Advanced users • HDMI pixel clock derivation - Raspberry Pi 5

Hi,

I'm having some issues with EMC (radiated emissions) related to the HDMI output of a Raspberry Pi 5, and I want to try different modelines/pixel clocks, as that seems to have a fairly significant effect on the peaks.

I'm wondering how the pixel clock is derived? Is there some fractional-n PLL somewhere that it's derived from? What are "valid values" for the pixel clock in a modeline? Where can I see the coefficients set for that PLL?

I can't change screens, I can't change the pi, and I can't do any more with shielding and grounding, so I'd love to talk with folks about the software aspects of the HDMI signal generation in particular.

I'm able to use xrandr to add modes, and cvt/gtf to create arbitrary modelines, and vcgencmd to measure the pixel clock.

I guess various questions are:
How 'arbitrary' can those modelines actually be?

Can I just build modelines that set the pixel clock to whatever I like (within limits?) and just sweep through them at the test house and pick the "quietest" one (from a radiated emissions standpoint)??

Thanks!

Statistics: Posted by robot_army — Wed Apr 17, 2024 1:38 am — Replies 0 — Views 5

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