DS90UB948-Q1: S90UB948 sync problems

Hi,

If your system works with a twisted pair but does not work with a coaxial cable, then the issue is caused by signal integrity issues. Twisted pair cables have a better noise performance compared to coaxial cables. Is there noise that could be coupling onto the GND plane or the signal causing this issue?

Is there layout differences between the coaxial cable and the Twisted pair cable? 

How long is the twisted pair cable and how long is the coaxial cable?

Have you reviewed the datasheets of both cables to see what the signal integrity performance of both cables is?

Regards,

Michael W.

The length of the twisted pair cable is 5m. Works fine.

I am using an RG-174 / U cable with a length of 0.5 or 1.5 meters. I think that at this length there should not be much signal attenuation. According to the datasheet, the cable has an attenuation of 1.02 dB / m at a frequency of 1 GHz and an impedance of 50 ohms.

The second connection option is with a coaxial cable through the U.FL connector. U.FL-2LPHF6-04N1TV-A-200 cable 20cm long. The cable has an attenuation of 5.4 dB / m at 3GHz and an impedance of 50 ohms. There are also sync issues with this cable.

On the serializer I am using a 330nF on DOUT + and 150nF on DOUT- capacitor. The DOUT + track between the serializer and the connector is short, with a ground reference layer underneath.

Maybe the problem is not with the cable?

Can you share the source for the DS90UB921-Q1EVM PCB?

Hi,

Can you provide the schematic and layout for the SER and DES? 

The DS90UB921-Q1EVM PCB schematic and layout is in the EVM users guide: https://www.ti.com/lit/ug/snlu205/snlu205.pdf?ts=1629733873208&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FDS90UB921-Q1EVM

Schematic and layout in the archive.

ser.deser.rar

Hi,

Can you remove the XW connectors? The extra stub will be causing signal distortion. Can you also swap the 330nf and 150nf AC caps on the 921's FPD-Link Lines to 100nf and ~50nf? 

How were you able to test this board with a twisted pair?

When I connect through the XW1 connector, there is no stub.

When I connect through the XW3 connector, there is a 5mm long stub, but I tried to cut that part of the track. It did not help.

I tried to change the capacitors to 100nf and ~ 50nf, it didn't help either.

I also noticed that the image flickers only when turned on cold.

After the board has been turned on, the display does not flicker.

If the serializer and deserializer are turned on, then when the cable is connected to the hot, the display works stably.

What kind of experiments are possible more?

Hi,

can you verify  that the 921 and the 948 are following the recommended power up sequence?

Power up sequence is correct.

It looks like I fixed the problem. The Pixel Clock Edge Select (TRFB) bit helped. In register 0x03, bit 0 must be 1.

Tell me which capacitors should be correctly set when working on the coaxial of the 921 ser and the 948 des in a pair.