Low voltage on eDP AUX channel drivers

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Low voltage on eDP AUX channel drivers

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tonybryant
Contributor II

Problem: Unreliable EDID/setup negotiation with eDP panel on our iMX8MQ board.   (It works only 70% of the time.  When it works, everything works fine until the next power cycle)

Probable cause:   Low voltage on AUX channel output.   Approx 100mV Peak-to-peak with a 300mV centre (i.e. well out of spec).   Signal back from panel is the expected full 1V p-p.   

My Circuitry:  AUX lines connected to panel via simple series 0.1uF caps, with 100K pull up/down resistors.  Trace are 50 Ohm impedence, and are fairly short.   Panel is connected by a 3" microcoax cable via the standard IPEX connectors.

Have checked:   HDMI_AVDDIO = 1.8V   HDMI_AVDDCORE and HDMI_AVDDCLK = 0.9V

SW:  I'm using uboot 2018.07,  ATF v2.0,  Freescale Linux 4.14.98,  everything else is functional.

The question I have is:   Is there any power gating that needs to be enabled to get the full output voltage on the AUX pads?

Also,  which power pad supplies the power to the AUX pads?  HDMI_AVDDIO?

Thanks.

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igorpadykov
NXP Employee
NXP Employee

Hi Tony

> Is there any power gating that needs to be enabled to get the full output voltage on the AUX pads?

no

 

>which power pad supplies the power to the AUX pads?  HDMI_AVDDIO?

right.

In general one can check power-up sequence both i.MX8M and panel (using its datasheet).

One can check hardware connections using below link:

i.MX8 Display Port Interface 

and general hdmi requirements, like clocks in

i.MX8M Hardware Developer’s Guide

From software side issues were reported with some panels as described in

Table 14. Known issues and workarounds for i.MX 8M Family SoC

Release Notes document included in Linux 4.14.98_2.3 documentation

Documentation

So one can try different panels.

Best regards
igor
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1,265 Views
tonybryant
Contributor II

From the display port spec:

vgo.jpg

Are the 50 Ohm resistances (and obviously their connection to VbiasTX) supposed to be supplied externally to the SoC?

I've been through multiple vendors iMX8M schematics (including the EVK adaptor documented in the line), and haven't seen it on anyone's board, so I'm wondering if it just slipped through the cracks...

But if we do add these to our board, the signal levels on the cable become within spec.  (I am assuming VbiasTX is intended to be HDMI_AVDDIO, is this a fair assumption?)

Regards,

Tony.

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igorpadykov
NXP Employee
NXP Employee

>Are the 50 Ohm resistances (and obviously their connection to VbiasTX) supposed to be supplied externally to the SoC?

schematic provided on below link is recommended by nxp for eDP usage and it was tested internally :

i.MX8 Display Port Interface 

Best regards
igor

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