u8g by Oliver Kraus with extensions for ARM and the Marlin HAL.

The extensions allow access to the u8g library by "non native" CPUs/systems.

AVR, Arduino, and Due access to the library is not changed.

Enabling the extensions by defining the flag U8G_HAL_LINKS. This can be done in platformio.ini by adding -DU8G_HAL_LINKS to any environment's build flags.

The LCD_defines.h header brings in the top level link file which:

• Defines link files for SPI routines, delay routines, I2C routines and class definitions.
• Connects u8g com generic com defines to the desired HAL com routine.

Contains code or declarations for the u8g routines:

• void U8g_delay(int msec)
• void u8g_MicroDelay(void)
• void u8g_10MicroDelay(void)

Contains code or declarations for the u8g routines:

• void u8g_SetPinOutput(uint8_t internal_pin_number)
• void u8g_SetPinInput(uint8_t internal_pin_number)
• void u8g_SetPinLevel(uint8_t pin, uint8_t pin_status)
• uint8_t u8g_GetPinLevel(uint8_t pin)

Contains code or declarations for the u8g routines:

• void u8g_i2c_init(uint8_t options)
• uint8_t u8g_i2c_wait(uint8_t mask, uint8_t pos)
• uint8_t u8g_i2c_start(uint8_t sla)
• uint8_t u8g_i2c_send_byte(uint8_t data)
• void u8g_i2c_stop(void)

• Empty constructors are now provided so the u8g instance can be initialized later. Data members are still constructed early when the u8g object is declared static global.
• Constructors that just take an "options" argument now require the argument (e.g., U8G_I2C_OPT_NONE) or they will be treated as default empty constructors and won't be initialized.