I can now draw on the screen for real! This means all of the cores are working with both reads and writes, and the the LED matrix is showing the state of the cores in realtime, with an overall refresh rate of 12ms. The following image shows the extent of the flux from the cylindrical magnet on the end of the stylus.

This milestone has been a long time coming. The breakthrough I made was the discovery that my shared use of a GPIO pin for one of the matrix drive transistors and the LED array was causing interference. The effect was that quickly strobing the matrix transistor fooled the LED array into thinking it was receiving valid data. The array is driven by a series of pulses, and if they are close enough together (less than about 150 us) the first few WS28128 LED pixels in the string start to illuminate. This is a design problem that is obvious now. But fortunately everything is still happening fast enough when I added a necessary delay, a firmware change overcomes the issue. In the next prototype, I plan to implement two 3-8 binary decoders to free up some IO on the Teensy LC. I was already overlapping some of the IO usage, and could use some headroom for other functionality that I need to add. The core matrix itself requires 20 transistors on its own.

There are several more hurdles to resolve related to the functionality outside of the cores/LEDs, but the last  two hurdles I have to solve for the cores is the sensing signal and the half-select currents. Those need to be tuned up to operate 100% reliably with a wider range of power supply voltage. I am seeing occasional flashes on the LED matrix which are indicating that sometimes the core flip is not being sensed. The allowable input voltage range right now is 3.3 to 4.0V, which works OK with the LiPo battery I'm using. But it needs to be more reliable.

Also looking forward to the next PCB layout, so I can update it to say 2020!