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TMDS361B driving short or longer cable to another sink

Other Parts Discussed in Thread: TMDS361B, DS34RT5110, DS16EV5110A

We are having some trouble with implementing a simple DVI-D (HDMI) switch function. This is an embedded application. Our PCBA is an intermediary intended to select between 3 sources and source forward to a Silicon Image sink device. Si91250, on another board. Two of the TMDS361B sources come from the outside world (outside our enclosure). Typical cable runs are 10m to 20m. So, we choose the TMDS361B with its adaptive cable equalization, extended reach. The third source is local inside our chassis from a Silicon Image HDMI transmitter device routed over 150mm of HDMI cable, twisted and individually shielded, to our board with the TMDS361B. The output of the TMDS361B is routed to another Silicon Image HDMI receiver device using another 150mm segment of HDMI cable. Typical clock rates are a 148.5Mhz. Data rates are around 1080 Mbps.

 

The TMDS361B appears to do a pretty good job performing equalization. Looking at the output of the TMDS361B it looks okay. Except, there is some deviation, jitter, in the amplitude of its outputs, clock and data pairs. It appears that the Silicon Image device is having trouble staying locked onto the output of the TMDS361B. Problem occurs with all three sources passed through the TMDS361B.

 

If I feed the S.I. receiver other sources, including short or longer up to 10m of cable, it works. If I take the output of the TMDS361B run it over 300mm of HDMI cable to a DS34RT5110 evaluation kit, then through another HDMI cable of any reasonable length, it works. This leads to suspecting that the output of the TMDS361B is unable to drive even a very short 150mm segment of cable well enough to satisfy the S.I. receiver. Unfortunately, we do not have access to the receiver schematics nor the datasheet for the S.I. device.

 

Testing the TMDS361B with different TMDS rise and fall times does not cure the issue. Works best with the default setting, fastest. Testing indicates the TMDS361B performs good cable equalization. We suspect the source outputs are not up to the requirments of this “system”.

 

The TMDS datasheet says; “TMDS361B is not intended for source side applications such as external switch boxes.”, on the first page. Okay. But, what does that mean? Does this mean that the drive strength of TMDS361B is not adequate to drive cables? Does this mean that it cannot drive our very short 150mm cable? Is it only capable of chip to chip on the source side? If this is the case, what data points in the datasheet should I inspect to determine this with this device and others?

 

It is interesting that the TMDS361B can drive a DS34RT5110 over a longer 300mm cable. I have not tested all lengths. But, it works at 2m. Yet, cannot satisfy the S.I. device over only 150mm (.15m) of cable. Comparing feeding other sources directly to the S.I. device verses the TMDS361B using various lengths of cable, the TMDS361B works over longer runs than the other device. This indicates that the adaptive equalization of the TMDS361B out performs the other device. But, our testing and results don’t explain why we can drive the “other” part, the SI 91250 from the TMDS361B over any length of cable?

 

We’ve tested using various sources including a pseudo random pattern originated from an SDI generator and converted to HDMI. The pseudo random test is the most telling. Eye patterns are what you would expect. Except, the previously mentioned amplitude jitter on the output of the TMDS361B. There is some clock jitter, U.I. interval. But, it appears reasonable. We have been unable to measure at the sink due to lack of schematic and datasheet to the other device. We do not have control over the sink. So, we have to find a way to satisfy what it wants with our TMDS361B circuit. We are considering a board spin that would add a DS16EV5110A to the output of the TMDS361B. But, this is not yet tested. (It appears the DS34RT5110 that we used for testing is in the process of be discontinued. Why?)

 

Suggestions and advice?

  • Hello Howard,
    thanks for providing such a detailed information, is it possible for you to share your eye diagrams of the TMDS361B output?
    We are going to review this and provide feedback shortly.
    Does the issue happens with different resolutions? (let's say lower resolutions)

    Regards.
  • Thanks Elias,
    I've posted a couple scope pictures. They are not the best. I only have a 1Ghz scope.

    The issue only occurs at higher resolutions, 148.5Mhz clock. We only operate at two frequencies. It seems to work fine at 74.25Mhz.

    I failed to mention in the initial posting that I tried numerous Vadj levels ranging from 3.6K to 8.6K. All values between the recommended 4.02K and 8.6K behaved the same. Lower resistor values caused some different artifacts to appear in the video.

    We also did more empirical testing after the above posting. The problem seems to come down to compatibility with the Silicon Image SiL9125 receiver. While the TMDS361B was not able to drive longer cables in our setup, it successfully sourced clean video and patterns into various other devices over a 2m cable, including Dell monitors, HDMI to SDI converters and a TV. Using the same cabling and varying the length from the short 150mm, 2m, and 5m, the TMDS361B was unable to source clean video with the Si9125. At 150mm video would appear, but lose sync. Sync was never obtained with any of the longer cables.

    Thanks,
  • Hello,

    Some of the Sil devices have issues with jitter, I will ask around to see if someone have ever tested that device. I will be helpful if you could get the Sil datasheet.
    Regards.
  • Hi Howard,

    I would appreciate if you could send me an email with the business case for this opportunity.
    josecarlos.gil@ti.com

    Regards.
    JC
  • I asked SiL for a datasheet. They require purchasing a license for a minimum of $8K USD in order to gain access to datasheets. That's not going to happen.

    I agree, the evidence points to the SiL part not being very forgiving, sensitive to jitter. This is one of their older parts.
  • We buy the board with the SiL device. We have good relations with the other company’s engineering. We’ve just found out that there is a replacement board with a newer SiL device in the works. That may provide another path to a solution. Although, the newer board will not be available until mid-2015. That’s a problem.

    We have a couple other approaches under consideration: Further empirical testing indicates that when the connections (cable) between the TMDS361B and the other board is optimal it works, probably with very little margin. We are in the process of having several precise, matched impedance, inter and intra pair matched individually shielded, wires fabricated to see if we can get just enough margin for the current TMDS361B board to work, “as-is”. Our next option, is to re-spin with another T.I. device acting as a buffer between the TMDS316B and the SiL. The system works well with the DS34RT5110 evaluation card in the middle acting as a buffer. With the DS34RT5110 re-clocking PLL disabled, it also works well. Re-clocking with the PLL does extend cable reach. Since the DS34RT5110 is NRND, we would probably try the DS16EV5110A. If that combination works, we have a solution. We could add the DS16EV5110A to the output of the TMDS361B on our board. Although, we don’t like the cost of the re-spin and scrapping current inventory.

    Without knowing the characterization of the SiL device, we can only speculate. My theory is that the slew rate of the TMDS361B is less than the DS34RT5110 and the other sources we’ve tested, so far. I’m guessing, when the DS34RT5110 is placed in the path, it sharpens (faster slew) up the edges, giving us slightly more jitter margin, enough to work with the SiL9125. Do you think there is validity to this theory?

    If we end up spinning our board, adding a buffer between the TMDS361B and the SiL device, the question would be; What device do you suggest? DS16EV5110A?

    Thanks,
    Howard
  • Hi Howard,

    DS16EV5110A is a TMDS equalizer, and can be used as a TMDS driver.

    Regards
    JC
  • Jose,

    Thanks. I am aware that the DS16EV5110A is an equalizer and can be used as a driver. Two questions:
    1. What differentiates it from the TMDS361B that makes it a "driver"?
    2. In solving this issue, do you believe the DS16EV5110A can replace the DS34RT5110? (I know the speed and PLL difference.) In other words, do you believe the DS16EV5110A is compatible with the SiL device?

    I don't know if it will help in this case. But, I do like the flexibility of having the ability to change various settings via i2c in the DS16EV5110A.

    Howard
  • Hi Howard,

    The TMDS361B is a 3:1 DVI/HDMI Switch with adaptive equalization and the DS16EV5110A is a Video Equalizer (3D+C) for DVI, HDMI Source/Repeater/Sink Applications, both are equalizers but with the TMDS361B you can connect up to 3 sources to one sink.
    And yes, the DS16EV5110A can replace the DS34RT5110, it is compatible with Sil device.

    Regards.
    JC
  • Thanks Jose, I'm ordering a DS16EV5110A evaluation board to test, to verify it works as well as the DS34RT5110 does with the SiL device. We are using the TMDS361B as you describe, as a 3:1 switch. It is looking more likely that we will be spinning our board to add the DS16EV5110A to look like this:

    3 sources ->TMDS361B->DS16EV5110A->150mm cable->SiL device.

    Thanks for the advice.

    Howard

  • Hi Howard,

    Any time help is needed let us know.

    Regards.
    JC