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Update to HDMI_EXTSWING pull down value

Other Parts Discussed in Thread: TEST

A little background information: we are having some trouble passing test ID 7.2 (TMDS - VL) of the HDMI CTS 1.4b.  We seem to be ~100mV low on the spec (2.6V when passing is between 2.7V and 2.9V.  Our signal routing is very short and pretty clean so we don't believe this is a routing issue and I've found very little control over the HDMI Phy from it's registers.

In researching a solution, we noticed there was a recent change to the DM8168 Datasheet (SPRS614E released in Feb. 2014) that changed recommended resistance value of the pull down resistor on HDMI_EXTSWING from 5.9K +/- 1% to 6.8K +/- 1%.   

1) Does anyone know the reason for this change?

2) What effects does this change have on the HDMI Phy?

3) What is the purpose of the HDMI_EXTSWING pin?

Thanks,

 - Mark 

  • Hi Mark,

    HDMI_EXTSWING is HDMI voltage reference pin. See the below links for more info:

    http://processors.wiki.ti.com/index.php/DM816x_Design_Resources#HDMI

    http://e2e.ti.com/support/dsp/davinci_digital_media_processors/f/717/t/194352.aspx

    http://e2e.ti.com/support/dsp/davinci_digital_media_processors/f/717/t/191339.aspx

    http://e2e.ti.com/support/dsp/davinci_digital_media_processors/f/717/t/375044.aspx

    Regards,
    Pavel

  • Mark,

    The value was changed since it was determined that 5K9 was an incorrect value. The PHY was designed to use a 6K8 resistor. Some original EVMs and very old documentation used yet a different value which was completely wrong. 6K8 is the correct value.

    This resistor affects both the output swing and the Voh & Vol levels. We found that 5K9 would achieve a good eye diagram but failed on the Voh & Vol CTS. The correct value if 6K8 should pass all CTS tests.

    Be aware that any EMI and/or ESD protection will degrade the eye diagram though so care needs to be taken with those components if used.

    BR,

    Steve

  • Thanks Pavel.  I had seen the forum posts but none of those addressed why the resistor was changed or what its effects are so I just wanted to see if we could get more detail.

  • Mark,

    Mark Leonhardt said:
    Thanks Pavel.  I had seen the forum posts but none of those addressed why the resistor was changed or what its effects are so I just wanted to see if we could get more detail.

    Have you check the wiki page that I provide you in my first post? I think no.

  • Steve,

    Thanks you for the reply!  This is exactly what we were looking for.  We do have ESD protection but we had no issues on the Eye diagram tests with the 5.9K in there.  We will test with a 6.8 and hopefully that fixes our issues.

    Thanks again for the quick reply!

     - Mark