Hi!
We are having some EMC troubles with a display connected to our imx8 board. I’ve read here that its possible to change the drive strength by adding a device tree overlay to get better EMC resutls. However, I’m struggling on how to find what pins I need to configure and how to do it on the imx8 board.
Where can I find some documentation on what pins I need to configure? And where can I find something about how they are named and what they refer to?
Can you change this via the pinctrl_hdmi_ctrl in imx8-apalis-v1.1.dtsi « freescale « dts « boot « arm64 « arch - linux-toradex.git - Linux kernel for Apalis, Colibri and Verdin modules ?
Thanks for any help,
Pontus
Hi @pontusoim,
since you did not specifically mention it, I am not quite sure what display you are connecting. I guess it will be an HDMI display is that correct?
The iMX8 Chip is on many occasions a bit different to handle than its predecessors. There is a reference manual that you can download from NXP after you created an account.
Reference Manual iMX8
Now, regarding the drive strength. You could change the drive strength on “pinctrl_hdmi_ctrl”, but this will not have the effect that you expect. Changing it there only changes it for one specific pin. In this case, this will be the pin MIPI_CSI1_GPIO0_00. If I am understanding right though, that is not your goal.
To change the drive strength of the HDMI output in general. You can check Chapter 14.6 in the reference manual for some more details about the HDMI interface. For further insight, like necessary configurations to change the drive strength, you would need to contact NXP directly since this information is not available to us.
Did this help?
Feel free to reach out again.
Best Regards
Kevin
Hi!
Yes, I’m working with a HDMI interface.
Okay, but is it possible to change the drive strength of all the HDMI pins? Oe does that not work either?
Hmm, I’m looking thruogh the Reference Manual you linked to but these is no chapter 14.6…
Best regards,
Pontus
Hello pontusoim,
whats the reason for changing the levels? EMC ssues?
On the imx6 there was an why how to change the signal level on HDMI but on the IMX8 it is more complex since you now always have the firmware in the middle as well so we don’t have a documentation there yet.
But if you have EMC issues our experience shows that lowering the level does not help. The EMC issues are most likely a bad PCB Layout and Grounding.
Best Regards,
MAtthias
Hi Matthias,
Yes, it is EMC issues. I have read on multiple places that people have EMC issues with HDMI screens and that lowering the drive strength should help.
All the EMC issues we are having are located on frequencies that are multiples of the refresh rate of the screen. Also, when I disconnect the screen the issues go away so I definitly think it’s connected to the screen.
But if you’re saying changing the drive strenght won’t help it would be very helpful with some other suggestions on how to solve this.
Best regards,
Pontus
If you want I would like to look at your schematic. And the layout too. You can post this here or sent it to Support@toradex.com
Very often the Grounding concept does not include a separation from signal and chassis GND or no current return paths.
Best Regards
Matthias
