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MK_ABQ's avatar
MK_ABQ
Icon for Contributor rankContributor
2 years ago
Solved

LVDS

Hi,

I am using Intel Arria 10 GX FPGA development kit.

DK-DEV-10AX115S-A

The development kit has two FMC connectors FMC A and FMC B. There are LVDS standard pins in the connector.

I am trying to use three pins from FMC B.

My design is simple. I am just driving the three pins with hard coded 1 and 0, and configured as LVDS standard in the qsf.

pin 1 <= '1';

pin 2 <= '0';

pin 3 <= '0';

just few lines of code and no LVDS IP used. I believe this is enough to see something from the connector, but when I probe with scope I see that all three pins are just showing 1.8V. Am I missing something?

Thanks,

MK

Configuration
Generic Io
Other Ips
  • FvM's avatar
    FvM
    2 years ago

    You don't need to connect differential termination to see specified LVDS voltage range. Unterminated pins simply swing to VCCIO and GND for 1 and 0 state. In so far termination isn't required to see LVDS output working. There must be another elementary problem in your test, e.g. probing wrong FMC pins.

7 Replies

  • FvM's avatar
    FvM
    Icon for Super Contributor rankSuper Contributor
    2 years ago
    Hi,
    your setup isn't clear. If you chose LVDS IO standard, a pin pair is used. So how do you measure three pins? There 6 pins involved with 3 LVDS outputs.
  • MK_ABQ's avatar
    MK_ABQ
    Icon for Contributor rankContributor
    2 years ago

    Hi,

    Lets take just one pin for example. Pin 1 refers to an LVDS pin, and yes both P and N of this pin is defined in QSF. My question is, lets say if I drive the LVDS pin as "logic 0" or "logic 1" or say I just toggle 1 and 0 in regular interval, I expect it to show in the acutal hardware when I probe.

    I am wondering if just declaring the signal as an output port in VHDL, and assigning the correct pin in qsf is just enough for LVDS?. or I should use any intel IP which has configurations to be able for the signal to propogate in LVDS pin.

    Thanks

    • FvM's avatar
      FvM
      Icon for Super Contributor rankSuper Contributor
      2 years ago

      Hi,
      check in the Quartus .pin file which pi s have been actually assigned.

      The general way is to use a single port pin in top level entity, assign the positive pin of an available pin pair to it in pin planner or assignment editor and specify LVDS IO standard. The negative pin will be assigned automatically, both pins are shown in pin planner.

  • MK_ABQ's avatar
    MK_ABQ
    Icon for Contributor rankContributor
    2 years ago

    Hi,

    Yes I double checked quartus pin planner and also fitter report and the pins looks okay and have been assigned correctly, but I am not able to see anything in the LVDS pin.

    So, I don't need an LVDS IP ?

  • _AK6DN_'s avatar
    _AK6DN_
    Icon for Frequent Contributor rankFrequent Contributor
    2 years ago

    LVDS (low voltage differential signaling) is normally implemented as a current source/sink between the P and N driver.

    So you need a (typical) 100ohm differential load at the receiver, and measure the voltage across that.

    So put 100ohms between your P and N outputs and measure the differential voltage across that as you drive a '1' and '0' internally.

    Measuring the unloaded driver pins is not generally meaningful as no current can flow thru an effective infinite load resistance.

    • FvM's avatar
      FvM
      Icon for Super Contributor rankSuper Contributor
      2 years ago

      You don't need to connect differential termination to see specified LVDS voltage range. Unterminated pins simply swing to VCCIO and GND for 1 and 0 state. In so far termination isn't required to see LVDS output working. There must be another elementary problem in your test, e.g. probing wrong FMC pins.

  • AqidAyman_Altera's avatar
    AqidAyman_Altera
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Hi,


    I seen that you accepted the answer as solution. I’m glad that your question has been addressed, I now transition this thread to community support. If you have a new question, Please login to ‘https://supporttickets.intel.com’, view details of the desire request, and post a feed/response within the next 15 days to allow me to continue to support you. After 15 days, this thread will be transitioned to community support. The community users will be able to help you on your follow-up questions.