cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 
Highlighted
Visitor
Visitor
262 Views
Registered: ‎04-27-2009

GTH recovered clock issue with HDMI

Hello,

I have built a HDMI receiver using GTH transceivers that is working great with most sources.  I have run across a couple of sources that I see buffer over and underflows in the transceiver.  I used the GTH transceiver wizard to build the HDMI interface using 3 GTH RX channels.  I have a fixed reference clock.  The buffer over/under flows occur on the transceivers not listed as the master in the transceiver wizard.  I brought out the recovered clock (RXOUTCLKPMA) for each of the RX blocks.  I measure, using a frequency counter, slightly different recovered clocks on each channel.  A source that works has exactly the same frequency on the recovered clock for each channel and no over/under flows on the transceiver buffer.

Here is an example of the measured recovered clocks:

HDMI input channel 0: 74.24846 MHz

HDMI input channel 1: 74.24848 MHz

HDMI input channel 2: 74.24852 MHz

I believe the buffer over/under flows are caused by the read out clock from the master channel being slightly different from the other channels.  I moved the master channel to a different channel and get the same result, over/under flows on the channels that are not the master.  

It appears that the compatibility issue only occurs with a specific HDMI transmitter IC company.  That HDMI source works fine with other monitors I have, just not with my implementation.  I can't use the Xilinx HDMI implementation due to logic size and transceiver placement constraints.

Thanks for any suggestions!

John

Tags (3)
0 Kudos
2 Replies
Highlighted
Moderator
Moderator
194 Views
Registered: ‎07-30-2007

Typically recovered frequencies are off in this manner when an RX reset is done before the RX input is present.  Without a frequency stable input to the RX the clock data recovery circuit can't lock to the correct frequency.  For this reason you can't do a reset all on a channel that is being looped back.  The RX will be interrupted by the TX reset and often won't lock properly.




----------------------------------------------------------------------------
Don't forget to reply, kudo, and accept as solution
----------------------------------------------------------------------------


0 Kudos
Highlighted
Visitor
Visitor
179 Views
Registered: ‎04-27-2009

Thanks for the suggestion.  In this case, I'm using an external HDMI transmitter (video switcher) that is supplying the HDMI signal.  I did try to add in a manual rx PLL and datapath reset via a VIO connection to reset after the signal was stable, but the result is the same.

0 Kudos