A general summary of project progress as of February 12th, 2022.
Electrical
Completed a detailed high level block diagram of all required peripherals with all the signals that the microcontroller needs to drive. The total number of pins required to drive all the peripherals is 40, with 3 pins per motor controller for EN, STEP, DIR, 8 pins for the LCD, 2 pins for USB and one pin each for the thermistors, limit switches, fans and heaters.
From the pin requirements, Marlin firmware recommendations and part availability, the STM32F401RC microcontroller was chosen for this board.
The stepper motor drivers were chosen as A4988s because TMC stepper drivers were largely out of stock, and since A4988s are basic parts, they are more readily available. Additionally, the stock Ender 3 uses A4988 stepper motor drivers so they should work well with the stepper motors already.
Software
Marlin has had previous attempts to add 5 axis capabilities.