RetroDevEm

RetroDevEm (Retro Device Emulator) is a free (as in Free Software) input devices (mouse and joystick) emulator for retro consoles and computers (Atari 2600, Atari ST, Amstrad CPC, ...). It allows using any input device, recognized by Linux on the Raspberry Pi, on unmodifed retro machines. Programs collect events from input devices and send the corresponding signals to the console or computer.

The project consists of 2 components:

• Hardware: A daughter printed circuit board, to be plugged on a Raspberry Pi;
• Software: Programs running on the Raspberry Pi.

The daughter board can be connected to the retro machines via DB-9 flat cables.

Project focus

This project focuses on:

• Using off the shelf hardware ;
• Devices accuracy and low latency ;
• Simplicity ;
• Low CPU usage.

Rationale

I bought a 30 Euros Atari ST mouse adapter from a hobbyist. It only works with ps/2 mice and the accuracy is okish. Especially with slow movements, the Atari ST pointer accuracy is bad. So why not do something better and make it free software ?

Project details

Hardware

A PCB (Printed Circuit Board), with:

• 2x ULN2003AN ICs (7 Darlington transistors per IC)
• 14x 15 KOhms resistors (optional)
• 1x 40 pins socket (Raspberry Pi connector)
• 2x 10 pins headers (Atari ports connectors)

About the resistors

Without resistors, the design works perfectly well. The current drawn on the Raspberry PI GPIO pins is 706 uA, for a 440 uA current flowing out of the Atari ST mouse/joystick port.

With 15 KOhms resistors, the design works perfectly as well. The current on the Raspberry Pi GPIO pins in 113 uA, for a 440 uA current on the Atari ST port side.

Board wiring

RPI sig.	J1 pin	J2/J3 pins	Atari ST signals	Amstrad CPC sig.	Atari 2600 sig.
GPIO2	3	J2 9	P0 Right Button	COM2 (GND Joy 2)	Pot 1 (analog)
GPIO3	5	J2 5	P0 Port 0 enable	Fire 3 (undoc)	Pot 0 (analog)
GPIO4	7	J2 4	P0 YB / Right	Right	Right
GPIO17	11	J2 3	P0 YA / Left	Left	Left
GPIO27	13	J2 2	P0 XA / Down	Down	Down
GPIO22	15	J2 1	P0 XB / Up	Up	Up
GPIO10	19	J2 6	P0 Left Button / Fire	Fire 2 (default)	Fire
GPIO9	21	J3 9	P1 Right Button
GPIO11	23	J3 5	P1 Port 0 enable
GPIO0	27	J3 4	P1 YB / Right
GPIO5	29	J3 3	P1 YA / Left
GPIO6	31	J3 2	P1 XA / Down
GPIO13	33	J3 1	P1 XB / Up
GPIO19	35	J3 6	P1 Left Button / Fire

Generating the gerber files

In Kicad PCB editor:

• Run the DRC (Design Rule Checker)
• File -> Fabrication outputs -> Gerbers
• Select the output directory
• Include every layer except Margin, F.Courtyard and B.Courtyard
• Keep only the following "General Options" checked:
  • Plot the reference designators
  • Check zone fills before plotting
• Keep the default "Gerber Options"
• Click "Plot" button
• Then click on "Generate Drill Files..."
• In the "Drill File Format" section, select "Gerber X2"
• Then click "Generate Drill File"
• Create a zip archive from the gerber folder.

The generated archive can be checked in Kicad's Gerber viewer.

Software

The RetroDevEm programs need to be installed on a Raspberry Pi connected to the Atari ST through the RetroDevEm board (an RPI daughter board). The programs process events from input devices (mouse, gamepad) and send corresponding signals to the retro machine connected to the board.

Prerequistes

On a fresh raspbian (Linux raspberrypi 6.12.47+rpt-rpi-v8), the following 2 packages need to be installed: swig, liblgpio-dev, evtest.

$ sudo apt install swig liblgpio-dev evtest

Installation

You need to create a virtualenv on your Raspberry Pi and activate it:

$ virtualenv venv/retrodevem
created virtual environment CPython3.11.2.final.0-64 in 353ms
  creator CPython3Posix(dest=/home/florent/venv/retrodevem, clear=False, no_vcs_ignore=False, global=False)
  seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/florent/.local/share/virtualenv)
    added seed packages: pip==23.0.1, setuptools==66.1.1, wheel==0.38.4
  activators BashActivator,CShellActivator,FishActivator,NushellActivator,PowerShellActivator,PythonActivator
$ source venv/retrodevem/bin/activate

Then RetroDevEm can be install with pip:

$ pip install retrodevem
Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple
Collecting retrodevem
  Using cached retrodevem-0.2-py3-none-any.whl (22 kB)
Collecting click
  Using cached https://www.piwheels.org/simple/click/click-8.1.7-py3-none-any.whl (97 kB)
Collecting gpiozero
  Using cached https://www.piwheels.org/simple/gpiozero/gpiozero-2.0.1-py3-none-any.whl (150 kB)
Collecting lgpio
  Using cached lgpio-0.2.2.0-cp311-cp311-manylinux_2_34_aarch64.whl (364 kB)
Collecting colorzero
  Using cached https://www.piwheels.org/simple/colorzero/colorzero-2.0-py2.py3-none-any.whl (26 kB)
Requirement already satisfied: setuptools in ./venv/retrodevem/lib/python3.11/site-packages (from colorzero->gpiozero->retrodevem) (66.1.1)
Installing collected packages: lgpio, colorzero, click, gpiozero, retrodevem
Successfully installed click-8.1.7 colorzero-2.0 gpiozero-2.0.1 lgpio-0.2.2.0 retrodevem-0.2

Usage

Identify the input device files using evtest:

$ evtest
No device specified, trying to scan all of /dev/input/event*
Not running as root, no devices may be available.
Available devices:
/dev/input/event0:      vc4-hdmi-0
/dev/input/event1:      vc4-hdmi-0 HDMI Jack
/dev/input/event2:      vc4-hdmi-1
/dev/input/event3:      vc4-hdmi-1 HDMI Jack
/dev/input/event4:      Sony Interactive Entertainment Wireless Controller
/dev/input/event5:      Sony Interactive Entertainment Wireless Controller Motion Sensors
/dev/input/event6:      Sony Interactive Entertainment Wireless Controller Touchpad
/dev/input/event7:      Logitech USB Optical Mouse
Select the device event number [0-7]: ^C

Run the atarist-mouse program to emulate a mouse on the Atari ST:

$ atarist-mouse --device /dev/input/event7 &
[1] 1947

Run the atarist-joystick program to emulate a joystick on the Atari ST:

$ atarist-joystick --device /dev/input/event4 &
[2] 1951

Available options for the atarist-mouse program:

$ atarist-mouse --help
Usage: atarist-mouse [OPTIONS]

  Send mouse events to an Atari ST connected to the RetroDevEm board.  Usage
  example: atarist-mouse --board v2.0 --device /dev/input/event0 --port 0
  --speed 4

Options:
  --board TEXT          Board revision.  [default: v2.0]
  --device TEXT         Input device to use.  [default: /dev/input/event0]
  --port INTEGER        Board/Atari port to connect the mouse to.  [default:
                        0]
  --speed INTEGER       Mouse speed divider (more = slower).  [default: 2]
  --debug / --no-debug  Display debugging information.  [default: no-debug]
  --help                Show this message and exit.

Available options for the atarist-joystick program:

$ atarist-joystick --help
Usage: atarist-joystick [OPTIONS]

  Send joystick/gamepad events to an Atari ST connected to the RetroDevEm
  board.  Usage example: atarist-joystick --board v2.0 --device
  /dev/input/event0 --port 0

Options:
  --board TEXT          Board revision.  [default: v2.0]
  --device TEXT         Input device to use.  [default: /dev/input/event0]
  --port INTEGER        Board/Atari ST port to connect the joystick to.
                        [default: 1]
  --debug / --no-debug  Display debugging information.  [default: no-debug]
  --help                Show this message and exit.

Additional information

• Sourcing
• Mechanical details

Consoles and computers DB9 wiring

Atari ST

DB9 pins	Mouse P0	Joystick P0	Joystick P1
1	XB	Up	Up
2	XA	Down	Down
3	YA	Left	Left
4	YB	Right	Right
5			P0 enable
6	Left But	Fire	Fire
7	+5V	+5V	+5V
8	GND	GND	GND
9	Right But

Amstrad CPC & Atari 2600 Joystick

DB9 pins	Amstrad CPC	Atari 2600
1	Up	Up
2	Down	Down
3	Left	Left
4	Right	Right
5	Fire 3 (undoc)	Pot 0 (analog)
6	Fire 2 (default)	Fire
7	Fire 1 (extra)	+5V
8	COM1 (joy1)	GND
9	COM2 (joy2)	Pot 1 (analog

Sources:

• cpcwiki
• Atari ST Internals - page 73

The first version will focus on the following use cases:

• Atari 2600: 1 joystick
• Atari ST: 1 mouse + 1 joystick
• Atari ST: 2 joysticks
• Amstrad CPC: 1 joystick

Schematics

Current status

Atari ST

• Mouse emulation is working. Latency is below 20 ms (i.e 1 frame at 50 Hz) and CPU usage is below 10% of a core, for normal usage.
• Joystick emulation is working. No latency, CPU usage below 1% of a core.

Todo

• Configure RPI GPIO pins to pull-down input by default;
• Write blog article about the project.
• Implement auto-fire;
• Use python-evdev instead of custom inputdevice module.

Links

• Incremental encoder on Wikipedia: describes the mouse signal expected by the Atari ST ;
• ATARIPiMouse Github: inspiration for writing the emulator in Python ;
• Yaumataca: inspiration for the "quadrature encoder" ;
• Atari-Quadrature-USB-Mouse-Adapter