ST7796 LCD Controller driver

Overview

The ST7796S is a single-chip controller/driver for 262K-color, graphic type TFT-LCD. It consists of 960 source lines and 480 gate lines driving circuits. The ST7796S is capable of connecting directly to an external microprocessor, and accepts 8-bit/9-bit/16-bit/18-bit parallel interface, SPI, and the ST7796S also provides MIPI interface. Display data can be stored in the on-chip display data RAM of 320x480x18 bits. It can perform display data RAM read-/write-operation with no external clock to minimize power consumption. In addition, because of the integrated power supply circuit necessary to drive liquid crystal; it is possible to make a display system with fewest components.

The ST7796 LCD controller driver is a platform-agnostic driver, based on the generic MIPI driver. It implements display initialization, supports display rotation and implements the display flush callback. The user needs to implement only two platform-specific functions to send a command or pixel data to the controller via SPI or parallel bus. Typically these are implemented by calling the appropriate SDK library functions on the given platform.

Prerequisites

There are no prerequisites.

Configuring the driver

Enable the ST7796 driver support in lv_conf.h, by cmake compiler define or by KConfig

#define LV_USE_ST7796  1

Usage

You need to implement two platform-dependent functions:

/* Send short command to the LCD. This function shall wait until the transaction finishes. */
int32_t my_lcd_send_cmd(lv_display_t *disp, const uint8_t *cmd, size_t cmd_size, const uint8_t *param, size_t param_size)
{
        ...
}

/* Send large array of pixel data to the LCD. If necessary, this function has to do the byte-swapping. This function can do the transfer in the background. */
int32_t my_lcd_send_color(lv_display_t *disp, const uint8_t *cmd, size_t cmd_size, uint8_t *param, size_t param_size)
{
        ...
}

To create an ST7796-based display use the function

/**
 * Create an LCD display with ST7796 driver
 * @param hor_res       horizontal resolution
 * @param ver_res       vertical resolution
 * @param flags         default configuration settings (mirror, RGB ordering, etc.)
 * @param send_cmd      platform-dependent function to send a command to the LCD controller (usually uses polling transfer)
 * @param send_color    platform-dependent function to send pixel data to the LCD controller (usually uses DMA transfer: must implement a 'ready' callback)
 * @return              pointer to the created display
 */
lv_display_t * lv_st7796_create(uint32_t hor_res, uint32_t ver_res, lv_lcd_flag_t flags,
                                                                lv_st7796_send_cmd_cb_t send_cmd_cb, lv_st7796_send_color_cb_t send_color_cb);

For additional details and a working example see the generic MIPI driver documentation.

Note

You can find a step-by-step guide and the actual implementation of the callbacks on an STM32F746 using STM32CubeIDE and the ST HAL libraries here: Step-by-step Guide: How to use the LVGL v9 LCD drivers with STM32 devices