Soggy-1000 is an open-source clone of the Sega SG-1000 home videogame system. The eventual goal of the project is to produce a copy of the SG-1000 main console, and the SK-1100 keyboard, so that the software made for the related Sega SC-3000 home computer can be preserved and enjoyed.

License

The Soggy project is licensed under the Creative Commons Attribution-NonCommercial 4.0 license.

Sub-Projects

This repository contains multiple KiCad projects that are related to the overall Soggy project. These are:

• keyboard: An implementation of the SK-1100 keyboard, using modern key switches (Under development)
• cartridges/romcart: An implementation of a bare-bones (ROM-only) cartridge for SG-1000
• cartridges/basic-iiib: An implementation of the SEGA BASIC Level IIIB cartridge, used for testing purposes in order to make sure cartridges can take over the entire memory map (Untested)

Assembly Guide and BOM

Please match the BOM with the appropriate version - the files here in master are currently post-v4 improvements and some parts may be missing or changed their values when compared with the ones in sg1000-v0.4.bom.csv

TODO

Programmer's Guide

The Soggy is basically identical to the existing SG-1000, with the addition (in v0.3 and onwards) of memory mapping hardware. This hardware allows programmers to take advantage of the full 32k of main RAM on the system.

TODO: Memory map diagram TODO: Use IOW + $10 to set it

Special Thanks

The following people helped me figure out some aspect of the SG-1000, fixed a bug in my hardware, or otherwise contributed.

• Enri for providing SC-3000 and SG-1000 schematics
• Chartreuse for spotting a bug with the latch responsible for page switching
• Nick Hook from SC-3000 Survivors helped with providing diagrams, pictures, and SF-7000 information
• Fabio Dalla Libera (sms power) helped with the M1 transistor
• asynchronous (sms power) also helped with the M1 transistor

Version History

v0.5 (in development)

This version contains experimental features.

• Wider holes on the boot ROM socket footprint, to accept a Textool/AliExpress-style ZIF socket for easier ROM changes during development
• 1206 ferrite bead on video path to prevent colour burst noise

v0.4 (in testing)

This version is in active development and may not be suitable for testing yet.

• /JOY_SEL signal connected to keyboard edge, allowing keyboard 8255 to force polling of joysticks when row 7 of matrix is requested.
• Converted to DIP Z80 from PLCC, to allow cheaper AliExpress parts to be used rather than expensive new PLCC Z80 part. New DIP Z80s are also still available from Digi-Key, Mouser, etc.
• Debounce circuit on NMI pin using a Schmitt trigger (design from this Tynemouth Software blog entry, thank you)
• Weird overpour onto expansion edge fixed, as were some missing silkscreens for '574 and '04
• Rudimentary support for Sega Genesis/Mega Drive controllers. No mode switching, so it is a similar limitation to Master System games on the Genesis.
• Trim pot for fine tuning of TMS9918 input clock (and therefore colourburst/Z80/sound clock.) Can be replaced with a simple wire link if you are feeling brave.

v0.3

Many new features added on the base of the v0.2, and some bugs fixed. This was intended to be the first public release, and is recommended for most people.

• /IORD and /IOWR signals (IORQ + WR/RD) synthesized and passed to cartridge port. This prevented the SF-7000 from working, as did an I/O map collision with the keyboard. Both problems are now fixed.
• The full 62256 SRAM is wired up, providing 32K of work RAM to the system. The first 8K are accessible without touching the memory management unit; the second 8K can be switched between four "pages." This should allow Sega BASIC and many other titles relying on SC-3000 functions to work. Previous boards had a 2K mod, matching the SC-3000, but this was not tested.
• An onboard BIOS ROM socket has been added, which runs when a cartridge is not inserted (detected through the "B2" pin on the cartridge slot, which is usually connected to B1 inside cartridges)

Known bugs:

• /JOY_SEL is not connected to the keyboard edge
• Load input on memory management 74ls573 is inverted and doesn't work

v0.2

• SK-1100 keyboard edge connector added.
• Sound chip-induced lockup under /M1 fixed.
• Extremely weak video signal fixed.
• Backwards controller ports fixed.
• Experimental debounce cap on "pause" button added.
• Reset button added.
• USB power connector shield grounded.
• Several problems with sporadic interrupts and waits fixed by adding pull-ups.

Known bugs:

• /JOY_SEL is not connected to the keyboard edge

v0.1

Initial release.

Known bugs:

• Too many to list.