Arcade-CityConnection_MiSTer/README.md

Arcade-CityConnection_MiSTer

FPGA core for City Connection (Jaleco, 1985) targeting the MiSTer FPGA platform (Terasic DE10-Nano).

City Connection is a horizontal driving/action arcade game running on a single MC6809 main + MC6809 sound CPU board with a 32×32 tilemap, 5bpp foreground with virtual palette per-scanline (the famous "trail" effect), 4bpp background with half-rate scrolling, 4bpp sprites, and AY-3-8910 + YM2203 audio. This core reimplements the hardware in SystemVerilog from MAME references and pixel-accurate validation against original PCB footage.

Status

Current version: 1.0 (May 2026) — first public release.

The core runs the full game with audio and has been validated pixel-by-pixel against the original Jaleco PCB (reference video courtesy of Janet's channel, see Acknowledgements).

Features

• MC6809 main CPU @ 2 MHz (Greg Miller's cycle-accurate core)
• MC6809E audio CPU @ 625 kHz (separate sound subsystem)
• AY-3-8910 (jt49) and YM2203 (jt03) sound chips with MAME-accurate mixer
• 247×224 active video area (PCB-accurate, not MAME's 240×224 clip)
• Tilemap with virtual palette per-scanline writeback FSM (the "trail")
• Sprite line buffer with PCB-accurate wrap behavior
• 6 PAL16L8 chips decompiled (h7, l5, u7) with gate-level reimplementation
• Inputs (P1/P2 8-way joystick + 2 buttons) and full DIP support
• MiSTer OSD with video offset knobs

ROM sets supported

• City Connection (set 1) — reference set (citycon)
• City Connection (set 2) — alternate set (citycona, c11b variant)
• City Connection (Cruisin) — Kitkorp license variant (cruisin)

The 15-day positioning bug

Two weeks of pixel hunt: sprites and the platform "trail" effect were all consistently a few pixels off vs MAME. Every offset added (sprite +4X, sy −1, FG +1, BG +1) felt like a band-aid. After disassembling the original 6809 ROM and mapping every routine that touched the spriteram and tilemap, none of the formulas matched.

The breakthrough came from a YouTube video of the real PCB on a Supergun by Janet's channel "Janet의 고전게임오락실". One frame revealed it: the PCB outputs 247 pixels wide, not 240 as MAME's clipping suggests. MAME crops 7 pixels and adjusts everything internally to fit. Once the RTL matched the actual PCB width and the sprite line buffer used proper PCB-style wrap, every offset suddenly made sense.

Reference video: https://www.youtube.com/watch?v=9D2OQHAQmV8

Hardware emulated

Component	Spec
Master clock	20 MHz crystal
Main CPU	HD68B09 (MC6809) @ 2 MHz (MASTER/4 internal E)
Audio CPU	HD68A09EP (MC6809E) @ 625 kHz (MASTER/32)
Sound chip 1	AY-3-8910 (jt49) @ 1.25 MHz (MASTER/16)
Sound chip 2	YM2203 (jt03) @ 1.25 MHz (MASTER/16)
Video resolution	247×224 active, 320×262 total
Refresh rate	59.63 Hz
FG layer	5bpp tilemap with virtual palette per-scanline
BG layer	4bpp tilemap with scroll/2 ratio
Sprites	4bpp 8×16, 64 slots, PCB-accurate line buffer wrap

Hardware requirements

• Terasic DE10-Nano
• MiSTer I/O board (recommended)
• HDMI display (recommended), or HDMI→VGA adapter for 31 kHz VGA monitors

Building from source

Requires Quartus Prime 17.0 (free Lite Edition).

Open CityConnection.qpf in Quartus → Processing → Start Compilation

Output bitstream is generated in output_files/CityConnection.rbf (~3.5 MB).

Running on MiSTer

The releases/ folder contains the pre-built bitstream and the parent MRA for the reference ROM set:

• CityConnection.rbf — pre-built core bitstream
• City Connection (set 1).mra — parent MRA (reference set)

Alternative ROM sets are provided in releases/alternatives/:

• City Connection (set 2).mra — citycona (c11b variant)
• City Connection (Cruisin).mra — Kitkorp license

Following the MiSTer-devel convention, the alternative sets are also mirrored to the official MRA-Alternatives_MiSTer repository, where they are picked up automatically by Update_All.

Steps:

1. Copy the .rbf to _Arcade/cores/ on the MiSTer SD card.
2. Copy the desired .mra file(s) to _Arcade/ on the MiSTer SD card.
3. Provide your legally-owned City Connection ROM files where each MRA expects them (usually in games/mame/).

ROMs are NOT included in this repository. You must provide them yourself.

Repository layout

Arcade-CityConnection_MiSTer/
├── rtl/
│   ├── citycon/      City Connection-specific core RTL
│   │   └── pld/      Decompiled PAL16L8 + TTL gate-level support
│   ├── jt03/         YM2203 + AY-3-8910 (Jose Tejada)
│   ├── jtframe/      JTFRAME framework modules
│   ├── mc6809/       MC6809 cycle-accurate core (Greg Miller)
│   ├── pll/          Clock PLL
│   └── sdram.sv      SDRAM controller (Sorgelig)
├── sys/              MiSTer framework (Sorgelig / MiSTer-devel)
├── releases/         Pre-built .rbf + parent MRA
│   └── alternatives/ MRA files for alternate ROM sets
├── docs/             Documentation
├── CityConnection.qpf  Quartus project
├── CityConnection.qsf  Quartus assignments
├── Template.sv       Top-level wrapper
├── Template.sdc      Timing constraints
├── files.qip         HDL file list
├── build_id.v        Build version stamp
└── README.md         This file

Acknowledgements

• Janet (Janet의 고전게임오락실) for the invaluable PCB reference video that revealed the actual 247-pixel output of the original hardware. This single piece of footage solved weeks of pixel-positioning mysteries.
• Greg Miller for the cycle-accurate MC6809 core used as both main and audio CPU.
• Jose Tejada (@jotego) for JT03 (YM2203) and JT49 (AY-3-8910) sound chip implementations.
• Sorgelig and the MiSTer-devel team for the framework, SDRAM controller and Template.
• The MAME project for invaluable hardware reference.

Support this project

If you enjoy this core and want to support its development:

• Ko-fi — one-time support
• Patreon — monthly support
• PayPal — one-time donation

Follow

• Twitch — live streams
• YouTube — playlists and videos

License

The RTL source code in this repository is provided as-is for educational and preservation purposes. Original ROM data is not included; users must provide their own legally obtained copies.

Original City Connection arcade hardware © Jaleco Co., Ltd., 1985.