Display interface

To create a display for LVGL call lv_display_t * display = lv_display_create(hor_res, ver_res). You can create a multiple displays and a different driver for each (see below),

Basic setup

Draw buffer(s) are simple array(s) that LVGL uses to render the screen's content. Once rendering is ready the content of the draw buffer is sent to the display using the flush_cb function.

flush_cb

An example flush_cb looks like this:

void my_flush_cb(lv_display_t * display, const lv_area_t * area, void * px_map)
{
    /*The most simple case (but also the slowest) to put all pixels to the screen one-by-one
     *`put_px` is just an example, it needs to be implemented by you.*/
    uint16_t * buf16 = (uint16_t)px_map; /*Let's say it's a 16 bit (RGB565) display*/
    int32_t x, y;
    for(y = area->y1; y <= area->y2; y++) {
        for(x = area->x1; x <= area->x2; x++) {
            put_px(x, y, *buf16);
            buf16++;
        }
    }

    /* IMPORTANT!!!
     * Inform LVGL that you are ready with the flushing and buf is not used anymore*/
    lv_display_flush_ready(disp);
}

Use lv_display_set_flush_cb(disp, my_flush_cb) to set a new flush_cb.

lv_display_flush_ready(disp) needs to be called when flushing is ready to inform LVGL the buffer is not used anymore by the driver and it can render new content into it.

LVGL might render the screen in multiple chunks and therefore call flush_cb multiple times. To see if the current one is the last chunk of rendering use lv_display_flush_is_last(display).

Draw buffers

The draw buffers can be set with lv_display_set_buffers(display, buf1, buf2, buf_size_px, render_mode)

  • buf1 a buffer where LVGL can render

  • buf2 a second optional buffer (see more details below)

  • buf_size_byte size of the buffer(s) in bytes

  • render_mode

    • LV_DISP_RENDER_MODE_PARTIAL Use the buffer(s) to render the screen in smaller parts. This way the buffers can be smaller then the display to save RAM. At least 1/10 screen size buffer(s) are recommended. In flush_cb the rendered images needs to be copied to the given area of the display. In this mode if a button is pressed only the button's area will be redrawn.

    • LV_DISP_RENDER_MODE_DIRECT The buffer(s) has to be screen sized and LVGL will render into the correct location of the buffer. This way the buffer always contain the whole image. If two buffer are used the rendered areas are automatically copied to the other buffer after flushing. Due to this in flush_cb typically only a frame buffer address needs to be changed. If a button is pressed only the button's area will be redrawn.

    • LV_DISP_RENDER_MODE_FULL The buffer(s) has to be screen sized and LVGL will always redraw the whole screen even if only 1 pixel has been changed. If two screen sized draw buffers are provided, LVGL's display handling works like "traditional" double buffering. This means the flush_cb callback only has to update the address of the frame buffer to the px_map parameter.

Example:

static uint16_t buf[LCD_HOR_RES * LCD_VER_RES / 10];
lv_display_set_buffers(disp, buf, NULL, sizeof(buf), LV_DISP_RENDER_MODE_PARTIAL);

One buffer

If only one buffer is used LVGL draws the content of the screen into that draw buffer and sends it to the display via the flush_cb. LVGL then needs to wait until lv_display_flush_ready is called (that is the content of the buffer is sent to the display) before drawing something new into it.

Two buffers

If two buffers are used LVGL can draw into one buffer while the content of the other buffer is sent to the display in the background. DMA or other hardware should be used to transfer data to the display so the MCU can continue drawing. This way, the rendering and refreshing of the display become parallel operations.

Advanced options

Resolution

To set the resolution of the display after creation use lv_display_set_resolution(display, hor_res, ver_res)

It's not mandatory to use the whole display for LVGL, however in some cases the physical resolution is important. For example the touchpad still sees the whole resolution and the values needs to be converted to the active LVGL display area. So the physical resolution and the offset of the active area can be set with lv_display_set_physical_resolution(disp, hor_res, ver_res) and lv_display_set_offset(disp, x, y)

Flush wait callback

By using lv_display_flush_ready LVGL will spin in a loop while waiting for flushing.

However with the help of lv_display_set_flush_wait_cb a custom wait callback be set for flushing. This callback can use a semaphore, mutex, or anything else to optimize while the waiting for flush.

If flush_wait_cb is not set, LVGL assume that lv_display_flush_ready is used.

Rotation

LVGL supports rotation of the display in 90 degree increments. You can select whether you would like software rotation or hardware rotation.

The orientation of the display can be changed with lv_disp_set_rotation(disp, LV_DISPLAY_ROTATION_0/90/180/270). LVGL will swap the horizontal and vertical resolutions internally according to the set degree. When changing the rotation LV_EVENT_SIZE_CHANGED is sent to the display to allow reconfiguring the hardware. In lack of hardware display rotation support lv_draw_sw_rotate can be used to rotate the buffer in the flush_cb.

Color format

The default color format of the display is set according to LV_COLOR_DEPTH (see lv_conf.h)

The color_format can be changed with lv_display_set_color_depth(display, LV_COLOR_FORMAT_...). Besides the default value LV_COLOR_FORMAT_ARGB8888 can be used as a well.

It's very important that draw buffer(s) should be large enough for any selected color format.

Swap endianness

In case of RGB565 color format it might be required to swap the 2 bytes because the SPI, I2C or 8 bit parallel port periphery sends them in the wrong order.

The ideal solution is configure the hardware to handle the 16 bit data with different byte order, however if it's not possible lv_draw_sw_rgb565_swap(buf, buf_size_in_px) can be called in the flush_cb to swap the bytes.

If you wish you can also write your own function, or use assembly instructions for the fastest possible byte swapping.

Note that this is not about swapping the Red and Blue channel but converting

RRRRR GGG | GGG BBBBB

to

GGG BBBBB | RRRRR GGG.

User data

With lv_display_set_user_data(disp, p) a pointer to a custom data can be stored in display object.

Decoupling the display refresh timer

Normally the dirty (a.k.a invalid) areas are checked and redrawn in every LV_DEF_REFR_PERIOD milliseconds (set in lv_conf.h). However, in some cases you might need more control on when the display refreshing happen, for example to synchronize rendering with VSYNC or the TE signal.

You can do this in the following way:

/*Delete the original display refresh timer*/
lv_timer_delete(disp->refr_timer);
disp->refr_timer = NULL;


/*Call this anywhere you want to refresh the dirty areas*/
_lv_disp_refr_timer(NULL);

If you have multiple displays call lv_disp_set_default(disp1) to select the display to refresh before _lv_disp_refr_timer(NULL).

Note

that lv_timer_handler() and _lv_disp_refr_timer() can not run at the same time.

If the performance monitor is enabled, the value of LV_DEF_REFR_PERIOD needs to be set to be consistent with the refresh period of the display to ensure that the statistical results are correct.

Events

lv_display_add_event_cb(disp, event_cb, LV_EVENT_..., user_data) adds an event handler to a display. The following events are sent:

Further reading

API

lv_refr.h

lv_display.h

lv_types.h