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Visitor acolletta
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Registered: ‎05-31-2019

VC707: Using both differential pairs and singled ended signals on the same FMC connnector

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Hello,

I want to use both differential pairs and single-ended signals on the same FMC connector. Are there limitations to which sets of pins can be differential pairs and which sets of pins can be single-ended? I was not able to locate any mention of such limitations in the docs.

For those curious, I want to send ADC signals and clocks over diff-pairs and the rest of the slower, less critical IO over single ended.

Thanks!

 

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Registered: ‎01-22-2015

Re: VC707: Using both differential pairs and singled ended signals on the same FMC connnector

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@acolletta 

    Are there limitations to which sets of pins can be differential pairs and which sets of pins can be single-ended?

Yes, the differential pin-pairs on the FMC connector must be connected to differential pin-pairs on the FPGA.  For example, from the VC707 schematic, you will find the FMC-1 pins labelled:

  • FMC1_HPC_HA23_P
  • FMC1_HPC_HA23_N  

are connected to the FPGA pins labelled:

  • IO_L4P_TO_35_A35 
  • IO_L4N_TO_35_A36. 

From Table 1-12 of Xilinx document UG475, you will find that “L4P” and “L4N” in these FPGA pin labels identify them as a differential pin-pair. 

Finally, differential pin-pairs can instead be used as two single-ended pins.

Cheers,
Mark

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295 Views
Registered: ‎01-22-2015

Re: VC707: Using both differential pairs and singled ended signals on the same FMC connnector

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@acolletta 

    Are there limitations to which sets of pins can be differential pairs and which sets of pins can be single-ended?

Yes, the differential pin-pairs on the FMC connector must be connected to differential pin-pairs on the FPGA.  For example, from the VC707 schematic, you will find the FMC-1 pins labelled:

  • FMC1_HPC_HA23_P
  • FMC1_HPC_HA23_N  

are connected to the FPGA pins labelled:

  • IO_L4P_TO_35_A35 
  • IO_L4N_TO_35_A36. 

From Table 1-12 of Xilinx document UG475, you will find that “L4P” and “L4N” in these FPGA pin labels identify them as a differential pin-pair. 

Finally, differential pin-pairs can instead be used as two single-ended pins.

Cheers,
Mark

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Visitor acolletta
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Registered: ‎05-31-2019

Re: VC707: Using both differential pairs and singled ended signals on the same FMC connnector

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Thanks for the insight! Quick question: do you think there is any benefit from only using the "P" pins for the single ended signals? I would think with less dense signals, perhaps there is less risk in noise/crosstalk.
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Registered: ‎01-22-2015

Re: VC707: Using both differential pairs and singled ended signals on the same FMC connnector

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     do you think there is any benefit from only using the "P" pins for the single ended signals?

You are correct to think that "less dense" = "less crosstalk".  However, "less dense" in the FMC cable is more important than "less dense" on the FMC connector - because cable wires are near each other over longer distances.  

Try googling "FMC cable pinout" to learn how FMC connector pins match-up with wires in the FMC cable.  You will find that the standard FMC cable has ground wires placed between wires that carry different signals - which is a way to reduce crosstalk.

Mark

Visitor acolletta
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Registered: ‎05-31-2019

Re: VC707: Using both differential pairs and singled ended signals on the same FMC connnector

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Thanks for your help, Mark.

 

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