When this was originally planned there was no Arduino support at all, but it was planned.  This made me think that Marlin would get support for this chip, but this has not happened and there seems to be no movement on the feature request.  On top of that, the current design uses a MCP3008 ADC to handle the thermistor inputs (because the stock ADC on the RasPi Pico isn't very good).  This same MCP3008 handles the endstop inputs as well, because there are not enough pins available on the RasPi Pico.

So I am a bit disappointed that it doesn't appear Marlin will get support for the RP2040 chip any time soon, and that even if it did I am not sure the 3008 would work.  At this point this project seems to be a dead-end as far as 3D printers go.  Thus half of the original goal seems to be out of reach.

The other half of the original goal is also looking bad.  The Arduino support really isn't very good and I don't feel like learning MicroPython when there are no shortage of boards that work correctly with Arduino.

Based on that, the current state of the RasPi Pico based design is effectively dead.  If someone wants to play with one and get the code working I'm happy to send my sample board along...otherwise it's end of development.

So...what board to replace the Pico with?  Back to my old favorite store...PJRC.  The Teensy 4.0 is supported by Marlin.  It also has the same footprint as the Teensy LC, so it still meets the low cost add-on board goal.  Based on this I designed a slimmed down board.  This board still has everything you need for a 3-in-1-out delta printer, but not much more.  Also within the same 100x100mm design is a pendant keypad and screen breakout!  Naming this T-RAMPS, as it's a bit like a RAMPS board for the Teensy.

SOURCE & CAM FILES UPLOADED, BOARDS ORDERED BUT NOT YET TESTED.