How to integrate a trackpoint in a mechanical keyboard

This is a summary of what I needed and learned while integrating a trackpoint into my custom built hand-wired split mechanical keyboard. It took quite a lot of research and planning, so I thought it would be helpful to others to create this guide.

Before starting this project, the main questions I had were:

• How do I identify the pinout of my trackpoint?
• What should I use for the trackpoint stem?
• Will I need to ruin my keycaps to make space for the stem?
• Can I use the same teensy to power both the keyboard and TP?
• Can I use the TP and keyboard mousekeys concurrently?
• Should I use busywait, interrupt or usart in TMK?
• How do I build the reset circuit?
• Will I have enough clearance inside the case?

There is a lot of useful information scattered around the deskthority and geekhack forums, and TMK issues, so I'm also including all the pinouts I found during my travels, datasheets, and TMK keyboard code tweaks. (Unfortunately I didn't take note of where I found the above mentioned resources, so I can't give credit where it is due, for that I apologize. I'll try retrace my steps, but if you can let me know, I'd be most welcome).

Thanks to hasu for his awesome TMK firmware, and the deskthority and geekhack communities - if you have posted something to do with a trackpoint, I've read it, and it most likely helped :)

Pull requests are welcome, especially new pinouts.

Table of contents

• Target audience
• FAQ
  • Q: What should I use for the trackpoint stem/stick?
  • Q: How do I identify the trackpoint pinout?
  • Q: Should I use busywait, interrupt or usart in TMK?
• What you need
• Steps
  • 1. Solder leads to trackpoint
  • 2. Create the reset circuit (RC)
  • 3. Solder leads to reset circuit (RC)
  • 4. Testing on a breadboard
  • 5. Make trackpoint stem hole in case
  • 6. Make space for trackpoint in keyboard
  • 7. Test trackpoint clearance and stem length, glue stem
  • 8. Solder reset circuit (RC) to teensy
  • 9. Finish up
  • 10. Update tmk_keyboard firmware and flash
• TMK firmware changes and tweaks

Target audience

It's hard to cover every use-case, keyboard and firmware so I'm limiting this for folks who have a hand-wired mechanical keyboard that uses a teensy (2.0) with TMK_keyboard firmware, and who want to integrate a trackpoint for just mouse movement (and take advantage of the keyboard for mouse keys).

That said, the information here should still be useful for others.

FAQ

Q: What should I use for the trackpoint stem/stick?

A labret cheek piercing! (yes, I know, weird, but it's awesome!)

• Relatively cheap.
• Super strong (surgical steel, titanium).
• Really thin in diameter (16G is perfect for between keycaps).
• They come in different lengths (8mm to 26mm).
• They have a flat bottom (great for gluing to TP).
• They have a screw on top ball, in different sizes.

I ordered a few sets of different lengths and different diameters (I wasn't sure if 16G would be strong enough - it is), as well as different ball sizes from ebay for relatively cheap.

My trackpoint pointer base sits flush under my 3mm switch plate, and with a cherry MX switch having a height 10.2mm above the switch plate, and about 1-2mm PBT cherry keycap on top, the 14mm stem with 3mm ball is kind of perfect.

For extra friction on the ball, I used some sandpaper.

Q: How do I identify the trackpoint pinout?

With some luck, or a multimeter and a steady hand. (A scope would be better, but unfortunately I don't have one).

If you're lucky, someone has already identified the pinout of the trackpoint you have (see pinouts), or at least a similar one.

If not, you'll need to identify the pinout yourself, here are some tips:

• The largest tracer is most likely GND.
• The second largest tracer is most likely VCC.
• Using a multimeter and the PTPM754 datasheet, test for connectivity:

CLK (INT0)    - PTPM754 pin 24
DATA (TXD/1)  - PTPM754 pin 2
RST           - PTPM754 pin 5
GND           - PTPM754 pin 8
VCC           - PTPM754 pin 22

• Alternatively, if you have a scope and know what you're doing:

CLK  - should have a frequency reading
DATA - should have a 5V reading
RST  - should have a 0V reading

Q: Should I use busywait, interrupt or usart in TMK?

As far as I know, USART is the recommended implementation, as it uses hardware for PS/2 signal receive.

You will need to use D2 (data) and D5 (clock) on the teensy though.

What you need

• Keyboard with at least 6mm clearance between switch and base plates, depending on the trackpoint module height.

• TMK_keyboard firmware source code and development environment.

• Teensy 2.0 (pins D2 and D5 available for USART).

• Trackpoint module (salvaged from old thinkpad or new tp keyboard).

• Veroboard (for reset circuit).

• Reset circuit components (4.7k resistors x2, 100k resistor, 2.2uF capacitor).

• Stem for trackpoint (labret cheek piercing 16G).

• Double sided tape (for testing stem lengths before gluing).

• Glue (stem to trackpoint, I used Loctite super glue-3)

• Cutting knife (x-acto, nt-cutter, japanese knife, etc.).

• Sandpaper (extra friction to stem ball, smooth veroboard cuts).

• Thin wire (I used wire wrap 30 gauge).

• Wire cutter and stripper.

• PCB drill / thin rounded file (to make a 1.2mm hole).

• Insulation tape (prevent shorts, protect trackpoint circuitry).

• Soldering equipment.

• Multi-meter.

• Patience.

• Bonus: extra teensy and breadboard for testing.

Steps

1. Solder leads to trackpoint

• What you need:

  • Thin wire (I used wire wrap 30 guage wire)
  • Wire cutter and stripper
  • Soldering equipment
  • Bonus: helping hand

• Cut and strip 5 leads.

• Clean and tin the trackpoint pads to be soldered.

• Use a helping hand to hold the leads on the tinned pads, and heat the stripped leads so they solder to the pad.

2. Create the reset circuit (RC)

• What you need:

  • 4.7k resistors x2
  • 100k resistor
  • 2.2uF capacitor
  • Veroboard
  • Cutting knife (x-acto, nt-cutter, japanese knife, etc.)
  • Sandpaper
  • Soldering equipment
  • Insulation tape

• Cut a piece of veroboard (6x5 - the 5 is for rails)

  • Using the cutting knife, perform 5-10 cuts on both sides.
  • Using a little pressure, snap the board on the cut.
  • Use sandpaper to smooth out the cut edges.

• Place the components on the veroboard, and solder

  • Resistors

    • Resistors don't have polarity, no need to worry.
    • Bend component legs, insert into veroboard.
    • On the back-side, slightly bend legs so components stay in position.
    • Use insulation tape to hold resistors in position if needed.
    • Snip legs (leaving about 3-5mm) and solder.
    • Snip the protruding legs for extra clearance.

  • Capacitor

    • Capacitors have polarity, usually marked with a minus (negative).
    • Positive leg must go in the VCC rail.
    • Insert capacitor legs partially, so it can be bent off the veroboard for clearance inside the keyboard case.
    • On the back-side, bend legs slightly so it stays in position.
    • Use insulation tape to hold capacitor in position.
    • Snip legs and bend negative over to 100k resistor pad, and solder.
    • Snip the protruding leg for extra clearance.

3. Solder leads to reset circuit (RC)

• What you need:

  • Thin wire (I used wire wrap 30 guage wire).
  • Wire cutter and stripper.
  • Soldering equipment.
  • Bonus: helping hand.

• Solder trackpoint leads to reset circuit.

• Cut, strip and solder leads to reset circuit that will go to the teensy.

4. Testing on a breadboard

• Insert teensy in breadboard.
• Insert leads from RC into breadboard connected to teensy pins.
• Update firmware to enable PS2 mouse and USART (see below).
• Flash firmware to teensy, and test.

5. Make trackpoint stem hole in case

• What you need:

  • PCB drill / thin rounded file (to make a 1.2mm hole).

• Decide where you want the trackpoint stem - I prefer using my middle finger and have a row staggered keyboard, so I positioned it on the cross section between U/I/K (QWERTY layout).

• Locate position for trackpoint stem, and scratch or mark the space available.

• Remove the keycaps, make the hole (slowly and carefully), replace keycaps.

• Test stem diameter in hole. Should be rigid with a slight amount of slack.

6. Make space for trackpoint in keyboard

• What you need:

  • Insulation tape.
  • Wire cutter.

• Add insulation tape to trackpoint to prevent shorts.

• Rewire diodes / column wires if needed for trackpoint clearance.

• Trim switch legs if needed for trackpoint clearance.

• Relocate D2 and D5 pins if they are already being used.

7. Test trackpoint clearance and stem length, glue stem

• What you need:

  • Stem for trackpoint (labret cheek piercing 16G, different lengths).
  • Double sided tape.
  • Glue (loctite super glue-3).
  • Sandpaper.

• Remove red cap.

• Stick a stem to trackpoint pointer base with double side tape.

• Insert trackpoint with stem into hole.

• Close case, flip keyboard, screw on stem ball, test.

• Rinse and repeat for other stems until satisfied with length.

• Remove double sided tape, glue stem.

• Use sandpaper to roughen up the stem ball for added friction.

8. Solder reset circuit (RC) to teensy

• Solder leads to teensy:

VCC  -> teensy VCC
GND  -> teensy GND
CLK  -> teensy D5
DATA -> teensy D2

9. Finish up

• Verify trackpoint is positioned squarely, and close case.
• Screw on stem ball.

10. Update tmk_keyboard firmware and flash

• What you need:

  • TMK keyboard firmware and development environment.

• Update firmware with PS2 mouse and USART support (see below).

• Plug in keyboard, flash and test.

TMK firmware changes and tweaks

• Configure PS2 mouse and USART support: diff
• Configure mousekeys: keycodes, keymap
• Enable concurrent trackpoint movement and mousekeys: diff
• Trackpoint polling interval delay for dropped key strokes: diff1, diff2
• Todo: auto-enable mouse-layer on tp movement.