Problem when I use external SRAM in touchgfx. Wrong pixels.

[fernandogamax]

Hello.

I have made a custom project with STM32H735IGT. I use Touchgfx.

I am using external flash and external RAM. The problem is when I place the framebuffer in external RAM. Some green pixels appear as in image 1. When I place the framebuffer in internal RAM, the image is painted well.

I am using IS66WVH16M8DBLL-100B1LI RAM. I attached hardware configuration, the RAM is OCTOSPI2.

video of the problem and .ioc:

https://drive.google.com/drive/folders/1I96eqnh-5-xjDEKxfkxy-vVqeLvuVDY3?usp=sharing

Where is the problem?

[MM..1]

Dont check memory with debuger , write test code and repeat write read compare 10000x ... and show only error for example with led or gpio toggle or uart report etc.

[fernandogamax]

I have done the test of writing and reading in memory.

And turn on an LED when they are different.

I have some errors.

[fernandogamax · Accepted solution]

Hello,

it seems that it is a signal synchronization problem. I have changed the resistances of the CLK and DQS. For resistances of 120. It seems that the problems have been solved. We have ordered the new 4-layer PCB. When I do tests on the new board, I will write to you.

Thank you very much to all.

[fernandogamax]

Hello. I have designed the PCB with 4 layers, and I have placed the APS12808L-3OBM-BA memory. The board is functioning correctly. However, I need to add a 250-ohm resistor to the clock line, while the other lines have a 33-ohm resistor. Can anyone tell me why this is necessary? Could it be a configuration issue?

[FBL]

Hello @fernandogamax 

This is due to signal integrity considerations. The clock line typically has a higher frequency and therefore requires a higher impedance to maintain signal integrity. The 250-ohm resistor helps to reduce reflections and noise on the clock line, which can cause signal degradation and errors in data transmission. You can find more details in the datasheet of the memory. Some general guidelines about Interface signal layout are provided in AN4938.

[duchuylee]

Hello @fernandogamax 
I'm also having the same problem as you. I used STM32H723ZGT6, OSPI PSRAM APS12808L-3OBM, LCD RGB666 LTDC. The image always display noisy green pixels (see image).
My custom board is layout with 2 layers, RAM wires is same length, but I don't add any resistor on any wire (CLK, DQS, DQ0..7). Would adding a 120 ohm resistor on CLK solve the above problem? (Because of budget and time issues, I don't want have to re-layout the circuit board with 4 layers)
Thank you so much.