Image displayed using Touchgfx for LCD RGB666 is noisy with green pixels when using OSPI RAM

[duchuylee]

Hi all,
As title, I have a custom board using STM32H723ZGT6, RGB666 LCD (480 * 480) with LTDC interface, the framebuffer used external Octo SPI PSRAM APS12808L-3OBM-BA of APMemory. I can display image using TouchGFX, but the image always noisy with some green pixels ramdom, I try with others image, but it still appear (see image)

Below is my configuration for system clock, OSPI, LTDC
APS12808LTDCRGB LCDSystem clock

I have performance test random write then read for OSPI PSRAM with log:

Size of FrameBuffer = 460800

Writing Sequence of 460800 bytes OK with 0.1870s
Read 460800 bytes in 0.0690s. Total error = 0, Error rate 0.00 %

===== 1. Write Read random 1 block PSRAM OK =====
Size of FrameBuffer = 460800
Writing Sequence of 460800 bytes OK with 0.1880s
Read 460800 bytes in 0.0690s. Total error = 0, Error rate 0.00 %
===== 2. Write Read random 1 block PSRAM OK =====

============================================

Writing Sequence of 16777216 bytes OK with 9.2790s
Read 16777216 bytes in 2.3420s. Total error = 0, Error rate 0.00 %
===== Write Read random full memory of PSRAM OK =====

Does anyone have a solution for this problem? Please help me 
Thank you so much!

[KDJEM.1 · Accepted solution]

Hello @duchuylee ,

Your issue looks like this that have been talked in the forum: Solved: Problem when I use external SRAM in touchgfx. Wron... - STMicroelectronics Community.

For that could you please take a look at this thread and let me know if the issue is solved or not?

I hope this help you.

Kaouthar

[duchuylee]

Hello @KDJEM.1 ,
Thank you for reply,
That seems like exactly what I encountered. It will take time for me to adjust the hardware according to the problem that guy over there solved. 

I will update if there is any new information
Thank you!

[duchuylee]

Hello @KDJEM.1 
The good news

I tried as you recommended in the link above: adding a 120-ohm resistor to the CLK and RWDS pins of the PSRAM. The green pixels are no longer appearing (although there are still some white pixels in a line, but this problem is really solved) (see image below). Once again, I am very grateful for your help! :smiling_face_with_smiling_eyes:

P/S: I have a question. Can you help me with it? I referred to the schematic of the STM32H735-DK board provided by ST, and they did not add any resistors to the OSPI pins (or later updates did not mention the fix). However, it seems that this green pixel error does not appear on their LCD. Why might that be?

[KDJEM.1]

Hi @duchuylee ,

Glad to know that the issue is solved.

The STM32H735-DK board Altium_Designer project is designed with an optimized line length and thickness to connect the external memory.

It is recommended to use the same STM32H735-DK board  Altium design or you can base on the signal integrity analysis to choose the optimal resistor value if it is necessary.

Thank you.

Kaouthar

[duchuylee]

Hi @KDJEM.1 ,
I got it, thank you for info