LVDS signal flickers when DFS is enabled

Technical Support
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1

Hello again,

Now I updated Apalis T30 to BSP2.2b4 and have the Problem, that LVDS Signal flickers. When I disable DFS with Toradex Taskmanager Signal is ok. What could that be? The flickering is only, when CPU is in low power.

I made a small video for you.

video lvds flickering

2

Dear @JanP

The flickering is caused by temporary insufficient memory bandwidth.
DFS adjusts the memory bandwith based on average bandwidth requirements, therefore short bandwidth peaks cannot be covered in time.
We saw such issues only at higher resolutions than what your video shows. However, this might be related to the exact system configuration, or running background tasks.

There are two ways to avoid the flickering

  1. Reduce the bandwidth
  2. Increase the minimum allowed memory frequency

Reduce the Bandwidth

  • By default, the Apalis T30 drives two displays - LCD(LVDS) and CRT(VGA). If you don’t need VGA, you can disable it by setting BootupStyle=dword:0x01. This will reduce the display memory bandwidth by 50%
    Refer to the developer page article Display Driver Registry Settings for details.
  • If you don’t need high color pictures, you can limit the display to 16-bit colors (instead of 32), by setting MainPanelBpp=dword:0x10. This will also cut the display bandwidth in half. Again, for details refer to the developer page article Display Driver Registry Settings.

Increase the minimum allowed memory frequency

Try to force the EMC frequency to go no lower than 400MHz.

  • You can achieve this directly in the Task manager: Select EMC from the dropdown box, edit the bottom left textbox to 400, then press the [Set] button.
  • There’s a registry setting DfsEMCLowCornerKHz=400000 which has the same effect. For details, refer to the developer page Resource Manager Registry Settings.
  • There’s also a way to change limits dynamically in your application code. Let me know if you need details about this.

Regards, Andy