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Altera_Forum
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10 years ago

LVDS problems (Cyclone IV)

Is there any problem with using direct LVDS connections between two Altera devices (Cyclone IV) residing on different boards (without external transceivers)? We are facing strange behavior that is...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
10 years ago

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