h1. Grbl - An embedded g-code interpreter and motion-controller for the Arduino/AVR328 microcontroller
    
Grbl is a no-compromise, high performance, low cost alternative to parallel-port-based motion control for CNC milling. It will run on a vanilla Arduino (Duemillanove/Uno) as long as it sports an Atmega 328. 

The controller is written in highly optimized C utilizing every clever feature of the AVR-chips to achieve precise timing and asynchronous operation. It is able to maintain more than 30kHz of stable, jitter free control pulses.

It accepts standards-compliant G-code and has been tested with the output of several CAM tools with no problems. Arcs, circles and helical motion are fully supported, as well as, other basic functional g-code commands. Functions and variables are not currently supported, but may be included in future releases in a form of a pre-processor.

Grbl includes full acceleration management with look ahead. That means the controller will look up to 18 motions into the future and plan its velocities ahead to deliver smooth acceleration and jerk-free cornering.

*Changelog for v0.8 from v0.7:*
  - *BETA status: _Under development but nearing completion. Code state may change with each push as new features are integrated or when bugs are squashed._*
  - Major structural overhaul to allow for multi-tasking events and new feature sets.
  - Run-time command control: Feed hold (pause), Cycle start (resume), Reset (abort), Status reporting
  - Controlled feed hold with deceleration to ensure no skipped steps and loss of location.
  - After feed hold, cycle accelerations are re-planned and may be resumed.
  - Advanced homing cycle with direction and speed configuration options. (Requires limit switches.) When enabled, homing is required before use to ensure safety.
  - Limit pins are held normal high with internal pull-up resistors. Wiring only requires a normally-open switch connected to ground. (For both ends of an axis, simply wire two in parallel into the same pin.)
  - Hard limits option and plays nice with homing cycle, so switches can be used for both homing and hard limits.
  - New switch commands: Dry-run, block delete, single block mode, optional stop. A check g-code switch has also been added to allow users to error check their programs. This feature can also be used as a "program restart" by enabling the check g-code switch, begin streaming, and disable the check g-code switch at the desired point with minimal additional g-code.
  - Re-factored g-code parser with robust error-checking.
  - 6 work coordinate systems (G54-G59), offsets(G92), and machine coordinate system support. Work coordinate systems are stored in EEPROM and persistent.
  - G10 L2 and L20 work coordinate settings support. L2 sets one or more axes values. L20 sets the current machine position to the specified work origin.
  - G28.1 and G30.1 set home position support. These set the internal EEPROM parameter values to the current machine position. (G28 and G30 no longer perform homing cycle, '$H' does. They move to these stored positions.)
  - Program stop(M0,M1,M2,M30) support.
  - Coolant control(M7*,M8,M9) support. (M7 is a compile-time option).
  - System reset re-initializes grbl without resetting the Arduino and retains machine/home position and work coordinates.
  - Settings overhauled and dozens of new settings and internal commands are now available, when most were compile-time only.
  - New startup line setting. Allows users to store a custom g-code block into Grbl's startup routine. Executes immediately upon startup or reset. May be used to set g-code defaults like G20.
  - Misc bug fixes and removed deprecated acceleration enabled code.  
  - Advanced compile-time options: XON/XOFF flow control (limited support), direction and step pulse time delay, and up to 5 startup lines.
  

*Important note for Atmega 168 users:* Going forward, support for Atmega 168 will be dropped due to its limited memory and speed. However, legacy Grbl v0.51 "in the branch called 'v0_51' is still available for use.

_The project was initially inspired by the Arduino GCode Interpreter by Mike Ellery_