STM32 LTDC Not Displaying Image

Question

I am currently using the STM32N657 on the NUCLEO-N657X0-Q board to drive an 800×480 pixel display with 24-bit color.

The display that I am using is the NHD-7.0-800480EF-ASXV#-T. I’m using the built-in LTDC controller on the chip. I do know that the controller is working because the entire screen is black, the screen would be showing the backlight if it wasn’t. The problem is that I am unable to display anything at all when writing to the screen buffer.

Here are the configuration parameters that I am using based on the data sheet:

Here are the layer settings that I am using:

Here is the clock configuration for the LTDC:

Here is the code that I am running:

#include "main_application.h"

__attribute__((section(".screen_buffer")))
uint8_t screen_buffer[FRAME_BUFFER_SIZE];

void main_app(hal_pointers_t hal_pointers){

    for (uint32_t i = 0; i < FRAME_BUFFER_SIZE; i++){
        screen_buffer[i] = 0;
    }


    HAL_LTDC_SetAddress(hal_pointers.hltdc, (uint32_t) screen_buffer, 0);
    //HAL_LTDC_ReloadLayer(hal_pointers.hltdc, LTDC_RELOAD_IMMEDIATE, 0);
    //HAL_LTDC_SetOutputDisplay(hal_pointers.hltdc, LTDC_OUT_RGB);

    //Make a square
    for (uint32_t i = 0; i < FRAME_BUFFER_SIZE/3; i++){
        //for (uint32_t j = 0; j < 20; j++){
            //screen_buffer[(i*3) + (j*SCREEN_WIDTH*3)] = 255;
            screen_buffer[i*3] = 0x55;
        //}
    }

    while(1){
    }

}

Here is what I’ve observed:

• I have run the code in debug mode and can see data being written at the screen buffer location

• I’ve hooked up a logic analyzer to the red data pins, HSYNC, VSYNC, DE, and DCLK (below is a snapshot). When the red data pins are not active HSYNC seems to be acting somewhat normally. When the data pins are active, HSYNC does not act normal at all. It pulses almost randomly. Please note that the logic analyzer is a cheap one from Amazon, so it may not even be reading this data correctly.

Here's what I’ve tried:

• I’ve tried using the following HAL functions:

  • HAL_LTDC_ReloadLayer(hal_pointers.hltdc, LTDC_RELOAD_IMMEDIATE, 0),
  • HAL_LTDC_SetOutputDisplay(hal_pointers.hltdc, LTDC_OUT_RGB).

  No change.

• I’ve tried slightly adjusting the pulse width settings for both HSYNC and VSYNC. No change.

Let me know if I’ve forgotten to provide any information.

Answer 1

To anyone who stumbles upon this post. I figured out what my issue was. I was using AXISRAM to store my screen's buffer memory, which by itself is fine. What I didn't realize is that the LTDC peripheral does not initially have access to this memory. In order for the display data to get to the LTDC peripheral, you need to use the RIMU.

In order to enable the RIMU you need to enable RIF in the hardware configuration. Make sure to enable it whether you are running your main code in application or in FSBL.

Then go to the RIF tab on the top, go to "Domains (RIMU)" tab on the side, and then give LTDC_L1 (Layer 1, or whichever Layer(s) you are using) privilege and secure status. The way that I understand it, you are telling the microcontroller that the LTDC can have full access to system resources (privilege) and is a trusted peripheral (secure).

Once you do that, you need to add these lines of code.

RIMC_MasterConfig_t RIMC_master = {0};
RIMC_master.MasterCID = RIF_CID_1;
RIMC_master.SecPriv = RIF_ATTRIBUTE_SEC | RIF_ATTRIBUTE_PRIV;
HAL_RIF_RIMC_ConfigMasterAttributes(RIF_MASTER_INDEX_LTDC1 , &RIMC_master);

HAL_RIF_RISC_SetSlaveSecureAttributes(RIF_RISC_PERIPH_INDEX_LTDCL1 , RIF_ATTRIBUTE_SEC | RIF_ATTRIBUTE_PRIV);

These lines of code basically do what the hardware configurator is supposed to do when giving the LTDC layers security status and privilege status. I'm not really sure why the hardware configurator doesn't write these lines of code for you, unless it's not being written somewhere else that isn't main.c. You will want to write these lines of code before you set the memory address or addresses for any of your LTDC layers.

As soon as I made these changes, I immediately saw color on my screen.