Hi - I'm trying to figure out how grbl (and the grbl shield) deals with spindle control. I've seen this in the docs for grbl
If you have a desire or need for spindle or coolant control, Grbl will toggle these output pins (D12, D13, A3) high or low, depending on the g-code commands you send to Grbl. Since these are all application dependent, we'll leave it to you on how to control and use these for your machine.
Which implies that the spindle pins for enable and direction are digital pins and are simply toggled based on passed gcodes - but this leaves me to wonder if or how speed is handled. I wondered if it used PWM - but those pins don't support PWM.
(I realize this may be too grbl-specific but I've been searching all over and haven't found any clear info concerning spindle speed - except for stray comments which imply that it is handled somehow.
Other than that, the grbl shield works great!