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.

Target audience
FAQ
What you need
Steps
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.

alonswartz/trackpoint

alonswartz

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