LVDS problems (Cyclone IV)

There are no issues connecting two FPGAs directly, on the same or different boards, using LVDS, providing you've terminated the lines as necessary. Unless you're driving a significant distance there's no need for any external transceivers.

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strange behavior that is reflected by low impedance (few ohms) between the LVDS signal (usually the negative line) and GND

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How are you measuring this? Assuming the board(s) is(are) off then the FPGA isn't going to know that it's supposed to be driving/receiving LVDS. However, you certainly shouldn't be measuring a low impedance to GND from either trace. So, I'd be looking for a board/build issue. If your board is on and operating then you're likely to be able to detect a low impedance path to GND depending on the state of the lines - that's more or less how LVDS works.

Cheers,

Alex

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Ideally there should not be issue interfacing since both are same device family and with similar LVDS IO standard. Have you had a chance to check on the electrical values ie VOD, VID, VCM at the TX and RX of both devices to see if still within the specs?

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Thanks. We have checked the voltage values and they all seem to be within spec

The measuring was done of course with no power using a DVM.

Replacing the Altera component usually solves the issue, but we still don't understand the behavior because the phenomena repeats on different boards and on different LVDS lines

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The measuring was done of course with no power using a DVM.

Replacing the Altera component usually solves the issue, but we still don't understand the behavior because the phenomena repeats on different boards and on different LVDS lines

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If replacing the FPGA fixes the low impedance pin then you must be causing damage to the pins. I'm always very wary of connecting FPGAs on different boards directly, or exposing any FPGA pins directly to external connectors for that matter. I'm assuming these two boards are not in the same chassis with the same power supplies and ground planes? Depending on the setup between the boards you could easily get an inadvertent over-voltage situation on those lines.

Can you give more information about the physical setup between the boards?

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If replacing the FPGA fixes the low impedance pin then you must be causing damage to the pins. I'm always very wary of connecting FPGAs on different boards directly, or exposing any FPGA pins directly to external connectors for that matter. I'm assuming these two boards are not in the same chassis with the same power supplies and ground planes? Depending on the setup between the boards you could easily get an inadvertent over-voltage situation on those lines.

Can you give more information about the physical setup between the boards?

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The two boards are not on the same chassis but one of the boards is supplying power to the other, such that the power and ground levels should be the same.

Regarding the damage, in one of the cases we have replaced the FPGA and noticed that the FPGA that was removed is still having these low impedance pins.

Thanks,

Ilan.