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AR# 7937

Foundation 2.1i + SP2: How to instantiate LVDS I/O in Foundation Schematics

Description

Keywords: Foundation, fndtn, LVDS,Virtex-E

Urgency: Standard

General Description: With SP2 support for Virtex-E has been added to Foundation 2.1i. However the schematic libraries do not contain LVDS
I/O buffer symbols. How can LVDS pins be instantiated in schematics?

Solution

Instead of using LVDS buffers use regular IBUFS and OBUFS. Note that for LVDS inputs only one signal is needed whereas for LVDS
outputs both, the positive and the negative output (with an inverter) must be drawn.

In the ucf set the IOSTANDARD=LVDS attribute for all IBUFS and OBUFS that should be LVDS.

INST <INPUT_BUFFER reference name> IOSTANDARD = LVDS; # LVDS input
INST <OUTPUT_BUFFER_POSITIVE reference name> IOSTANDARD = LVDS; # POSITIVE LVDS OUTPUT
INST <OUTPUT_BUFFER_NEGATIVE reference name> IOSTANDARD = LVDS; # NEGATIVE LVDS OUTPUT

You can see the reference name in the schematic editor by double clicking on the buffer symbol.

For example, a valid line for an input buffer would be:

INST $I234 IOSTANDARD = LVDS;

Also, for all LVDS I/O the pin locations must be fixed. The implementation
tools can not automatically select an I/O pair.

A LVDS input must be LOCed to a positive LVDS pin. The negative pin is
selected by the tools. For LVDS outputs both, the positive and the negative
pins must be LOCed.

NET INPUT_LVDS LOC = A5; # LVDS INPUT, POSITIVE PIN LOCATION
NET OUTPUT_LVDS_POSITIVE LOC = D8; # LVDS POSITIVE OUTPUT PIN LOCATION
NET OUTPUT_LVDS_NEGATIVE LOC = C8; # LVDS NEGATIVE OUTPUT PIN LOCATION

Refer to the Virtex-E pinout tables to find the LVDS pin pairs.
AR# 7937
Date Created 10/26/1999
Last Updated 03/12/2002
Status Archive
Type General Article