According to its manual, the DM566 is capable of 1/128 microsteps when you use the DIP switches
In the spreadsheet you can configure it to either 1/128 or for 1/64 (we can provide input if you attach
the spreadsheet).

However, here is an important point: since it is an external driver, OnStep will only tell it "take a step"
using the STEP and DIR pins ONLY.

So your Config.h will have the driver model as GENERIC, which means that MICROSTEPS should be
set to OFF.