SN75LVDS83 LVDS Problem

Kagan Calisal

Other Parts Discussed in Thread: SN75LVDS82, SN75LVDS83, SN75LVDS83B

Hi everyone,

We are using SN75LVDS83 as transmitter and SN75LVDS82 as receiver in our LVDS project. We are using twisted cables between transmitter and receiver. Connection between transmitter and our PXA270 development board and between receiver and LCD is provided with ffc cables. When we connect the full system, image is visible for a second on the LCD and then dissappers. From then on we are seeing only colourful lines on the screen even if we switch off and on our system power. When we change the two chips the same procedure is repeated. I can send you our schematics. I will be waiting to hear from somebody.

Kagan

over 14 years ago

0 David Zhang16358 over 14 years ago

Expert 1860 points

There are so many possible reasons for this result.

Did your PCB layout follow differential signal layout rules?

Did you terminate LVDS signal properly?

Did you control the impedance of signal traces on PCB?

Did you have integral power/ground plane?

Did you have extra power noise?

0 Kagan Calisal over 14 years ago in reply to David Zhang16358

Prodigy 30 points

Hi David,

Thanks for your fast reply. We paid attention to differential signal layout rules in our design. And we used 100 ohm termination resistors in the receiver side. We don't have inegral power or ground plane. Our supply power is so clean as to drive the chips. May the problem be because of frequency range of the SN75LVDS83 transmitter. Its frequency range is between 31 and 68 MHz. We want to drive a VGA (640x480) screen. Does SN75LVDS83 support VGA screens?

Best regards,

0 David Zhang16358 over 14 years ago in reply to Kagan Calisal

Expert 1860 points

Please check this link:

http://www.epanorama.net/documents/pc/vga_timing.html

The clock rate of VGA is 25.175MHZ.

Check the attached image it is from datasheet of SN75LVDS83, looks like SN75LVDS83 do not support VGA.

It is really the problem of clock rate, what is your video source, is the video from DVI?

How about use high frame rate video source to meet frequency requirement of SN75LVDS83 and chop video inside processor then show the chopped video on LCD?

0 veli demirel over 14 years ago in reply to David Zhang16358

Prodigy 40 points

Dear David,

We preferred SN75LVDS83B chip rather than SN75LVDS83 because of frequency limit. Our problem is now that when we touch the clockin singal of the chip with finger via a needle our image is begining to go well. What is the solution? May the problem be a capacitance or resistance problem.

Best regards,

0 David Zhang16358 over 14 years ago in reply to veli demirel

Expert 1860 points

That clkin signal is a single end signal, how did you terminate this signal?

How about using a scope to check the quality of clkin signal with and without needle?

I think when needle is touched with clkin pin, needle founctions as a parallel termination resistor.

Hope I am right.

0 veli demirel over 14 years ago in reply to David Zhang16358

Prodigy 40 points

Our clkin signal is a single end signal and we didn't use any termination resistor in our pcb layout. Is it necessary to use termination resistor in the transmitter side?

We also try 400k - 1M parallel resistor to clkin signal and result is same. Up to you, which magnitude of resistor we must use...

0 David Zhang16358 over 14 years ago in reply to veli demirel

Expert 1860 points

the value you used is too big.

the attached PDF files are regarding to clock termination, hope they are helpful.4760.AB023.pdf

1541.CTS_AN1025.pdf

Usually we use 50ohm PCB trace and use termination resistors to reduce reflections on signal traces.

0 Kagan Calisal over 14 years ago in reply to David Zhang16358

Prodigy 30 points

We didn't pay attention for 50 ohm pcb trace in layout. Then which clock termination method you suggest us?

0 David Zhang16358 over 14 years ago in reply to Kagan Calisal

Expert 1860 points

If the trace of PCB layout is even, you only get different impedance such as 80ohm.

You can use online PCB trace impedance calculator to check based on your PCB trace parameters then based on references paper to decide which clock termination you would to implement.

Hope it is helpful.

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SN75LVDS83 LVDS Problem