Working with STM32 MCUs, especially when building a board that uses LTDC, external SDRAM, and a TFT LCD panel, often leads to debugging sessions that look very similar across different projects. Even though the peripherals are well-documented, the interaction between SDRAM timing, LCD clock configuration, GPIO drive strength, and TouchGFX task scheduling can create issues that are not immediately obvious.
This article summarizes several recurring problem areas engineers frequently encounter when bringing up a custom STM32-based display system. The goal is to provide a technical overview—not product promotion—and link to a few community discussions that illustrate these cases in practice.
1. SDRAM Initialization and LTDC Framebuffer Stability
SDRAM configuration is a common source of problems when the framebuffer is stored externally. LTDC relies on continuous access to the memory bus, and even small timing inaccuracies can result in:
- Rows of corrupted pixels
- Flickering during the initial refresh
- Black screens with no visible activity
- LTDC underflow interrupts
These symptoms often appear when SDRAM timing parameters (RAS, RC, RP, RCD, refresh rate) do not match the memory device's specification. Incorrect FMC clock settings can also cause unstable transactions.
A typical example of this situation is discussed here:
SDRAM and TFT LCD issue
https://community.st.com/t5/stm32-mcus-touchgfx-and-gui/stm32f7-discovery-sdram-and-tft-lcd/td-p/419734
In many cases, correcting the refresh counter or adjusting CAS latency fixes the issue.
2. Display Timing and Panel Compatibility
Not all TFT displays behave identically—even panels with the same nominal resolution and interface can require slightly different synchronization timings. Engineers often assume LTDC "typical" values are valid, but real-world panels may need adjustments for:
- Horizontal sync width
- Vertical sync width
- Back porch timing
- Front porch timing
- Pixel clock frequency
If these are off by even a small margin, the display may light up but show no content, shift the image, or simply stay black.
This is well demonstrated in the following discussion on panel compatibility:
LTDC display compatibility question
https://community.st.com/t5/stm32-mcus-touchgfx-and-gui/ltdc-display-compatibility-question/td-p/114201
Engineers often need to scope the pixel clock and HSYNC/VSYNC signals to confirm the panel requirements.
3. Initial Screen Flicker and Black Screen During Startup
A common symptom when bringing up a custom board is that the display shows a short flicker when the system first boots, and then transitions into a black screen. This can happen before TouchGFX starts drawing UI elements, typically due to:
- The framebuffer not being initialized yet
- Layer settings being applied before the clock is fully ready
- SDRAM not stable during the first memory writes
- The LTDC being enabled before a valid buffer address is set
Here is a community example of this exact behavior:
LCD showing black screen with initial flicker
https://community.st.com/t5/stm32-mcus-touchgfx-and-gui/lcd-tft-display-getting-black-screen-w-initial-screen-flicker/td-p/103804
Ensuring that LTDC is enabled only after SDRAM and clock trees are fully configured usually prevents this.
4. TouchGFX TaskEntry Not Running or Stalling in OS Wrappers
On custom boards, it's also common to encounter an issue where TouchGFX never leaves the OS wrapper layer. This often indicates that the framework is waiting for a system event such as:
- VSync interrupts not firing
- The HAL tick not incrementing
- The display refresh callback not being triggered
- FreeRTOS priorities causing the task to starve
A useful reference for this scenario:
TouchGFX TaskEntry never leaving OSWrappers
https://community.st.com/t5/stm32-mcus-touchgfx-and-gui/touchgfx-custom-board-touchgfx-taskentry-never-leaves-oswrappers/td-p/105903
This usually requires checking interrupt priorities (LTDC, DMA2D, SysTick), ensuring the RTOS tick frequency is correct, and verifying that TouchGFXHAL::initialize() is called at the right point.
Conclusion
Many problems that appear during STM32 display bring-up share the same root causes: timing mismatches, incomplete SDRAM configuration, missing synchronization, or incorrect RTOS task scheduling. Reviewing these areas systematically often leads to a quicker resolution.
The community posts linked above illustrate realistic debugging journeys. They serve as useful references for engineers working with LTDC, external SDRAM, and TouchGFX on custom hardware. While each board design is different, the failure patterns tend to be very similar, and understanding these typical behaviors can significantly shorten development time.
If you work with custom STM32 graphics hardware, reviewing these cases early in the design and bring-up phase is often a practical investment.
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