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Inverting LVDS Signals for Efficient PCB Layout in Spartan-3 Generation FPGAs

Posted: 13 Sep 2008 ?? ?Print Version ?Bookmark and Share

Keywords:Spartan-3 FPGAs? inverter? receiver datapath? accidental PCB trace swapping?

Differential signals, such as LVDS or LVPECL, can be difficult to route on simple, four-layer or six-layer PCBs without excessive use of vias. The reason has been because the positive pins on the driver must drive the corresponding positive pins on the receiver, and the negative pins must drive the receiver�s negative pins. Sometimes the traces end up with the wrong orientation, in effect, adding an inverter to the circuit.

This application note shows how Spartan-3 Generation FPGAs, with just the inclusion of an inverter in the receiver data path, can avoid excessive use of vias and fix accidental PCB trace swapping without requiring a PCB respin. The technique is equally applicable to the case where the FPGA is the driver, and swapping traces allows easier PCB routing to another device or to a connector.

View the PDF document for more information.

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