Is there a set of basic routines for TMC428 access available as a minimalist C example code of a TM428 control program available?
Yes, the tmc248demo C example enfolds a set of basic routines to access the TMC428 registers via its SPI interface. These routines all together build a simple example application to control the TMC428. One can directly run it together with an TMC428 Evaluation Kit. These sources are distributed in the hope that they will be useful. They might be a basis for your own application, even on pure micro controller applications without PC.