Fixed on Mega2560 and Due
https://github.com/firepick1/FireStep/blob/master/CHANGELOG.md
Tested with:
RAMPS1.4 with A4988 drivers @ 200 steps; 16 microsteps
EMC02 with A4988 drivers @ 400 steps; 16 microsteps
Arduino IDE 1.6.7 (NOTE: earlier versions of Arduino are NOT supported)
Arduino Mega2560, Arduino Due