I have multiple Dell P2425H monitors in my lab, all work except one that won't allow a CM5 to boot in my custom device or a CM5IO.
After investigation, it was found to be putting out +5.25V on it's DDC lines. With a +5.1V supply to the CM5, it will not boot. If I boost that +5.1V to +5.3V to the pi supply it will boot. I believe this to be a backdrive problem.
I have replicated this behavior minus the monitor by applying +5.3V between just a DDC pin and ground on the HDMI0 connector and it also keeps a CM5IO from booting.
While that particular Dell monitor is running high compared to every other monitor I've tested, it is my *understanding* that is still with the +5.3V max allowed by the HDMI spec. (I don't have access to the HDMI spec,.. if some else could confirm that). If this is the case, it appears to be a bug in the CM5.
..Jack
After investigation, it was found to be putting out +5.25V on it's DDC lines. With a +5.1V supply to the CM5, it will not boot. If I boost that +5.1V to +5.3V to the pi supply it will boot. I believe this to be a backdrive problem.
I have replicated this behavior minus the monitor by applying +5.3V between just a DDC pin and ground on the HDMI0 connector and it also keeps a CM5IO from booting.
While that particular Dell monitor is running high compared to every other monitor I've tested, it is my *understanding* that is still with the +5.3V max allowed by the HDMI spec. (I don't have access to the HDMI spec,.. if some else could confirm that). If this is the case, it appears to be a bug in the CM5.
..Jack
Statistics: Posted by sel_lab — Mon Feb 02, 2026 7:21 pm — Replies 2 — Views 38