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Best way to connect a third party display

Wed Dec 11, 2019 3:53 am

Hi all,

We're in the final stages of a project and connecting a display is giving us a bit of grief.

Our original prototype used a Sharp LS055R1SX04 as the display which was driven by a generic HDMI to 50 Pin MIPI LCD driver board that came with it. We were using the Raspberry Pi Zero W as our test platform. This worked ok for a proof of concept but could not be put into production due to RF emissions from the LCD driver board.

We are now developing a version 2 which incorporates a CM3+ onto a custom carrier board and we're in the validating stage to find out how we're going to interface everything. From researching, it seems HDMI is the simplest way to go, but making the circuit to drive the panel doesn't seem all that straight forward and I'm wondering if there is a better way.

Has anyone had experience in using a 3rd party display like the Sharp screen with the Compute Module?
If so, what was your method to interface them?
Or is it not a simple thing to implement and I should search for another plan for the display?

Thank you in advance!

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Re: Best way to connect a third party display

Wed Dec 11, 2019 11:32 am

The pannel you use is very high resolution, I'm not sure it is possible to drive it through MIPI DSI directly as the pannel requires 2 mipix4 and the CM3 has only 1x2 mipi and 1x4 mipi.

You would need to put a scaler in the middle and drive it with a lower resolution or change the pannel for a lower resolution. In any case not en easy solution.
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Re: Best way to connect a third party display

Wed Dec 11, 2019 12:24 pm ... 0#p1361021

bridging HDMI to LVDS. There are HDMI to MIPI bridges available to the market (as well as DPI to MIPI) .. look at your 'generic driver board'.

Sure, you can use MIPI DSI as well; there are posts on the forum where folks got it working. But keep in mind the CM3's GPU has some limitations in the supported display size (total no of pixels).

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Re: Best way to connect a third party display

Wed Dec 11, 2019 2:00 pm

So you can add any DSI display to the vc4-kms-v3d stack by writing a suitable Linux kernel panel driver. It requires a modicum of understanding of DSI and how KMS hooks together, and a complete datasheet for the panel, but ... lay-panels details most of what is required.

Switching to vc4-kms-v3d does use OpenGL for rendering the X desktop. On CM3 this is limited to resolutions less than 2048x2048 as that is the biggest output buffer the GPU can write to. If using the console or adding layers directly with DRM/KMS, then that isn't a restriction. You should also be able to run the DRM/KMS side without the OpenGL acceleration. It all depends on what your use case is.

I can't find a full datasheet for the panel as to how "MIPI (2 ch, 4 data lanes)" should be interpreted. If it requires 2 4-lane DSI interfaces then you are out of luck as the Pi SoC only has a 2-lane and a 4-lane interface, and no way to combine the two interfaces. If you can run it at a lower rate, then you may be able to interface it directly. The datasheet, if you have one, may give more detail.
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