#define EXT0_TEMPSENSOR_PIN THERMOCOUPLE_1_PIN

It would be nice to be able to set the board fan PWM via a #define. It is already being PWMed but with a hard coded value of 255 for the speed.

#define DEFAULT_AXIS_STEPS_PER_UNIT {80.00,80.00,4000.00,606.09}

#define X_HOME_DIR -1

#define EXT0_DECOUPLE_TEST_PERIOD 0

I'd say #define HEATED_BED_MAX_TEMP 110 is the reason.

#define HEATED_BED_HEAT_MANAGER 2 

#define YAXIS_STEPS_PER_MM 80

#define MIN_HARDWARE_ENDSTOP_X true

#define DRIVE_SYSTEM 0

#define SDA_PIN         20  

firmware  hangs when i use #define UI_HAS_I2C_KEYS.

#define DRIVE_SYSTEM 3

#define

#define COMPILE_I2C_DRIVER

Easy question: Is it possible to disable the UI_RESET_PIN by setting it to "#define UI_RESET_PIN -1"? Or will I encounter any problems with that?

#define MCP_IN_ADDRESS 0x20

#define UI_DISPLAY_I2C_ADDRESS 0x4e but the firmware freezes again.

#define UI_DISPLAY_RS_PIN _BV(0)

#define EXT1_HEATER_PIN HEATER_0_PIN