paranoid about switching-induced noise, you can use the above alternatives as a first-stage voltage regulator and then make a second stage through a linear regulator, which should "eat" any remaining noise and provide the SID with very stable power.
• Separate grounds: A separate audio ground plane was introduced and connected to the signal ground plane through a single ferrite bead, preventing noise from spreading from the latter to the former. A second ferrite bead was inserted on the incoming +5V power rail in order to make it cleaner.
• Alternative BJT footprint: An alternative footprint for the output amplifier transistor was added, allowing for the mounting of the 2SC1815 BJT used in the original C64 audio circuit.
• SID clock delay circuit: This allows use of CD74HCT4520B chips in place of the CD4520B.
• Clock divider reset: Makes sure the SID clock always has the correct phase upon startup/reset.
• Short-circuit protection: In the original Solder design, a short-circuit can happen on the joystick port if it is written to. Series resistors were added in order to avoid this situation.

Modifications Introduced in V1

• Supports both MOS 6581 and 8580 SIDs: this depends on mounting different components though, the board cannot be switched "on-the-fly", see below.
• A volume control for the DigiFix was added and even a switch to completely bypass it.
• The output switch was removed: audio is automatically redirected to the Plus/4's EXT_AUDIO when nothing is connected to the output jack.
• The second stage of the amplifier was removed and the first stage modified so that it uses the same circuit and component values as the C64: the original card was known to introduce some distortion to the output signal as it was trying to amplify it, these days you can easily do better amplification outboard, so we chose to preserve sound fidelity.
• A decoupling capacitor was added close to every IC.
• The values for the step-up converter were recalculated in order to make the generated voltage less noisy and as ripple-free as possible.
• Last but not least, the design was documented and made open.

Assembly

If you want to assemble a card, please read the assembly notes on the Wiki carefully or you might destroy your SID chip!

SID Replacements

Most SID replacements will not work correctly. This is because for technical reasons the clock that is provided to the SID has a lower frequency than what it gets on the C64 (~886 vs ~985 kHz). Most SID replacements do not use this clock signal at all and just assume C64 frequency (I guess), leading to incorrect data transfers. This is the case for the SwinSID firmware, for instance. See below for a table of tested SID replacement devices.

Games & Software

You can get images of the disks Solder used to ship with his SIDcard here. These will help making sure that the card works as intended.

Here is also a list of a lot of C16/+4 games and software supporting the card.

Releases

If you want to get this board produced, you are recommended to get the latest release rather than the current git version, as the latter might be under development and is not guaranteed to be working.

Every release is accompanied by its Bill Of Materials (BOM) file and any relevant notes about it, which you are recommended to read carefully.

Enclosure

A 3D-printable enclosure is currently available for this board, it was kindly contributed by Starquake and it is available in the enclosure folder.

Usage should be self-explanatory, just note that there are two slightly different versions depending on the SID model, as the one for the 8580 has an extra hole in the upper shell for the Digifix). You will probably have to bend the ferrites and capacitor at the bottom of the board a little to the side.

2x countersunk head screws M3x5mm will be required for assembly.

Please see this thread on Forum64 for more information and printing recommendations.

SteveGuidi kindly contributed original-264-style labels for the completed cartridge.

License

The ReSeed documentation, including the design itself, is copyright © SukkoPera 2022-2023 and is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

This documentation is distributed as is and WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES whatsoever with respect to its functionality, operability or use, including, without limitation, any implied warranties OF MERCHANTABILITY, SATISFACTORY QUALITY, FITNESS FOR A PARTICULAR PURPOSE or infringement. We expressly disclaim any liability whatsoever for any direct, indirect, consequential, incidental or special damages, including, without limitation, lost revenues, lost profits, losses resulting from business interruption or loss of data, regardless of the form of action or legal theory under which the liability may be asserted, even if advised of the possibility or likelihood of such damages.

Support the Project

If you want to get some boards manufactured, you can get them from PCBWay through this link:

You get my gratitude and cheap, professionally-made and good quality PCBs, I get some credit that will help with this and other projects. You won't even have to worry about the various PCB options, it's all pre-configured for you!

Also, if you still have to register, you can use this link to get some bonus initial credit (and yield me some more).

You can also buy me a coffee if you want:

Thanks

• Solder, for designing the original card and sharing its schematics.
• Kinmami for his invaluable support
• Luca, Chronos and all the folks at Plus/4 World for helping with the troubleshooting
• thireo for the 3D model of the headphone jack