Corrupted data waveform in specific LTDC data lines

[Slh]

I am going to set up a RGB666 LCD with STN32F779 and Frame buffer in SDRAM. The problem I'm facing is that there are a series of parallel dark lines in the image.

I first tested the SDRAM and the reading and writing is done correctly. Then I realized that only in the R5, G5, B5 data lines of LTDC the signal form is different from the other lines and when I manually write '0' in which these lines, (for example 0b11011100) the image is displayed clear and without problems. Here is an image which there is '1' in R5/G5/B5 on some areas:

 

 

The data waveform for normal data lines like R/G/B-2, R/G/B-3, R/G/B4, ... is as follows:

 

And the corrupted data waveform in the R5, G5, B5 lines is as follows:

 

As seen in the photo, there is a 6.25MHz signal inside the main 31.05KHz signal which causes parallel lines in the image and I don't know the reason. The zoomed waveform is as follows:

I also moved the Frame buffer address from SDRAM to internal flash for testing without any change in SDRAM and LTDC settings, but this time it happened for R3, G3, B3!

Thanks for any help.

[KDJEM.1]

Hello @Slh,

 

What is the LTDC pixel clock frequency?

Could you please try to decrease the LTDC pixel clock and check if the issue is solved or not.

 

Thank you.

Kaouthar

 

[Slh]

Hello @KDJEM.1 ,

Thank you for your reply. I reduced the clock speed, but it had no effect. 

I made the second board and the problem was solved. So I think there is a hardware or controller issue that I will look into at the appropriate time.

[KDJEM.1]

Hello @Slh;

 

Thank you for updating post.

I think that it is hardware issue. May this application note AN4661 "Getting started with STM32F7 Series MCU hardware development" can help you for hardware checking.

Glad to know that the issue is solved.

Thank you for your contribution.

 

Kaouthar