Firmware for the Neotron I/O Controller (a Microchip AtMega328), as fitted to the Neotron 32.
- PS/2 Keyboard Input
- PS/2 Mouse Input
- 2x Atari/SEGA MegaDrive/SEGA Genesis compatible 9-pin Joystick inputs
- Simple ASCII protocol to the Host CPU over UART
| AtMega Pin | Arduino Name | Direction | Function |
|---|---|---|---|
| PB0 | D8 | N/C | Not used |
| PB1 | D9 | N/C | Not used |
| PB2 | D10 | In | START_C_JS2 |
| PB3 | D11 | In | RIGHT_GND_JS2 + SCK |
| PB4 | D12 | In | LEFT_GND_JS2 + MISO |
| PB5 | D13 | In | DOWN_JS2 + MOSI |
| PB6 | XTAL1 | In | A_B_JS2 |
| PB7 | XTAL2 | In | UP_JS2 |
| PC0 | A0 | Open-Drain | KB_CLK |
| PC1 | A1 | Open-Drain | KB_DAT |
| PC2 | A2 | Open-Drain | MS_CLK |
| PC3 | A3 | Open-Drain | MS_DAT |
| PC4 | A4 | Out | SELECT_JS1 |
| PC5 | A5 | Out | SELECT_JS2 |
| PC6 | RESET | In | /RESET |
| PD0 | D0 | Out | KBMS_TO_HOST |
| PD1 | D1 | In | KBMS_FROM_HOST |
| PD2 | D2 | In | START_C_JS1 |
| PD3 | D3 | In | RIGHT_GND_JS1 |
| PD4 | D4 | In | LEFT_GND_JS1 |
| PD5 | D5 | In | DOWN_JS1 |
| PD6 | D6 | In | A_B_JS1 |
| PD7 | D7 | In | UP_JS1 |
This version of Netron IO implements a basic set of commands over the UART. Each command is plain ASCII, and is terminated by a new-line character (\n). Carriage-return characters (\r) are ignored, and any raw data is sent hex-encoded.
B020
The B indicates that the device has booted. The following three hex digits are the version number, as major, minor and patch.
sxxxx
Indicates Joystick 1 has changed state. The 16-bit hex word xxxx indicates the state of the (up to) 16 supported pins on the Joystick interface.
The currently supported bits are (where 0 is the least significant bit):
- Up: Bit 0, (or 0x0001)
- Down: Bit 1 (or 0x0002)
- Left: Bit 2 (or 0x0004)
- Right: Bit 3 (or 0x0008)
- A: Bit 4 (or 0x0010)
- B: Bit 5 (or 0x0020)
- C: Bit 6 (or 0x0040)
- Start: Bit 7 (or 0x0080)
In the future we may support:
- X: Bit 8 (or 0x0100)
- Y: Bit 9 (or 0x0200)
- Z: Bit 10 (or 0x0400)
For example, a value of 0x0042 means:
- Down is pressed (
0x0002) - C is pressed (
0x0040) - No other buttons are pressed
txxxx
Indicates Joystick 2 has changed state. The 16-bit hex word xxxx indicates the state of the (up to) 16 supported pins on the Joystick interface. See the Joystick 1 Changed indication for more details.
kxx
Indicates that the Keyboard has delivered a byte over the keyboard PS/2 interface. The hex byte xx is the byte that was received.
The PS/2 Keyboard protocol is documented at http://www-ug.eecg.toronto.edu/msl/nios_devices/datasheets/PS2%20Keyboard%20Protocol.htm.
mxx
Indicates that the Mouse has delivered a byte over the mouse PS/2 interface. The hex byte xx is the byte that was received.
The PS/2 Mouse protocol is documented at https://isdaman.com/alsos/hardware/mouse/ps2interface.htm.
Kxx
The K command sends the following hex byte (xx) to the keyboard PS/2 port. The Neotron IO controller will automatically hold the PS/2 device's clock line to signify that there is data to be sent, and clock out each byte in turn.
The PS/2 Keyboard protocol is documented at http://www-ug.eecg.toronto.edu/msl/nios_devices/datasheets/PS2%20Keyboard%20Protocol.htm.
Mxx
The M command sends the following hex bytes (xx) to the mouse PS/2 port. The Neotron IO controller will automatically hold the PS/2 device's clock line to signify that there is data to be sent, and clock out each byte in turn.
The PS/2 Mouse protocol is documented at https://isdaman.com/alsos/hardware/mouse/ps2interface.htm.
The Neotron-32 hardware doesn't have a Crystal for the Neotron-IO chip. You must therefore configure it to use the 8 MHz internal-RC. Unfortunately the internal-RC is only accurate to ±10%, while for a functioning UART you need the clock to be within ± 5%. To work around this, if you boot the device with pin PB0 held low, it enter its custom Calibration Mode.
In Calibration Mode, the Neotron-IO chip emits a 244.12 Hz square wave on pin PB0 (8 MHz divided by 32768). If you hold the Up button on your joystick then tap A, the OSCCAL calibration value is increased. If you hold Down and tap A, the OSCCAL calibration value is reduced. If you tap START, the OSCCAL value is saved to EEPROM. Use an oscilloscope and tune OSCCAL up and down until it is within 232 Hz to 256 Hz (and as close to 244.12 Hz as possible).
This project is built using Arduino and the miniCore add-on package. It has been tested using Arduino version 1.8.12 and miniCore version 2.0.4.
Use the miniCore bootloader for the AtMega328P, configured to use the 8 MHz internal-RC. You have to load the bootloader with an AVR programmer (such as an Arduino running the ArduinoISP sketch, or a dedicated Atmel-ICE USB programmer). Once the bootloader is installed, you should be able to load a sketch over the UART immediately after reset (assuming your internal-RC is within tolerance). If you have trouble, use an AVR programmer.
The Arduino libraries are licensed under the GPL, and so this project is also licensed under the GPL (v3 or later version, at your choice).
See the LICENCE file.