Sega Mega Drive (Genesis) controller to Atari 8-bit adapter
Adapt a Sega Mega Drive controller to the 9-pin Atari joystick standard using a Microchip PIC 16F1847 microcontroller.
This adapter is compatible with most computers that use the Atari 9-pin joystick connector, supporting most of the features of Sega Mega Drive controllers, which use the same connector with a slightly different pinout. The Mega Drive was known as Genesis in the US.
Read all you need to? Here's how to build the adapter.
Features of this adapter
The adapter uses a microcontroller to talk to the Mega Drive controller and map its four buttons to the three inputs available on an 8-bit Atari.
The B button on the controller is mapped to the normal joystick button. Buttons A and C are connected to pins 5 and 9 on the joystick port; these can be read as extra joystick buttons by the computer. Most games that only support one extra button use the C button (pin 9).
"A is Up" toggle
The start button has no natural mapping since the Atari can only read three buttons, but a common complaint in platform games on the Atari (and later the Amiga) was that they often used joystick Up to jump because of the lack of extra joystick buttons.
A dedicated Up button would solve this problem, but the Start button is not usually in a good location to be used as a jump button. The adapter therefore supports this feature indirectly: holding the Start button down for a second maps the A button to Up.
Since the joystick button often starts games, a short press of the Start button is mapped to joystick button 1. This allows Start to both serve its normal function (in supported games) and switch the A button between modes.
SNACK enhanced mode
SNACK is an Atari adapter for SNES controllers. It has an enhanced mode that allows all of the buttons to be read by the Atari, and this adapter supports that mode if you have a six-button controller.
To switch to SNACK mode on a Sega controller, press Mode, Start, and C (the right shoulder button if you're using an 8BitDo M30 controller) at the same time. Mode+Start+Z (left shoulder on the M30) goes back to normal mode.
If you're using an 8BitDo Retro Receiver with an Xbox or PlayStation controller, the button layout is different. Press Mode+Start+X (left shoulder) to switch to SNACK mode on these controllers, and Mode+Start+Z (right shoulder) to go back to normal mode.
The SNES controller has four main buttons: A, B, X, and Y; left and right shoulder buttons; and START and SELECT.
The main buttons have the same labels and approximately the same diamond configuration as the A, B, X, and Y buttons on a Mega Drive controller, but the AB and XY buttons are swapped.
The Mega Drive controller does not have shoulder buttons, so the Z and C buttons are used instead. As noted above, the 8BitDo Retro Receiver maps Z and C to the shoulder buttons on the M30 controller.
Finally, the Mode button stands in for SELECT.
Notes
- SNACK mode uses the joystick ports in a completely different way to a normal joystick and will produce unexpected results in software that is not written to detect it.
- The buttons are not completely independent. Only one of START, SELECT (Mode), X, and Y can be detected at any time, and any of those buttons will override A and B. However, A and B are independent of each other, and the shoulder buttons are entirely independent.
- Centering the joystick relies on fairly precise timing to charge the Atari's pot capacitors to a specific range, so different computers and even cable lengths may make the joystick appear to be intermittently leaning in one direction.
- This feature is still experimental.
Autodetection (Atari 8-bit machines only)
Because of the way the paddle circuits are used to read the second and third button inputs, 8-bit Ataris read an unconnected second or third button input as if the button is being held down.
This allows* games running on the computer to detect the presence of a multibutton controller by assuming that a button does not exist until it has been seen to be released. The adapter supports this autodetection by leaving its button outputs open unless a Mega Drive controller is detected or the corresponding button input is pulled low.
To activate the second button when a Mega Drive controller is not connected, pin 9 on the controller input must be pulled low before the adapter starts driving the second button output. This means that the second button must be pressed once before it can be detected by the computer.
The third button output is open unless a Mega Drive controller is detected since there is no other source for a third button input.
* The astute reader will note that games are required to ignore button inputs until the button is released, otherwise a nonexistent button will appear to be continually held down.
Computer compatibility
8-bit Atari computers
The adapter has mostly been tested on 8-bit Ataris using Joy2B+ enhanced games.
8-bit Commodore computers
The C64, C128, and VIC-20 can also read the second and third buttons. The extra buttons on these machines are active high, which is opposite to Atari and Sega.
Pulling the /C64 pin (number 4) on the PIC low inverts the outputs for the extra buttons, allowing them to be read correctly by the 8-bit Commodores. The simplest way to do this is to install a jumper in the appropriate location on the circuit board.
Because the joystick inputs of the 8-bit Commodores are somewhat sensitive, all of the pins are normally open (floating). The extra button pins are pulled high when active, all other signals are pulled low when active.
Amiga
Unlike the 8-bit Commodores the extra buttons on the Amiga are active low, and unlike all the 8-bit computers they are digital inputs instead of repurposed paddle circuits. The Atari mode supports both Atari and Amiga by driving the pins for buttons two and three both high and low.
Atari ST (untested)
Button two registers as the second mouse button / port 1 joystick button if the adapter is plugged into port 0. The ST does not support additional joystick buttons on port 1.
Controller compatibility
Sega controllers
The adapter is compatible with three- and six-button Mega Drive controllers. On six-button controllers, buttons X, Y, and Z act as autofire versions of A, B, and C, while the Mode button is only used for SNACK enhanced mode.
Sega Master System controllers are theoretically supported, including the second button, but this is untested.
8BitDo Retro Receiver
The adapter is extensively tested with the 8BitDo Retro Receiver for SEGA. It's a nifty way to use wireless controllers on old hardware.
Other joysticks or controllers
A controller or joystick with a second button that is not normally recognised by 8-bit Atari and Commodore computers may work through the adapter.
CAUTION: the second button must be on pin 9 and not pin 5. Pin 5 is the power pin, and pulling it low may damage your computer or the adapter.
Single-button joysticks will work but derive no benefit from using the adapter.
Atari 7800 controller, Commodore multi-button joysticks, paddles, steering wheels, etc.
These devices will not work and should not be plugged in to the adapter.
Build
The schematics and parts list for the adapter are in the kicad directory. If you don't already have a preferred way to manufacture a PCB, you can order a set of three from OSH Park. You also need a way to program the microcontroller.
Program
JOY-2-PIC can program a PIC using an Atari 8-bit computer's joystick port. Boot from the .atr image attached to each release to start programming. If you have a USB PIC programmer connected to a PC, use the .hex file instead.
Modify
If you want to modify the firmware, you need the xc8
compiler from
Microchip to build it. Run rake or xc8-cc -mcpu=16f1847 -o sega-adapter.hex *.c to create a .hex file suitable for a PIC
programmer.
Rationale
All right, it's time for some history.
- In 1977, Atari released the 2600 game console.
- In 1981, Commodore released the VIC-20 home computer.
- In 1984, Sega released the SG-1000 II console with one main improvement from the SG-1000: detachable controllers.
These machines all used the same physical connector with an almost identical pinout for their controllers, and their manufacturers stuck with the standard through later generations, tempting people to swap controllers and joysticks between the systems.
Most of the time, this more or less worked. Buttons were not always completely compatible, but most Atari and Commodore games only supported one button so it didn't really matter as long as one button worked. One important caveat: Sega chose to put power on pin 5 instead of pin 7, so mixing and matching had the potential to cause damage to both machines and controllers.
Initially each signal was carried by an individual pin: four pins for directions, one for the button, two pins reserved for power and ground, leaving two pins free for other functions.
Mega Drive (Genesis)
For the Mega Drive, released in 1988, Sega wanted to have more buttons (four) than there were available pins on the controller port (three). They solved this problem by using one pin (7) as an output from the console to the controller, allowing two pins (6 and 9) to multiplex the A, B, C, and Start buttons.
Joy2B+
Fast forward 20 years, to January 2019. A poster on an Atari 8-bit forum introduced a design for a three-button joystick for the 8-bit Ataris, and -- crucially -- included patched versions of existing games that supported the extra buttons. This was the Joy2B+ project.
Suddenly, there was a reason to connect a joystick with more than one button to an Atari.
8BitDo Retro Receiver for SEGA
Finally, a month later, a Hong Kong-based company called 8BitDo released a device that allowed Bluetooth game controllers to be used with Sega consoles, the 8BitDo Retro Receiver for SEGA... tantalisingly close to being compatible with Atari joystick ports.
Pinout
PIC 16F1847
+---------_---------+
| |
Atari 5 -| 1 RA2 RA1 18 |- Atari 1
| |
Atari 6 -| 2 RA3 RA0 17 |- Atari 2
| |
Atari 9 -| 3 RA4 RA7 16 |- Atari 3
___ | |
C64 -| 4 RA5 RA6 15 |- Atari 4
| |
Atari 8 SEGA 8 -| 5 GND +5v 14 |- Atari 7 SEGA 5
| |
SEGA 1 -| 6 RB0 RB7 13 |- SEGA 6
| |
SEGA 2 -| 7 RB1 RB6 12 |- SEGA 7
| |
SEGA 3 -| 8 RB2 RB5 11 |- SEGA 9
| |
SEGA 4 -| 9 RB3 RB4 10 |- X
| |
+-------------------+
Acknowledgements
Information on reading Sega controllers came from https://github.com/jonthysell/SegaController/wiki/How-To-Read-Sega-Controllers and https://www.raspberryfield.life/2019/03/25/sega-mega-drive-genesis-6-button-xyz-controller/