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+# Documentation of variables 
+
+The following are the different variables that can be overwritten in the user makefiles.
+
+- [Global variables](#global-variables)
+- [Installation/Directory variables](#installationdirectory-variables)
+- [Arduino IDE variables](#arduino-ide-variables)
+- [Sketch related variables](#sketch-related-variables)
+- [ISP programming variables](#isp-programming-variables)
+- [Compiler/Executable variables](#compilerexecutable-variables)
+- [Avrdude setting variables](#avrdude-setting-variables)
+- [Bootloader variables](#bootloader-variables)
+- [ChipKIT variables](#chipkit-variables)
+
+## Global variables
+
+### ARDUINO_QUIET
+Suppress printing of Arduino-Makefile configuration.
+
+Defaults to unset/disabled.
+
+*Example*: 1
+
+Optional.
+
+## Installation/Directory variables
+
+### ARDMK_DIR
+Directory where the `*.mk` files are stored.
+
+Usually can be auto-detected as `AUTO_ARDUINO_DIR` (parent of Arduino.mk).
+
+*Example*: /usr/share/arduino
+
+Optional.
+
+### AVR_TOOLS_DIR
+Directory where tools such as avrdude, avr-g++, avr-gcc etc. are stored in the bin/ subdirectory.
+
+Usually can be auto-detected from `$PATH` as `SYSTEMPATH_AVR_TOOLS_DIR` or as `BUNDLED_AVR_TOOLS_DIR` within the Arduino distribution.
+
+*Example*: /usr or /usr/share/arduino/hardware/tools/avr
+
+Optional.
+
+### RESET_CMD
+Command to reset the MCU.
+
+Defaults to ard-reset-arduino with the extra --caterina flag for atmega32u4 boards.
+
+*Example*: ~/gertduino/reset
+
+Optional.
+
+## Arduino IDE variables
+
+### ARDUINO_DIR
+Directory where the Arduino IDE and/or core files are stored.
+
+*Example*: /usr/share/arduino (Linux) or /Applications/Arduino.app/Contents/Resources/Java (OSX)
+
+Optional.
+
+### ARDUINO_VERSION
+Version string for Arduino IDE and/or core.
+
+Usually can be auto-detected as `AUTO_ARDUINO_VERSION` from /usr/share/arduino/lib/version.txt
+
+*Example*: 105
+
+Optional.
+
+### ARDUINO_SKETCHBOOK
+Path to sketchbook directory.
+
+Usually can be auto-detected from the Arduino preferences.txt file or the default ~/sketchbook
+
+*Example*: ~/sketches
+
+Optional.
+
+### ARDUINO_PREFERENCES_PATH
+Path to Arduino preferences.txt file.
+
+Usually can be auto-detected as `AUTO_ARDUINO_PREFERENCES` from the defaults ~/.arduino/preferences.txt (Linux) or ~/Library/Arduino/preferences.txt (OSX)
+
+*Example*: ~/sketches/preferences.txt
+
+Optional.
+
+### ARDUINO_CORE_PATH
+Path to standard Arduino core files.
+
+*Example*: /usr/share/arduino/hardware/arduino/cores/arduino
+
+Optional.
+
+## Sketch related variables
+
+### ARDUINO_LIBS
+Any libraries you intend to include.
+
+Usually can be auto-detected from the sketch. Separated by spaces. If the library has a /utility folder (like SD or Wire library), then the utility folder should also be specified.
+
+*Example*: SD SD/utility Wire Wire/utility
+
+Optional.
+
+### BOARD_TAG
+Device type as listed in boards.txt or `make show_boards`.
+
+*Example*: uno or mega2560
+
+Mandatory.
+
+### MONITOR_PORT
+Path to serial (USB) device used for uploading/serial comms.
+
+*Example*: /dev/ttyUSB0 or /dev/ttyACM0 (Linux) or /dev/cu.usb* (OSX) or com3 (Windows)
+
+Mandatory.
+
+### USER_LIB_PATH
+Directory where additional libraries are stored.
+
+Defaults to 'libraries' directory within user's sketchbook.
+
+*Example*: ~/sketchbook/libraries (Linux)
+
+Optional.
+
+### ALTERNATE_CORE
+Non-standard core for Arduino-unsupported chips like the ATtiny.
+
+*Example*: attiny-master or arduino-tiny or tiny2
+
+Optional.
+
+### ARDUINO_VAR_PATH
+Path to non-standard core's variant files.
+
+*Example*: ~/sketchbook/hardware/arduino-tiny/cores/tiny
+
+Optional.
+
+### VARIANT
+Variant of a standard board design.
+
+Usually can be auto-detected as build.variant from boards.txt
+
+*Example*: leonardo
+
+Optional.
+
+### USB_VID
+Override USB VID for atmega32u4 boards.
+
+Usually can be auto-detected as build.vid from boards.txt
+
+*Example*: 0x2341
+
+Optional.
+
+### USB_PID
+Override USB PID for atmega32u4 boards.
+
+Usually can be auto-detected as build.pid from boards.txt
+
+*Example*: 0x8039
+
+Optional.
+
+### F_CPU
+CPU speed in Hz
+
+Usually can be auto-detected as build.f_cpu from boards.txt
+
+*Example*: 8000000L
+
+Optional.
+
+### HEX_MAXIMUM_SIZE
+Maximum hex file size
+
+Usually can be auto-detected as upload.maximum_size from boards.txt
+
+*Example*: 14336
+
+Optional.
+
+### MCU
+Microcontroller model.
+
+Usually can be auto-detected as build.mcu from boards.txt
+
+*Example*: atmega32u4
+
+Optional.
+
+### MCU_FLAG_NAME
+Override default MCU flags.
+
+Defaults to mmcu
+
+*Example*: mprocessor
+
+Optional.
+
+### MONITOR_BAUDRATE
+Baudrate of the serial monitor.
+
+Defaults to 9600 if it can't find it in the sketch Serial.begin()
+
+*Example*: 57600
+
+Optional.
+
+## ISP programming variables
+
+### ISP_PROG
+Type of ISP. Either a USB device or ArduinoISP protocol.
+
+*Example*: usbasp or usbtiny or stk500v2 or stk500v1
+
+Optional.
+
+### ISP_PORT
+Device path to ArduinoISP. Not needed for hardware ISP's.
+
+*Example*: /dev/ttyACM0 (Linux)
+
+Optional.
+
+### ISP_LOCK_FUSE_PRE
+Bootloader unlock bits.
+
+Usually can be auto-detected from boards.txt
+
+*Example*: 0x3f
+
+Optional.
+
+### ISP_LOCK_FUSE_POST
+Bootloader lock bits.
+
+Usually can be auto-detected from boards.txt
+
+*Example*: 0xcf
+
+Optional.
+
+### ISP_HIGH_FUSE
+SP_LOW_FUSE/ISP_EXT_FUSE - high/low/extended fuse bits.
+
+Usually can be auto-detected from boards.txt
+
+*Example*: 0xdf or 0xff or 0x01
+
+Optional.
+
+### ISP_EEPROM
+Whether to upload the EEPROM file or not.
+
+Defaults to 0
+
+*Example*: 1
+
+Optional.
+
+## Compiler/Executable variables
+
+### CC_NAME
+C compiler.
+
+Defaults to avr-gcc
+
+*Example*: pic32-gcc
+
+Optional.
+
+### CXX_NAME
+C++ compiler.
+
+Defaults to avr-g++
+
+*Example*: pic32-g++
+
+Optional.
+
+### OBJCOPY_NAME
+Objcopy utility.
+
+Defaults to avr-objcopy
+
+*Example*: pic32-objcopy
+
+Optional.
+
+### OBJDUMP_NAME
+Objdump utility.
+
+Defaults to avr-objdump
+
+*Example*: pic32-objdump
+
+Optional.
+
+### AR_NAME
+Archive utility.
+
+Defaults to avr-ar
+
+*Example*: pic32-ar
+
+Optional.
+
+### SIZE_NAME
+Size utility.
+
+Defaults to avr-size
+
+*Example*: pic32-size
+
+Optional.
+
+### NM_NAME
+Nm utility.
+
+Defaults to avr-nm
+
+*Example*: pic32-nm
+
+Optional.
+
+### OPTIMIZATION_LEVEL
+Linker's -O flag
+
+Defaults to s, which shouldn't really be changed as it breaks SoftwareSerial and usually results in bigger hex files.
+
+*Example*: 3
+
+Optional.
+
+### CFLAGS_STD
+Flags to pass to the C compiler.
+
+Defaults to -std=gnu99
+
+*Example*: <unset as per chipKIT.mk>
+
+Optional.
+
+### OVERRIDE_EXECUTABLES
+Override the default build tools.
+
+If set to 1 each tool (CC, CXX, AS, OBJCOPY, OBJDUMP, AR, SIZE, NM) must have its path explicitly defined. See chipKIT.mk
+
+*Example*: 1
+
+Optional.
+
+### MONITOR_CMD
+Command to run the serial monitor.
+
+Defaults to screen
+
+*Example*: minicom
+
+Optional.
+
+## Avrdude setting variables
+
+### AVRDUDE
+Path to avrdude utility
+
+Usually can be auto-detected within the parent of `AVR_TOOLS_DIR` or in the $PATH
+
+*Example*: /usr/bin/avrdude
+
+Optional.
+
+### AVRDUDE_CONF
+Path to avrdude.conf file
+
+Usually can be auto-detected within the parent of `AVR_TOOLS_DIR` 
+
+*Example*: /etc/avrdude.conf or /usr/share/arduino/hardware/tools/avrdude.conf
+
+Optional.
+
+### AVR_TOOLS_PATH
+Directory where tools such as avrdude, avr-g++, avr-gcc etc. are stored.
+
+Usually can be auto-detected from AVR_TOOLS_DIR/bin
+
+*Example*: /usr/bin or /usr/share/arduino/hardware/tools/avr/bin
+
+Optional.
+
+### ARDUINO_LIB_PATH
+Directory where the standard Arduino libraries are stored.
+
+Defaults to ARDUINO_DIR/libraries
+
+*Example*: /usr/share/arduino/libraries (Linux)
+
+Optional.
+
+### ARDUINO_CORE_PATH
+Directory where the standard Arduino cores are stored.
+
+Defaults to ARDUINO_DIR/hardware/arduino/cores/arduino
+
+*Example*: /usr/share/arduino/hardware/arduino/cores/robot
+
+Optional.
+
+### ALTERNATE_CORE_PATH
+Path to non-standard cores.
+
+Defaults to ARDUINO_SKETCHBOOK/hardware/ALTERNATE_CORE
+
+*Example*: ~/sketchbook/hardware/arduino-tiny/cores/tiny
+
+Optional.
+
+### BOARDS_TXT
+Path to boards.txt
+
+Defaults to ARDUINO_DIR/hardware/arduino/boards.txt
+
+*Example*: ~/sketchbook/hardware/boards.txt or /usr/share/arduino/hardware/arduino/boards.txt (Linux)
+
+Optional.
+
+### AVRDUDE_ARD_BAUDRATE
+Upload speed
+
+Usually can be auto-detected as upload.speed from boards.txt
+
+*Example*: 19200
+
+Optional.
+
+### AVRDUDE_ARD_PROGRAMMER
+Upload protocol
+
+Usually can be auto-detected as upload.protocol from boards.txt
+
+*Example*: stk500v1
+
+Optional.
+
+### AVRDUDE_ISP_BAUDRATE
+ISP speed if different to upload.speed
+
+Defaults to same as `AVRDUDE_ARD_BAUDRATE` or 19200
+
+*Example*: 19200
+
+Optional.
+
+### AVRDUDE_OPTS
+Options to pass to avrdude.
+
+Defaults to '-q -V -D' (quiet, don't verify, don't auto-erase). User values are not ANDed to the defaults, you have to set each option you require.
+
+*Example*: -v
+
+Optional.
+
+## Bootloader variables
+
+### BOOTLOADER_FILE
+File for bootloader.
+
+Usually can be auto-detected as bootloader.file from boards.txt
+
+*Example*: optiboot_atmega328.hex
+
+Optional.
+
+### BOOTLOADER_PATH
+Path to bootloader file.
+
+Usually can be auto-detected as a relative bootloader.path from boards.txt
+
+*Example*: optiboot or arduino:atmega or /usr/share/arduino/hardware/arduino/bootloaders/caterina/Caterina-Esplora.hex
+
+Optional.
+
+## ChipKIT variables
+
+### MPIDE_DIR
+Path to chipKIT MP IDE
+
+Usually can be auto-detected as `AUTO_MPIDE_DIR` from the defaults /usr/share/mpide (Linux) or /Applications/Mpide.app/Contents/Resources/Java (OSX)
+
+*Example*: ~/mpide
+
+Optional.
+
+### MPIDE_PREFERENCES_PATH
+Path to chipKIT preferences.txt file.
+
+Usually can be auto-detected as `AUTO_MPIDE_PREFERENCES_PATH` from the defaults ~/.mpide/preferences.txt (Linux) or ~/Library/Mpide/preferences.txt (OSX)
+
+*Example*: ~/chipkit/preferences.txt
+
+Optional.