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URGENT HELP NEEDED: LVDS

Hi everybody! I felt in a very strange situation: I'm working with an FPGA BOARD: -2 Virtex-4LX -1 Quick LVDS bus between the 2 FPGAs. -1 INPUT from an external board. -1OUTPUT to the same external board. I use for these quick interfaces ChipSync (Local clocking ressources

+ISERDES+OSERDES). When I make my tests for the internal bus (the connection between the 2 FPGAs) I have no problems: >1Gb/s for each LVDS line. Since the external board is not yet ready to use, I use a small loopback board to connect the output of my board to the input of the same boad (the FPGA board). This board INVERTS LVDS PAIRS ie: if I have an output {XP,XN} I get the input {XN,XP}. ({A,B}=A on P pin and B on N pin). When I make the same tests it doesn't work even on small frequencies. and the use of IDELAY DOESN'T HELP.

Please help me Mehdi

Reply to
GaLaKtIkUsâ„¢
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  1. I doubt anyone can understand your description above. Are you trying to say that the wires are crossed on the board?
  2. Use a scope.

/Mikhail

Reply to
MM

They are not crossed like X. the problem is the following if we try (using the loop back board) to get P pin on P pin and N pin on N pin we will be obliged to cross the wires (make an X on the board). To avoid this the guy ho made that loopback board decided to send the P to N and N to P and says that the only problem will the inversion of the logic levels. I reformulate my question in more simple terms: I LVDS pins are crossed what would be the result? only inversion of logic levels or more? Mehdi

Reply to
GaLaKtIkUsâ„¢

nd

to

If the loop-back board crosses the wires, then I assume that out-N --> loop board --> in-P and out-P --> loop-board --> in-N. If this is true, then yes, the signal is inverted. Make a simple test to verify this.

-Dave Pollum

Reply to
Dave Pollum

1) Are you using 8B10B or some similar coding? 2) use a scope
Reply to
John_H

nd

g to

8B10B is not used. using a scope is not possible!!

I rewrote my HDL code from the beggining and it worked. The problem was in wrong use of OSERDES. Now I'm transmitting at 16x1Gb/s.

Thank you for having taken time to answer Mehdi

Reply to
GaLaKtIkUsâ„¢

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