SN65LVDS93B: Differential clock lines loading issue

Hi,

How long is your LVDS cable or PCB? What is being shown on your screen?

Regards,

Michael W.

Hi Michael

We have 40 pin FPC connector on our board. right now for testing we are directly connecting LCD to it. attached image for reference. In actual scenario we are using 14 inch  FPC jumper cable.

Regards

shivakumar L V

Hi,

Thank you for the picture, Can you circle on your board where the SN65LVDS93B is? Where is the 14 inch flex PCB in your picture? where are you measuring the CLK with the oscilloscope? 

Hi 

Please find the image below. Please zoom in. I pointed the SN65LVDS93B and signal measurement point. As of now we are not using 14.5inch FPC cable. We directly inserted the LCD cable into 40Pin FPC connector.

What does the signal look like without the 14.5 in FPC cable?

Hi Michael

We didnt connected the 14.5inch cable. we directly connected the LCD to our FPC connector for Testing. Below image without connecting the LCD. I attached the one more image where signal gets attenuated.

1. without connecting the LCD

2. with connecting the LCD.

LCD is fine. we just tried to connect the 100ohm resistor to differential clock signals. then also signal getting attenuated.

Does the TCON of the LCD already have 100 Ohm terminations? are you accidently double terminating the signal?

Hi Micheal

No. We double checked we measured it is 100ohm. we dont have any termination option in our board. All we have is at the LCD side only.

What do you think will be the problem? 

Is there any other IC can we try out? or is there anything to do with firmware?

Regards

shivakumar L V

I think that your issue is with the layout of the traces, connector and the flex PCB. Have you made sure that your using the proper impedance matching on the PCB and Flex PCB? Can you send me your layout? I do not think switching to a different IC will fix this problem.

Hi Micheal

Here is the attached layout file. Please refer the J5 and U16 section in the layout.

The partnumber for the LCD FPC connector : ZF5S-40-01-T-WT-TR

The LCD partnumber : AFY1024600A0-7.0INTH-R

AFY1024600A0-7.0INTH-R_Orient display.pdf

.MAINBOARD.brd

I do not see anything obviously incorrect with your layout. Can you measure the CLKIN signal to ensure that it is correct?

Hi Michael

Can you confirm one thing? if you have any SN65LVDS93B based board. can you let me know what happens if we connect 100ohm termination to the differential pair.

Please let me know about signal level with or without connecting the LCD.

Regards

shivakumar l v

Hi Michael

I measured the signal before and after connecting LCD. Michael i thinkg signal levels are fine. its 400mVPk-PK with 1.2V common mode voltage. Please let me know your observation below. 

bef

Hi,

I assume the second picture is with the LCD connected, correct? The signal looks correct. I also assume that this signal was measured close to the SN65LVDS93B, correct?

Regards,

Michael W.

Hi Michael

Yes. second picture is when we connect the LCD. its near the 0E resistor. its near the connector. But I can see that 400mV peak to peak with 1.2V bias voltage( Common mode voltage).

Regards

shivakumar L V

This is how the LVDS signal should look when the LCD is connected.