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See TinyPS2 Revs 1-3 for ATTiny85 based build

TinyPS2 Front Rev4 3D.png


It is getting difficult to find ASCII serial keyboards for use in Retro-Computer projects. This card fills that need. The card is a PS/2 keyboard to TTL Level Serial adapter. PS/2 keyboards are still widely available.

Keyboard PS2 MicrosoftNatural.jpg


  • QT Py (RP2040 based)
    • Run at 125 MHz
    • 2MB of In System Programmable (ISP) flash
    • 264KB of SRAM
  • 115,200 baud serial
    • 57,600-250,000 are build time options
  • 49x49mm ODAS form factor
  • (4) 4-40 mounting holes
  • 3.3V or 5V operation
  • Current measurement
    • TBD @ 5V (without keyboard)
    • TBD @ 5V (including keyboard)
    • TBD @ 3.3V (including keyboard)
      • Does not require a keyboard that works at 3.3V

CPU Connections

TinyPS2 CPU Rev4 Conns.PNG

#define DATAPIN 27
#define IRQPIN  26


J1 - TTL Serial Connector

TinyPS2 J1 Rev4 Conns.PNG

  1. VCC (pin nearest to J1 silkscreem)
  2. SERTX
  3. N/C
  4. GND

J2 - PS/2

  • Standard PS/2 Mini-DIN-6 connector

TinyPS2 J2 Rev4 Conns.PNG

Firmware Build Card Rev 4



#define DATAPIN 27
#define IRQPIN  26
  • Example code produces warning
WARNING: library PS2KeyAdvanced claims to run on avr, sam, samd1, stm32, esp32 architecture(s) and may be incompatible with your current board which runs on rp2040 architecture(s).
Sketch uses 82336 bytes (0%) of program storage space. Maximum is 16777216 bytes.
Global variables use 43096 bytes (15%) of dynamic memory, leaving 227240 bytes for local variables. Maximum is 270336 bytes.
  • Code works well!



  • CircuitPython support issues
>>> import ps2io
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: no module named 'ps2io'



Rev 4

  • Changed CPU
    • Was 8 MHz ATTiny85
    • Changed to QT PY or XAIO
  • Changed pin usage
  • Uses Hardware Transmitter in CPU
  • Keyboard runs at 5V
  • CPU runs at 3.3VV
  • 3.3V to 5V level shifter

Assembly Sheet