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TMDS181: TX_TERM and VSADJ Relationship

Kevin Brown
Prodigy 10 points

Part Number: TMDS181

Hello,

We have designed an HDMI 2.0 to 12G SDI converter product that uses the TMDS181.  We successfully have it working with a 12G SDI monitor and it appeared to be robust, however we now have an SDI recorder that will occasionally show video dropouts or flicker.  

In our design, the TMDS181 is being used as a conditioner before going to an HDMI to SDI processor.  We are able to put the SDI processor in pattern generator mode and that works fine to the recorder with no flicker, so we feel like this might be on the input side. 

Trace lengths from the TMDS181 to the SDI processor are short (around an inch) and are AC coupled and pulled-up with 50 Ohm to 3.3V.  We were concerned as the whether termination is causing this, so we tried different VSADJ values with or without using termination (TX_TERM).  We found that lowering the VSADJ resister value would negate the need to provide termination, but this still did not fix this.  At higher VSADJ values, like 7.06k, we would have to add termination (Reg B [4:3] to 11) as expected to even get lock, but still occasional flickering.

We can see that the flickering does improve with higher values of VSADJ and termination even though it's never fully eliminated.  Does this behavior make sense? Are there other registers we should be trying?  How high can we go on the VSADJ resister?

Kevin  

  • 0
    TI__Guru**** 187811 points

    Kevin

    TX_TERM and VSADJ are two separate controls, but both impact the TX output signals.

    TX_TERM are set depending on the data rate. In TMDS181 with TX_TERM as NC, TMDS181 TX termination will set automatically

    For HDMI1.4 data rate < 1.65Gbps -> Open termination

    For HDMI1.4 1.65Gbps <= data rate < 3Gbps, 150-300ohm termination

    For HDMI2.0 3Gbps <= data rate < 5.94Gbps, 75-150ohm termination 

    VSADJ adjusts the TX output signal VOD and VL as shown below. The VSADJ ranges from 4.5k to 7.5k as shown below.

    Bigger VOD, lower VL. Smaller VOD, higher VL. The goal is to find one VSADJ setting that can meet both HDMI VL and VSADJ requirement.

    For this particular issue, if you switch TMDS181 from the re-timer mode to re-driver mode using its I2C register, do you see the flicking going away?

    Thanks

    David

     

     

  • 0
    Prodigy 10 points

    Hi David,

    Thank you for your quick response.  We have the TMDS181 in I2C mode where I2C_EN is pulled high, so i assumed we would need to set the termination register.  The Configuration Guide mentions that we need to set that manually.  TX_TERM is left floating, however, if this is not the case.  The device we're connecting to has an input amplitude requirement of 200mV to 1200mV.  

    I haven't tried switching to re-driver mode in this high of data rate.  I assume you want me to try and write bits [1:0] of register 0Ah to 00?

    Kevin

  • 0 in reply to Kevin Brown
    TI__Guru**** 187811 points

    Kevin

    Thank you for your quick response.  We have the TMDS181 in I2C mode where I2C_EN is pulled high, so i assumed we would need to set the termination register.  The Configuration Guide mentions that we need to set that manually.  TX_TERM is left floating, however, if this is not the case.  The device we're connecting to has an input amplitude requirement of 200mV to 1200mV.  

    Correct, in I2C mode the TX_TERM needs to be set manually.

    I haven't tried switching to re-driver mode in this high of data rate.  I assume you want me to try and write bits [1:0] of register 0Ah to 00?

    Correct

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Prodigy 10 points

    David,

    Re-driver mode didn't help. I still see occasional flickers on the recorder.  Let me know if there is anything else i should try.

    One other thing we've been confused about.  From the documentation, when changing certain parameters the APPLY_RXTX_CHANGES bit (bit 2 of register 0Ah) needs to be written to a 1.  I assume that this is so multiple changes can be accomplished at once.  I've seen that writing some registers listed in the APPLY_RXTX_CHANGES description happens immediately, without having to write bit 2 to a 1.  Is this expected?

    Kevin

  • 0 in reply to Kevin Brown
    TI__Guru**** 187811 points

    Kevin

    If you write to TX_TERM, HDMI_TWPST1, EQ_EN, EQ_ADA_EN, VSWING, and Fixed EQ, then you would need to write the APPLY_RXTX_CHANGES bit for the new settings to take effect. 

    Since TMDS181 is in the I2C mode, can you dump out the TMDS181 registers for me to review?

    What resolution are you seeing the video flicking? Any chance you can probe the TMDS181 output after the AC coupling capacitor?

    Is it also possible to share the schematic and layout for review?

    Thanks

    David 

  • 0 in reply to David (ASIC) Liu
    Prodigy 10 points

    David,

    We are only seeing the flickering in HDMI 2.0 (4K60p/4K/59.94p).  Everything works great in 4K30p and below.  Here is the dump of the 181 registers.

    [regnum-value(h)] 0-54, 1-4D, 2-44, 3-53, 4-31, 5-38, 6-31, 7-20, 8-01, 9-02, A-B1, B-1B, C-00, D-00, E-00, F-0F, 10-00, 11-00, 12-00, 13-00, 14-00, 15-80, 16-00, 17-80, 18-00, 19-00, 1A-00, 1B-00, 1C-00, 1D-00, 1E-00, 1F-00, 20-0A.

     I can get Engineering to share our schematic and layout.  Is there a way to do that privately?

    Kevin

  • 0 in reply to Kevin Brown
    TI__Guru**** 187811 points

    Kevin

    The register dump looks ok. 

    Would you please accept my friendship request so you can send me the schematic/layout in a private e2e message?

    Thanks

    David