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Corrupted Data in MIPI-DSI LCD interface

MGHTetaPower , 08-04-2023, 09:22 AM
Hello guys,
I have some issues with the MIPI-DSI graphical Interface.
In my case, we have a graphical Circuit with EXT SDRAM as FrameBuffer and MIPI-DSI as LCD Interface but the Data gets Corrupted. It's On Two Layer Board With STM32F7 On Top and EXT SDRAM On the Bottom side beneath the Microcontroller and MIPI DSI Lanes as close as possible to the Micro Pins (Around 23 mm Away). All SDRAM Signals Are Between 18 to 37 mm Long Traces, and Delays On ACC, BL0, and BL1 Groups are less than 100ps. What Will be the reasons for Corrupted Data, Data Gets to the LCD But It Gets Corruption in Nearby pixels.
How Much Length match will be fine for EXT SDRAM Signals?
using termination could help?
Using 4 Layer PCB and Impedance Matching Could Solve the Problem?

qdrives , 08-04-2023, 01:10 PM
What kind of corruption?
Is the timing correct?
How is the decoupling?

I have re-witten a display driver 2 times in order to solve some glitches on a STM32H7. Nothing helped.
It also had problems with SPI. The SPI communication only gave problems when SDRAM was used, either through the LCD or memcpy.

Layout was not good on that board, but I only discovered that when I was tasked to fix the EMC problem of that board too.
I.e. a Gnd via was about 10mm from some pins (combining 3 Gnd pins).
Board was not my design. A newer version silicon was available that had multiple fixes.
MGHTetaPower, 08-05-2023, 09:14 AM
It has to show a roller with "Option1, Option2" on it but as you can see the text is corrupted.Actually, I have a previous version that works fine but when I rotate the MCU and reroute the RAM and DSI, It gets this Distortion which means the timing is correct.decoupling Caps are very close to the RAM and MCU Pins.
qdrives , 08-05-2023, 03:32 PM
Is the image stable? Stable as in no flickering or (constantly) changing pixels.

Two layer board? That means you do not have a Gnd return plane. However, Your Altium screenshot show [1] L1 and [4] L4, which would indicate a 4 layer board. Looking at the layout I would say at least 4 layers too. Perhaps you mean to signal layers.

MGHTetaPower, 08-06-2023, 07:54 AM
the Image is stable without any flickering. Yes, it's a two-layer PCB. first implemented in 4 layers but finally changed to a two-layer PCB.
qdrives , 08-07-2023, 01:45 PM
If the image is stable, how do you know it is corrupted?
Have you also tried a special test image?
MGHTetaPower , 08-08-2023, 09:47 AM
Problem Solved by Increasing the Draw Buffer to some extent. but more testing indicates there is a problem with Ext RAM Data.
qdrives , 08-08-2023, 02:01 PM
Sounds a bit familiar.
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