There seems to be an error in the datasheet concerning FS_SPD. For starters the maximum value is h3FF and not hFF as stated. But if I am running in 128 microstep mode and the FS_SPD threshold is crossed it does not actually switch to full-step mode, it looks more like an in-between microstep mode. I have confirmed this on the scope, it's not the same as running the motor in full-step mode. BTW, I have managed to run the NEMA34 at up to 3400RPM.