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CDyer.1
CDyer.1
Associate III
December 7, 2020
Solved

Benefit of using LTDC if TFT LCD already has integrated display controller?

  • December 7, 2020
  • 5 replies
  • 2069 views

Hi, I'm looking to add a TFT LCD to a project and I've been looking at iltek controlled displays as a solution specifically the ili9488. Displays that integrate various driver ICs such as ili9486, ili9341, ili9488 are very popular on the arduino platform because it only requires simple SPI control by the master microcontroller. Looking at various examples using these types of displays on the stm32 platform LTDC is always used. My question is why use LTDC when the TFT screen already has a full integrated controller? What are the benefits to using LTDC to help drive the screen? Why not use simple SPI control like an Arduino? I'm sure the benefits are there but reading AN4861 it's not clear what the benefits are if an IC driver is already embedded into the display. Any help in clearing up this confusion would be great.

    This topic has been closed for replies.
    Best answer by waclawek.jan
    1. Maybe, there are many factors. thus no simple answer. If the framebuffer (GRAM) won't fit into the STM32 internal memory, one has to attach external memory, ant that's extra cost both for the memory and the mcu pins and PCB area. Many applications do just well with slower LCD update.
    2. Yes, that's its purpose.
    3. Increase no. You either have the memory in the controller, or entirely in the mcu if you use LTDC.
    4. There's a ceiling at which LCD refresh depletes the memory/bus bandwidth. Note, that if you want to actually change the displayed content, you have to write into the same memory from which LTDC continuously reads because of the display refresh. So, it may quite well display without CPU intervention a still picture, but you may not be able to change it without visible effects. Read AN4861.

    JW

    5 replies

    waclawek.jan
    waclawek.jan
    Super User
    December 7, 2020

    Display update through SPI may be too slow for high-FPS animation.

    Controller-less LCDs are also cheaper.

    JW

    CDyer.1
    CDyer.1Author
    Associate III
    December 7, 2020

    Thanks for the quick response, I just have a handful of follow up questions.

    1. ​If the SPI is too slow for high-FPS then you could alternatively use DBI type A or B (Motorola 6800 or Intel 8080) with FSMC to increase the speed of the LCD update. Would there be a benefit to using LTDC in this configuration?

    2. Can LTDC be used to completely drive a "dumb" TFT (i.e. act as an alternative to an on-chip display driver that has the GRAM/frame buffer, display controller commands etc.)

    3. If the GRAM of a display driver (ili9488 in my original question) isn't large enough for graphical requirements, can LTDC be used to increase GRAM by adding external SDRAM?

    4. Is there any performance hit at all in using LTDC? The application note states that the LTDC can be used with no CPU intervention.

    ​

    Any advice or help with any of the above questions would be appreciated. I'm just doing preliminary research at the moment. It seems strange that if the LTDC does everything you could possibly want as a display controller why one would even bother using TFTs with dedicated iltek controlled.

    waclawek.jan
    waclawek.janBest answer
    Super User
    December 7, 2020
    1. Maybe, there are many factors. thus no simple answer. If the framebuffer (GRAM) won't fit into the STM32 internal memory, one has to attach external memory, ant that's extra cost both for the memory and the mcu pins and PCB area. Many applications do just well with slower LCD update.
    2. Yes, that's its purpose.
    3. Increase no. You either have the memory in the controller, or entirely in the mcu if you use LTDC.
    4. There's a ceiling at which LCD refresh depletes the memory/bus bandwidth. Note, that if you want to actually change the displayed content, you have to write into the same memory from which LTDC continuously reads because of the display refresh. So, it may quite well display without CPU intervention a still picture, but you may not be able to change it without visible effects. Read AN4861.

    JW

      CDyer.1
      CDyer.1Author
      Associate III
      December 7, 2020

      You've been extremely helpful, thank you. Just as one further question if I may; aside from AN4861 are there any further ANs that you would recommend looking at for LCDs, graphics, art chrom etc.? I find STs site difficult to find exactly what may be useful. Again, thank you for taking the time to answer my above questions.

        waclawek.jan
        waclawek.jan
        Super User
        December 8, 2020

        I don't know of any, but I am not that interested either, so maybe just keep searching.

        There are some videos around, but videos I see as cheap and inferior substitute for technical literature.

        JW

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