libopencm3 VS grblHAL

Some of the vendor IDEs have the option to generate makefiles for a project. Depending on the processor, you could also opt out to a different system. A lot of the popular cortex-m stuff is supported by libopencm3.

You can also look at examples of HALs such as - libopencm3 (Though this is Cortex only) - RIOT OS (though this requires an RTOS)

Yeah by the looks of it I am going to have to expand my project do creating a portable driver interface as well. Abstracting hardware interfaces to create portable code is fairly common in embedded development. For example, libopencm3 does this and has great support for STM32 parts but very limited support for SAM parts.

TinyUSB or Libopencm3 are probably your best options (although check they support the SAME70).

that is fair. SPI isn't very portable across Arm cpu vendors because all peripherals are vendor specific, i.e. the registers will differ a lot and APIs reflect that. Finding common ground is hard unless using specifically designed portable libraries like https://github.com/libopencm3/libopencm3

https://github.com/terjeio/grblHAL/blob/master/doc/markdown/settings.md check this link there is an explanation of grbl code from A to Z

And it looks like pendant support was 'enabled' last year in grblHAL, but I don't see the "I'm with stupid guide" that I would need to connect it.

If you have time, check it out! https://github.com/terjeio/grblHAL/

You might want to look at grblHAL. It is the 32-bit evolution of Grbl. I make a 5 axis breakout board for it based on a Teensy 4.1. It supports step rates up to 400 kHz which allows you a lot of freedom in selecting microsteps and high speed rapids. All in with the Teensy, it costs about $65-70, depending on options. Here is a gallery with a number of machines based on grblHAL. More about grblHAL here. It is actively being developed with people adding features and porting to new processors (RaspBerry Pico is coming).