Extra step pins
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From my investigation into this topic I wasn't clear if step pins have to be associated with a specific part of pin configuration, but it looks like all step pins in the PIO spec as PD# -- so are external drivers 12-13 possible, I don't see any free PD pins I can repurpose.
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It's possible to use step pins on other ports, but more complicated and with a small loss of maximum step speed. Look at how it's done for the older Duets and for RADDS in those 3 functions at the end of Platform.h.