This cartridge enables the use of Atari 2600-style joysticks with the P2000T by emulating keyboard inputs. It uses an EEPROM and a few logic chips to translate joystick button presses into keystrokes, seamlessly integrating with the P2000T's keyboard polling process.

The P2000T features a matrix keyboard design, where each key is positioned at the intersection of a specific row and column. The rows are connected to the address pins, while the columns are connected to the data pins. The keyboard matrix consists of 10 rows and 8 columns.

To poll the keyboard, the P2000T employs a two-step process:

1. Initialize Rows: The P2000T writes a 1 to the BIT6 position of I/O port 0x10, which ties all rows to ground (GND).
2. Detect Key Presses: The P2000T then reads from I/O port 0x00. If at least one key is pressed, the read value will differ from 0xFF.

If a key press is detected, the P2000T proceeds with an additional step:

3. Detect specific key: Each row is sequentially pulled down by reading from I/O ports 0x00 to 0x0A. A zero-bit received during any of these read operations indicates the presence of a key press. The corresponding key is mapped to a specific keystroke based on its position in the matrix.

The described process is emulated using a single EEPROM and a few logic chips. The EEPROM’s address pins are connected to both the joystick buttons and the lower address bus used by the P2000T during the keyboard polling process. To detect the polling of the lines, a 74HC138 decoder is used. This decoder latches the D6 pin to capture the first phase of the keyboard polling process. During this phase, if any joystick button is pressed, the EEPROM generates the same output as if a key on the keyboard had been pressed. In the second phase, the EEPROM continues the emulation process. However, it now produces the output precisely when the corresponding row in the keyboard matrix is pulled low.

The use of an EEPROM makes this process fully programmable, allowing the user to create any desired joystick-to-keyboard mapping. With 10 joystick buttons (5 per joystick across 2 ports), only 32 KiB of EEPROM memory is required to store the mappings. Since even the smallest SST39SF0x0-type EEPROM has a capacity of 128 KiB, a single chip can accommodate up to four different joystick/keyboard mappings. To enable users to select their preferred mapping, a DIP switch is included, allowing easy switching between the stored configurations.

Download schematic

To generate the data for the EEPROM, you can use this script. The data can be written to the EEPROM using, for example, this hardware solution.