SN75DP149: Blinking issue of video screen

Hey Jeffrey,

Are you able to send the DDC log of the HDMI when we see this blinking?

Hi Vishesh,

Please let me know how to check or capture the DDC log of HDMI.

Regards,

Hi Vishesh,

As I checked it with customer, they are using this device through TMDS signal only without I2C interface, so the DDC log of HDMI couldn't be checked. Their configuration is as follows. Also, we tested long td2(VDD and VCC stable before OE deassertion) over 600ms, but it is not resolved. Is there any idea to resolve this issue? Please let me know that.

CPU -> TMDS(720p or 1080p) -> SN75DP149 -> TMDS -> Display

Regards,

Hey Jeffrey,

The DDC is a communication that happens between the HDMI source and sink. This is required for all HDMI links. You will need to pull this bus out and connect it to a I2C protocol reader. an example of a protocol reader is the TotalPhase Beagle: https://www.totalphase.com/products/beagle-i2cspi/?srsltid=AfmBOooSYKKTPRCMtuDJ6dkIimrOinkbmO-KChZ6FnP-kD-SeXtplae1

Hi Vishesh,

We checked the DDC log through I2C reader, but there was no data. Please let me know how to resolve this issue without I2C interface. There is no issue on TMDS signal transmission from CPU to display except the specific condition of power on and off. This device can't be used on their board if SCL/SDA_SRC and SCL/SDA_SNK must be connected to HDMI link.

Regards,

Hey Jeffrey,

The reason I ask for the DDC data is because in order for HDMI data to be sent from the source the following steps must take place:

1) a proper HPD signal is received by the source

2) The HDMI source reads the EDID via the DDC bus and determines resolution and refresh rate (if there is no data here that is the problem)

3) HDMI video is sent across

It seems you are able to send video across 99/100 times, but in the 1/100 blinking screen one of the pre-requisites of HDMI data transfer is not properly met.

Jeffrey Chung1 said:

Also, we tested long td2(VDD and VCC stable before OE deassertion) over 600ms

Additionally, try adding a 200nF capacitor to the OE pin to slow down the edge further. Its good that you verified the td2. Because you have this working 99/100 times this is most likely not a schematic issue as it works the vast majority of times.

Full disclosure, this problem will be very difficult to debug without using I2C to check the status and states of the device. Please try to convince customer to use I2C for debug.

One suggestion I have is to try disconnecting HPD as you power down the system, and reconnecting HPD as you power up. 

Hi Vishesh,

As we checked your suggestion of disconnecting and reconnecting HPD, there is no improvement on this issue. And they are trying to check the DDC data through I2C interface, it will take some time to verify the DDC data. Let me update the result after checking it.

Also, their PCB layout's layers is 2 layers. Could it be the cause of this issue?

Regards,

Hey Jeffrey,

Yes this may be the cause of the issue. Do you have a layout I can look at?

Hi Vishesh,

Please check the layout attached. There is SN75DP149 on the bottom side. Also, they are trying to change PCB layer from 2 layers to 4 layers.

SN75DP149_PCB Layout.pdf

Regards,

Hi Jeffrey,

This 2 layer PCB board will have too much noise and this may be why it fails link  training. Can you share the 4-layer PCB files once completed? I would like to review the updated layout before it goes to production. 

Hi Vishesh,

Let me share the updated layout of 4-layer with you once completed.

Regards,

Hi Vishesh,

Please review the updated layout of 4-layer below.

4-Layer Board.pdf

Regards,

Hi Jeffrey,

Any chance you can share with us the actual board file instead the .pdf format? This will really help us doing a proper layout review.

Thanks

David

Hi David,

I can share the layout file with you via email. Please let me know your email.

Regards,

Hi Jeffrey,

Was David able to provide the feedback needed? 

Hi Vishesh,

I got the original layout file of cadstar tool from customer, so I can provide it with you via email. Please let me know your email,

Regards,

Hi Jeffrey,

Please share it to v-pithadiya@ti.com

Hi Vishesh,

I sent it to you via email. Please review it and let me know your feedback.

Regards,

Sounds good, thanks!. I will follow up as soon as I can.

Could you complete this layout review by today? I'm sorry but it is very urgent issue, so please follow up ASAP.

HI Jeffrey,

I will follow up as soon as I can but the file provided is incompatible with the two PCB viewing programs I have. Altium and Allegro. I have submitted a ticked for CADstar download, but I am waiting on IT approval. 

Are you able to send a file compatible with Allegro or Altium?

Hi Jeffrey,

The layout looks ok .

but I have a few questions. what happened to the pullup resistors on the main link here:

If you are looking to simply do a HDMI to HDMI conversion then its best to use something like the TDP0604 and the TMDS181. If this is not an updated schematic as this is not what the pcb look like, can you send an updated schematic?

Hi Vishesh,

As I checked it with customer, there is no the change on their schematic.

Regards,

Hi Jeffrey,

I don't see the 49.9 Ohm pullup resistors on the main links in the layout. Can you highlight them for me?

It seems like that is missing. 

Hi Vishesh,

There are the pullup resistors next to J1 connector on top layer. Please check the layout pdf file which sent you before.

Regards,

Ah I see them in the email you sent.

I really want to push you to use the TDP0604 or the TMDS181 in this design as you are using DC coupled HDMI. The issue with using the DP149 in this application is that there are two termination networks present at the inputs. The DP149 terminated to ground as it is designed for AC coupled HDMI (DP++), and the external pull up termination to meet HDMI spec. These put together create a resistor divider network that divides the input signal to half really affecting signal quality.

Using devices that are designed for DC coupled HDMI will remove the need for the external resistor network and keep the design simpler and provide better signal quality. Please use the TDP0604 or the TMDS181 for this design. I see risks associated with using the DP149. 

Hi Vishesh,

Thanks for your kind response. But it is so difficult to change from DP149 to TDP0604 or TMDS181 because their design must be fixed for preparing of mass production. I will recommend them your recommendation, but I think that it is not enough time to change their design.

Regards,

Hi Vishesh,

Thanks for your kind support. I will close this thread and open it again if they have additional issue.

Regards,

Hi Vishesh,

They tested a modified board of 4 layers, but it was a same result. So I recommended to change from DP149 to TDP0604 or TMDS181. They want to know the position of TDP0604 or TMDS181, for example, HDMI source + TDP0604/TMDS181 as close as possible or TDP0604/TMDS181 + HDMI sink as close as possible. Also, please let me know if you have more advice.

Regards,

Hi Jeffrey,

The TDP0604 works better in a source applications. I recommend placing the part where it can compensate for the pre-channel optimally. The TDP0604 can do up to 12dB at a 3Ghz (6Gbps). I recommend placing the TDP0604 approximately 10dB of insertion loss away from the video source. For a source application you also want to configure the TDP0604 in limited mode, so we have control over the output swing, slew, and pre-emphasis for maximum tuning ability. 

The TMDS181 is a retimer so it will provide better signal quality than the TDP0604 as it is able to clean up jitter as well. Use the same principle at the TDP0604. This device has a larger RX EQ ability at 15dB.