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TMDS181: Register 0Ch reserved bits

Mark7389
Prodigy 170 points
Part Number: TMDS181


We have a design using the TMDS181 in between a source HDMI camera and an HDMI to SDI bridge. The output from the TMDS181 seems to be intermittent until we set bit 1 of register 0Ch of (MISC CONTROL Register Field). The problem is this register bit is marked as "Reserved". 

We found this purely by accident while setting the register to "00" - No de-emphasis and "01" 2dB de-emphasis. Option "11" was accidentally seleced and the video started working. Then all option for bit 0 and 1 were tried and it was found that the bit responsible fo the video output was bit 1 being set to 1 ("10").

Can you provide some insight as to what this bit is responsible for? Is there something else causing the video failure? What is it these "Reserved" bits arre actually controlling.

  • 0
    TI__Mastermind 31050 points

    Hi Mark,

    Bit 1 of register 0Ch should further increase the de-emphasis on the output signal. This can help with channel loss after TMDS181, but excessive de-emphasis can also cause failures in compliance testing. If the HDMI output is wired to an accessible HDMI port, I recommend validating that your design passes compliance with this setting before proceeding.

    The purpose of de-emphasis is to counter channel loss after the transmitter. If this bit is necessary, It suggests a lossy connection between the TMDS181 output and the HDMI to SDI bridge that the bridge is not able to recover itself with equalization. Perhaps there is a long cable or PCB trace between these?

    Best,

    Shane

  • 0 in reply to Shane Hauser
    Prodigy 170 points

    Shane,

    We don't go through any HDMI compliance testing with this product. The output goes to the HDMI to SDI bridge via the attached circuit section. The total length of the traces on the PCB from 181 to bridge are between 15.6mm and 17.2mm. Specifically the lengths before the caps range from 4.2 to 4.8mm and 11.9 to 12.5mm after the caps. The inline caps and resistor pullups are recommendations form the bridge manufacturer. Are those acceptable values? I believe the resistor connected to the VSADJ pin has something to do with this as well. The recommendation from the Bridge manufacturer was to use a 7.06 Ohm 1% resistor. Is that acceptable as well?

  • 0 in reply to Mark7389
    TI__Mastermind 31050 points

    Hi Mark,

    Thank you for the context. I'm curious if the swing is too large on the TMDS181 output for your HDMI->SDI bridge input.

    Can you try increasing the value of Vsadj by a few hundred Ohms? This should lower the swing from the TMDS181, which would have a similar effect to increasing the De-emphasis as you are currently doing in register 0Ch

    Best,

    Shane