i.MX8M DisplayPort AC-Coupling Capacitors

[eric_kuzmenko]

Hi,

The i.MX8M eval board as well as many other i.MX8M boards on the market use the HDMI interface and simply follow the Hardware Developer's Guide when it comes to adding 100nF coupling caps onto each of the diff pairs.

Could you please clarify whether or not these 100nF caps are needed if the application is using DisplayPort instead?

We also have 1μF capacitors on AUXP/N, should they be 100nF instead?

We are using 0Ω jumpers in place of the caps and were wondering if this could cause us any trouble.

Take a look at the attached screenshots to see what I am referring to.

Best regards,

Eric

[eric_kuzmenko]

I believe I may have found a conflicting answer to my question, we are using TI's HD3SS460 USB-C DisplayPort alt mode MUX IC and it suggests that for our application ("Type C Interface Using DP as Alternate Mode – Source/Host") we would not need any decoupling caps; and in fact, it could be detrimental if they were included. See figure 3 in its datasheet: http://www.ti.com/lit/ds/symlink/hd3ss460.pdf 

There's also a support thread on TI's site where they say it could cause problems to have them present: http://e2e.ti.com/support/interface/f/138/t/672016?HD3SS460-HD3SS460-AC-Coupling-CAP 

Specifically, they wrote:

"We suggest not putting AC coupling capacitors on the Source side so the DC bias can be controlled by the DP source which normally haves less variability than the connector side."

[weidong_sun]

Hi Eric,

  Yes, you can use 0.1uF ,

  BUT 1μF capacitors on AUXP/N should be changed to be 0.1uF, 1uF is too big.

Have a nice day!

Weidong

[eric_kuzmenko]

I found this app note which says in section 2.4 that 100nF coupling caps are required on the DisplayPort diff pairs, I assume this also applies to the i.MX8M: https://www.nxp.com/docs/en/application-note/AN10798.pdf 

Please let me know if this is correct, and if it also applies to the AUX lanes.