Problem with MIPI DSI on STM32MP157C CAD package?

[OL'He.1]

Hello!

We have a problem of LCD image quality with our custom STM32MP157C-based board: we see artefacts, wrong colours, ghosts of previous images and rainbows. The LCD is a Winstar WF40CTYAQM, linked with MIPI DSI, much like the Orise Tech LCD of the DK2.

What strange is: when on the DK2, the same LCD shows good images. On our custom board, it shows those artefacts and ghosts in particular. The artefacts happen even if we use the Video Pattern Generator in BER mode, grey stripes.

There is one difference between the boards: the DK2 has an STM32MP157CAC SoC, while our custom board has a STM32MP157CAD SoC, i.e. the package is different and has less pins.

Is there a DSI limitation on the CAD package?

Tests were done in 4 steps:

1. Show the ST splash screen, with the butterfly and Tux, during 10 minutes. Blue and green are too dark on the CAD package. "IMG_20200603_095341.jpg" <https://lh3.googleusercontent.com/u/0/d/1q3H5w46XKUTFXd5PLQ0CjDe-h4cuGvsA=w1920-h1006-iv1>
2. Show the SMPTE pattern: artefacts with the CAD package, we see the butterfly and Tux as ghosts through the pattern, colours are dull. "IMG_20200603_100759.jpg" <https://lh3.googleusercontent.com/u/0/d/1q8uZWfzRhZakB02Xp3BtDYwH5w1DWqaO=w1920-h1006-iv1>
3. Show the colour bars generated by the VPG: no artefact. "IMG_20200603_101357.jpg"<https://lh3.googleusercontent.com/u/0/d/1xVjw50ONyZOncxTFPP980MG1a6GPG3R5=w1920-h1006-iv1>
4. Show the grey bars generated by the VPG: the artefacts are back. "IMG_20200603_152709.jpg" <https://lh3.googleusercontent.com/u/0/d/1HybonO2isdtK8L4DbumIlCkx-TfcCLFO=w1920-h1006-iv1>

[OL'He.1]

PatrickF, we do not use "MIPI_DSI_MODE_VIDEO_BURST" yet, only "MIPI_DSI_MODE_VIDEO".

I have just seen the driver of the DK2's Orise Tech otm8009a LCD does use "MIPI_DSI_MODE_VIDEO_BURST" and "MIPI_DSI_MODE_LPM".

I'll try with those flags and share the device tree in a private message.

Thanks for the hint!

[OL'He.1]

I have added the "MIPI_DSI_MODE_VIDEO_BURST" flag to the current "MIPI_DSI_MODE_VIDEO". No difference, we have the same artefacts.

I have added "MIPI_DSI_MODE_LPM" as well, then I see nothing on the screen.

(My tests were a bit naive, just adding the flags in the driver, no time to check they were taken into account.)

[MM..1]

Hi, your images show as over pattern generator is alpha blended next layer. As in LTDC . Too i mean multicore as this MP can connect DSI to Cortex4 but too to Cortex7 and here can start trouble as this. Good is message from PatrickF to test show any change on display in sw when pattern is active.

Too try stop LTDC before start HAL_DSI_PatternGeneratorStart(&hdsi, 0, 1);

[OL'He.1]

Hello MM.1.

Thanks for your interest.

At first sight, it looks like an alpha blending indeed. However, I think I have ruled out alpha blending because:

1. The ghost effect clearly depends on the time during which the previous ghost image is displayed on the LCD. If displayed 2 seconds, no ghost, if displayed 5 minutes, strong ghosts. An alpha blending could not behave like that, AFAIK.
2. We see the ghosts even when displaying the pattern from the hardware Video Pattern Generator, which completely shortcut the LTDC.

BTW, we do not use the Cortex-M4 at all. It is neither programmed nor started.

[OL'He.1]

I have added the "MIPI_DSI_MODE_VIDEO_BURST" flag, no difference, our artefacts are still there. I have tried to add "MIPI_DSI_MODE_LPM" as well, like in the Orise Tech OTM8009a driver, but then we have nothing on screen. I suppose it is not supported.

We have privately shared the DTB, DTS and LCD driver.

[MM..1]

When is realy time depend , your trouble is LCD Vcc and Vcom stability HW or SW setup Vcom usw. in config initialisation.

[OL'He.1]

PatrickF, you wrote: "I assume you checked the 2.8-3.3V supply of the LCD as well as keeping DSI rate below 550Mbps (If you use it in dual data lanes)."

We have already checked the VCC and IOVCC supplies of our screen, they are at 2.8V and 1.8V respectively, and free of noise. We'll check it again.

I was not aware of a limitation of the DSI rate to 550 Mbps. Do you mean 550 Mbps per lane, or for the full DSI? Could you point me to the documentation that explains this limitation?

I have tried to lower the frame rate to 50 Hz, which brings the data rate to the same value as the Orise Tech LCD of the DK2. It does not change anything to the artefacts I see, with or without "MIPI_DSI_MODE_VIDEO_BURST".

Thanks!

[MM..1]

You use low limit VCI 2.8V , try go up or use other power source

[OL'He.1]

Hello MM.1 and PatrickF.

Thanks for your suggestion. We had already raised the VCI limit to 3.0V and 3.3V without improvements on our artefacts, but we had done it using the STPMIC1 on our custom board.

Yesterday, we have physically connected both the VCI and the IOVCC supplies of our LCD display to a lab power supply, and all LCD artefacts disappeared. 🙂

It was not a question of raising the voltage, the lab supply voltages were the same. When we reconnected the LCD's VCI and IOVCC supplies to the outputs of our STPMIC's BUCK4 and BUCK3 respectively, instead of LDO1 and LDO6, the LCD display showed almost perfect images, without any artefact.

We still need to understand this LDO limitation, but we have progressed a lot and know the problem is in the LCD supplies.

Thanks for your support!