Storing graphic primitives

Graphic primitives are basic elements (such as images or fonts) that can be combined to build the framebuffer content that is displayed.

Static data must be placed in a nonvolatile memory. When the amount of data to store is relatively low, the internal flash memory can be used. Otherwise, graphical contents must be placed in external memories.

The STM32 MCUs offer parallel (FMC) or serial (Quad SPI, Octo SPI, XSPI) interface for external NOR Flash memories (see Table 3 STM32 MCUs embedding an LTDC and their available graphic portfolio).

To build the framebuffer content, the DMA2D can directly read graphic primitives from a parallel NOR Flash or a Quad-SPI Flash.

Refer to the document AN4861_General-information.html#id_62127__entry_wzv_bky_31c and AN4861_General-information.html#id_62127__entry_psc_cky_31c for more details on storing graphic content on Quad SPI, Octo SPI,and XSPI memory.

To add graphic primitives to a user project, these primitives must be converted to C or header files. Some specific tools can be used to generate C or *.h files.

The LCD image converter is a very customizable free tool used to convert images to C files and to generate the C file in the desired format. An example is described in Display an image from the internal flash.

Two important hardware interfaces must be carefully designed: the LTDC interface and the external memory interface (used for framebuffer) such as FMC_SDRAM or FMC_SRAM.

When an application is in idle state and displays only a screen saver, it is important to drive the STM32 product in Sleep mode to reduce the power consumption. In Sleep mode, all peripherals can be enabled (FMC-SDRAM and LTDC for instance) while the CPU is stopped.

External memories, such as SDRAM or Quad-SPI Flash, can be driven in low-power modes whenever it is needed in order to avoid the waste of power.

If the application is in low-power state but requires to display graphics, the LTDC can be kept active, and the SDRAM can be put in self-refresh mode (in order to save power). If the application is set in power-down mode, it saves even more power.

The display can also be disabled or put in low-power mode if it is not needed when running the application.

This section provides:

• some graphic implementation examples considering the resources requirements
• an example on how to create a basic graphical application
• a summary of STM32 reference boards that embeds LTDC and features an on-board LCD-TFT panel

Thanks to their integrated SRAM, the STM32 MCUs can be used for graphic applications, without the need of an external SDRAM/SRAM memory for framebuffer. Also, thanks to their high-size internal flash (up to 2 Mbytes), graphic primitives can be stored in these memories. The use of internal memories allows reduced number of used pins, easy PCB design and cost savings.

In order to use a single-chip MCU for a graphical application, the following hardware configuration can be used:

• internal flash up to 2 Mbytes, storing user application code and graphic primitives
• framebuffer located in the internal SRAM

  Depending on the internal SRAM size for each STM32 MCU, the user can interface with a corresponding display size and color depth as illustrated below:

  • STM32F7x7: use SRAM1 (368 Kbytes) to support resolutions 400 x 400 16 bpp (313 Kbytes) or 480 x 272 16 bpp (255 Kbytes)
  • STM32F7x6: use SRAM1 (240 Kbytes) to support 320 x 320 resolution with 16 bpp (200 Kbytes)
  • STM32F469/F479: use SRAM1 (160 Kbytes) to support 320 x 240 resolution with 16 bpp (154 Kbytes)
  • STM32F429/F439: use SRAM1 (112 Kbytes) to support 320 x 240 resolution with 8 bpp (75 Kbytes).
  • STM32 MCU packages: LQFP 100 or TFBGA100

Figure 32 Low-end graphic implementation example illustrates a graphic implementation example, with a single chip and no external memories used.

In order to interface with higher resolution displays, an external memory connected to the FMC is needed for framebuffer. An external Quad-SPI flash memory can be used to store graphic primitives.

For mid-end or high-end graphical applications, the following hardware configuration example can be used:

• external Quad-SPI flash memory with up to 256 Mbytes addressable memory-mapped, used to store graphic primitives
• external SDRAM 32-bit memory used for framebuffer
• STM32 MCU packages: UFBGA169, UFBGA176, LQFP 176, LQFP 208, TFBGA216, WLCSP168, and WLCSP180.

Figure 33 High-end graphic implementation example illustrates a graphic implementation example where two external memories are connected to an STM32 MCU, one for the framebuffer and the other for graphic primitives.

Table 24 Example of graphic implantations with STM32 in different hardware configurations summarizes an example of graphic implementations in different STM32 hardware configurations.