Zybo HDMI startup issues

[aytli]

So I understand that the HDMI ports on the Zybo should be disconnected on startup, due to some power reflection issue. Does this problem also exist in other Digilent boards that can handle HDMI? I'm currently looking at Pynq, Arty-A7, and Nexys Video.

Alternatively, if I were to somehow route a video input into the Zynq board through the 15-pin PCAM port (either from a camera or an HDMI-CSI adapter), would it have the same issue?

[jpeyron]

Hi @aytli,

For the HDMI ports that is the case.  The PCAM Ipoer is locked in at 3.3V  rather than having pull-up resistors on all of the lines, but I don't doubt that if you provided a current source (i.e. an externally powered monitor/camera) you could theoretically back power the board. 

thank you,

Jon

 

 

[aytli]

Thanks for the response, is there something I can probe on the board to see if it's being back powered or not?

[jpeyron]

Hi @aytli,

I reached out to one of our design engineers about this thread and they responded saying that they think that the problem still exists on other boards, even the ones that have the HDMI hider. They looked at the datasheet and the HDMI hider used on some of the newer boards has internal pull-ups to the supply rail, which would allow some amount of current to flow into the 3.3V rail if the board isn’t already powered. We haven’t experimented with it to see how much current flows, but it wouldn’t surprise me if this causes problems.
 
Any Camera plugged into the PCAM connector should be powered by the 3.3V rail on that connector, and not externally. Plugging an HDMI-CSI adapter in prior to powering the board may cause a problem because you can potentially back power the board through the FPGA pins, which all have internal diodes that connect to the bank supply rail. You should be able to measure the internal pull-up resistors to the supply rail that are attached to the data lines to see if the board is being backpowered.

thank you,

Jon