Sync display refresh on vertical blank


hi, i am new here. hope i found the right place for my question.

i wonder if it is possible to refresh the display contents during the vertical sync/blank.

What MCU/Processor/Board and compiler are you using?

STM32F429 with a 480x272 TFT display.

What do you want to achieve?

avoid flicker, if memory is limited (no doublebuffer) and the framebuffer is directly displayed (direct_mode).

What have you tried so far?

  1. initialize the STM32F429 and its LTDC to generate an interrupt at the last line drawn on the TFT (about 1.2msec/20lines left until LTDC draws the first line)
  2. set a volatile variable in disp_drv.flush_cb and store all arguments
  3. within the ISR: copy the buffer contents to the framebuffer.

Code to reproduce

static volatile lv_disp_drv_t *mDispDrv;
static volatile lv_area_t *mArea;
static volatile lv_color_t *mColorP;
static volatile bool mRequest = false;

void lv_port_disp_init(void) {
    static lv_disp_draw_buf_t draw_buf_dsc_4;
    lv_disp_draw_buf_init(&draw_buf_dsc_4, (void *)alLtdcFb, NULL, MY_DISP_HOR_RES * MY_DISP_VER_RES);
    static lv_disp_drv_t disp_drv;
    disp_drv.hor_res = MY_DISP_HOR_RES;
    disp_drv.ver_res = MY_DISP_VER_RES;
    disp_drv.flush_cb = disp_flush_request;
    disp_drv.draw_buf = &draw_buf_dsc_4;
    disp_drv.direct_mode = 1;
static void disp_flush_request(lv_disp_drv_t *disp_drv, const lv_area_t *area, lv_color_t *color_p) {
        if (!mRequest) {
                mDispDrv = disp_drv;
                mArea = area;
                mColorP = color_p;
                mRequest = true;
static void disp_flush(lv_disp_drv_t *disp_drv, const lv_area_t *area, lv_color_t *color_p) {
        int32_t x, y;
        for (y = area->y1; y <= area->y2; y++) {
                for (x = area->x1; x <= area->x2; x++) {
                        alLtdcFb[x + y * disp_drv->hor_res] = color_p->full;
void lv_port_disp_flush_ISR(void) {
        if (mRequest) {
                disp_flush(mDispDrv, mArea, mColorP);
                mRequest = false;

… i know, the above code has some race conditions. but i wonder, if this could be a way to get flicker free (vblank synced) direct_mode.