LCD Character Drivers

The LCD driver exposes the LCD interface to userspace via ioctl() commands.

The LCD driver is intended to be used in the following scenarios:

On memory-constrained devices, as it doesn’t require a buffer to represent
the whole display:
- Hence, it’s an alternative to the Frame Buffer Drivers
For graphics libraries that draw specific areas of the displays, like LVGL

Binding

LCD drivers usually are not directly accessed by user code, but are usually bound to another, higher-level device driver. In general, the binding sequence is:

Get an instance of struct lcd_dev_s from the hardware-specific LCD screen driver, and
Provide that instance to the initialization method of the higher-level character driver.

Generic LCD Character Driver

This example will walk through the path from userspace to hardware-specific details on how an LCD screen is bound to an LCD character driver.

include/nuttx/lcd/lcd.h provides all structures and APIs needed to work
with LCD screens drivers:
- This header file also depends on some of the same definitions used for the frame buffer driver as provided in include/nuttx/video/fb.h
drivers/lcd/lcd_dev.c is the higher-level device driver. An instance of
struct lcd_dev_s will be provided to it:
- include/nuttx/lcd/lcd_dev.h prototypes public structures and functions;
- lcddev_register registers the LCD character driver as /dev/lcdN where N is the display number and,
- calls the board_lcd_getdev, an LCD-specific function usually defined in boards/<arch>/<chip>/<board>/src and prototyped in include/nuttx/board.h
Finally, the LCD screen drivers are usually available at drivers/lcd/ and implement the callbacks defined at include/nuttx/lcd/lcd.h:
- include/nuttx/lcd/lcd.h provides structures and APIs needed to work with LCD screens, whether using the framebuffer adapter or the LCD Character Drivers;

Supported devices

Re-usable LCD drivers reside in the drivers/lcd directory:

LCDs

mio283qt2.c

This is a driver for the MI0283QT-2 LCD from Multi-Inno Technology Co., Ltd. This LCD is based on the Himax HX8347-D LCD controller.
mio283qt9a.c

This is a driver for the MI0283QT-9A LCD from Multi-Inno Technology Co., Ltd. This LCD is based on the Ilitek ILI9341 LCD controller.
ssd12989.c

Generic LCD driver for LCDs based on the Solomon Systech SSD1289 LCD controller. Think of this as a template for an LCD driver that you will probably have to customize for any particular LCD hardware. (See also boards/arm/stm32/hymini-stm32v/src/ssd1289.c below).
st7567.c

LCD Display Module, ST7567, Univision Technology Inc. Used with the LPCXpresso and Embedded Artists base board.
memlcd.c

Sharp Memory LCD Suite, LS013B7DH01, LS013B7DH03, etc. There are some more different models, they are basically controlled by similar logics, thus this driver can be extended.
ra8875.c

RAiO Technologies RA8875 LCD controller. Contributed by Marten Svanfeldt.

OLEDs

p14201.c

Driver for RiT P14201 series display with SD1329 IC controller. Based on the SD1329 controller. This OLED is used with older versions of the TI/Luminary LM3S8962 Evaluation Kit. Example usage:
```
boards/arm/tiva/lm3s6965-ek/src
boards/arm/tiva/lm3s8962-ek/src
```
ug-2864ambag01.c

OLED Display Module, UUG-2864AMBAG01, Univision Technology Inc. Based on the SH1101A controller. Example usage:
```
boards/arm/stm32/stm32f4discovery
boards/arm/lpc214x/zp214xpa
```
ug-9664hswag01.c

OLED Display Module, UG-9664HSWAG01, Univision Technology Inc. Based on the SSD1305 controller. Used with the LPC Xpresso and Embedded Artists base board. Example usage:
```
boards/arm/lpc71xx_40xx/lpcxpresso-lpc1768
```
ssd1306.c

OLED Display Modules based on the SSD1306 controllers. This includes the UG-2864HSWEG01 and UG2832HSWEG04, both from Univision Technology Inc. The latter is used with the OLED1 module that comes with the Atmel SAM4l Xplained Pro board. This driver also supports Densitron Technologies DD-12864WO-4A which is based on SSD1309 LCD controller. Example usage:
```
boards/arm/stm32/stm32f4discovery
boards/arm/sam34/sam4l-xplained
```

Segment LCDS (SLCDs)

pcf8574_lcd_backpack.c

See pcf8574_lcd_backpack_readme.txt.

Examples

Examples apply to specific cases of the Generic LCD Character Driver:

TTGO T-Display ESP32 board

This board contains an ST7789 TFT Display (135x240). By selecting the ttgo_t_display_esp32:lvgl_lcd config, the lvgldemo example will be built with the LCD character interface.

boards/xtensa/esp32/ttgo_t_display_esp32/src/esp32_bringup.c registers the LCD character driver:

#ifdef CONFIG_LCD_DEV
 ret = board_lcd_initialize();
 if (ret < 0)
   {
     syslog(LOG_ERR, "ERROR: board_lcd_initialize() failed: %d\n", ret);
   }

 ret = lcddev_register(0);
 if (ret < 0)
   {
     syslog(LOG_ERR, "ERROR: lcddev_register() failed: %d\n", ret);
   }
#endif

board_lcd_initialize and board_lcd_getdev are defined at boards/xtensa/esp32/common/src/esp32_st7789.c;
- board_lcd_initialize initializes the LCD hardware on the board by defining the SPI interface which is connected to the display controller;
lcddev_register then calls board_lcd_getdev:
- board_lcd_getdev calls the st7789_lcdinitialize and returns a reference to the LCD object for the specified LCD;
- st7789_lcdinitialize is part of the LCD screen driver at drivers/lcd/st7789.c;
The LVGL demo application (lvgldemo) makes use of the ioctl system call to trigger an LCDDEVIO_PUTAREA request to the higher-level device driver to refresh the LCD screen with data:

ioctl(state.fd, LCDDEVIO_PUTAREA, (unsigned long)((uintptr_t)&lcd_area));;

NuttX Simulator

NuttX Simulator provides a X11-based LCD: character driver to simulate the LCD character displat usage into a X11-compatible host.

By selecting the sim:lvgl_lcd config, the lvgldemo example will be built with the LCD character interface.

boards/sim/sim/sim/src/sim_bringup.c registers the lcd driver the same way TTGO T-Display ESP32 board;
arch/sim/src/sim/up_lcd.c and arch/sim/src/sim/up_x11framebuffer.c will be built as CONFIG_SIM_LCDDRIVER = y and CONFIG_SIM_X11FB = y are set, respectively;
- up_lcd.c provides board_lcd_initialize and board_lcd_getdev:
  board_lcd_initialize calls up_x11initialize from up_x11framebuffer.c that initializes a X11-based window as an LCD character device. This is the underlying “driver”.
The LVGL demo application (lvgldemo) makes use of the ioctl system call to trigger an LCDDEVIO_PUTAREA request to the higher-level device driver to refresh the LCD screen with data as usual;

LCD Header files

include/nuttx/lcd/lcd.h

Structures and APIs needed to work with LCD drivers are provided in this header file. This header file also depends on some of the same definitions used for the frame buffer driver as provided in include/nuttx/video/fb.h.

struct lcd_dev_s

Each LCD device driver must implement an instance of struct lcd_dev_s. That structure defines a call table with the following methods:
Get information about the LCD video controller configuration and the configuration of each LCD color plane.
int (*getvideoinfo)(FAR struct lcd_dev_s *dev,
                    FAR struct fb_videoinfo_s *vinfo);
int (*getplaneinfo)(FAR struct lcd_dev_s *dev, unsigned int planeno,
                    FAR struct lcd_planeinfo_s *pinfo);
The following are provided only if the video hardware supports RGB color mapping:
int (*getcmap)(FAR struct lcd_dev_s *dev,
               FAR struct fb_cmap_s *cmap);
int (*putcmap)(FAR struct lcd_dev_s *dev,
               FAR const struct fb_cmap_s *cmap);
The following are provided only if the video hardware supports a hardware cursor:
int (*getcursor)(FAR struct lcd_dev_s *dev,
                 FAR struct fb_cursorattrib_s *attrib);
int (*setcursor)(FAR struct lcd_dev_s *dev,
                 FAR struct fb_setcursor_s *settings);
Get the LCD panel power status (0: full off - CONFIG_LCD_MAXPOWER: full on). On backlit LCDs, this setting may correspond to the backlight setting:
int (*getpower)(struct lcd_dev_s *dev);
Enable/disable LCD panel power (0: full off - CONFIG_LCD_MAXPOWER: full on). On backlit LCDs, this setting may correspond to the backlight setting:
int (*setpower)(struct lcd_dev_s *dev, int power);
Get the current contrast setting (0-CONFIG_LCD_MAXCONTRAST):
int (*getcontrast)(struct lcd_dev_s *dev);
Set LCD panel contrast (0-CONFIG_LCD_MAXCONTRAST):
int (*setcontrast)(struct lcd_dev_s *dev, unsigned int contrast);