There's a strange problem with steppers when the MCU runs at 20MHz as opposed to 16MHz. See this for details: https://github.com/ErikZalm/Marlin/issues/69#issuecomment-17548378 . Fix: https://github.com/ambrop72/Marlin-rrp/commit/562c9f1f3dc20a233085162a9a717b946c6c5152
Note: running at 20MHz improved the operation of the multimaterials firmware for me; at 16MHz there were occasional short stops during printing, especially noticable during retractions.
There's a strange problem with steppers when the MCU runs at 20MHz as opposed to 16MHz. See this for details: https://github.com/ErikZalm/Marlin/issues/69#issuecomment-17548378 . Fix: https://github.com/ambrop72/Marlin-rrp/commit/562c9f1f3dc20a233085162a9a717b946c6c5152
Note: running at 20MHz improved the operation of the multimaterials firmware for me; at 16MHz there were occasional short stops during printing, especially noticable during retractions.