motion5
Specialised variant of GRBL focused on motion of up to five axes
...It supports ATMega MCUs running with up to 20 MHz and provides jitter-free pulses with a frequency of up to 38 kHz.
Wherever usage and handling of motion5 is different to original GRBL, this is described in Wiki. For the G-code commands please note that motion5 supports more axes which are named "A" and "B". So a GRBL-command "G1X10.0Y20.0Z10.0" which moves its three axes would be e.g. "G1X10.0Y20.0Z10.0A20.0B30.0" in motion5 to move all five axes.