Using Differential Inputs - Xilinx Spartan-3E User Manual

Chapter 15: Expansion Connectors

Using Differential Inputs

LVDS and RSDS differential inputs require input termination. Two options are available.
The first option is to use external termination resistors, as shown in
board provides landing pads for external 100Ω termination resistors. The resistors are not
loaded on the board as shipped. The resistor reference designators are labeled on the
silkscreen, as listed in
bottom-side of the board, between the FPGA and the FX2 connector. The resistors are not
loaded on the board as shipped. External termination is always required when using
differential input pairs 15 and 16.
The second option, shown in
differential termination, which uses the DIFF_TERM attribute available on differential I/O
signals. Each differential I/O pin includes a circuit that behaves like an internal
termination resistor of approximately 120Ω . On-chip differential termination is only
available on I/O pairs, not on Input-only pairs like pairs 15 and 16 in
Figure 15-4
landing pads on the top and bottom side of the board.
associated with a specific differential pair.
118
Table 15-2.
Pads for 100Ω
surface-mount resistor
FPGA
PAD
LxxN_0
LxxP_0
a) External 100Ω termination resistor
Figure 15-3: Differential Input Termination Options
and Figure 15-5
Figure 15-4: Location of Termination Resistor Pads on Top Side of Board
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The landing pads are located on both the top- and
Figure 15-3b, is a Spartan-3E feature called on-chip
LxxN_0
Signal
LxxP_0
b) On-chip differential termination
show the locations of the differential input termination resistor
Spartan-3E Starter Kit Board User Guide
Figure 15-3a. The
Table 15-2.
Differential termination
(~120Ω)
FPGA
PAD
UG230_c12_03_022406
Table 15-2 indicates which resistor is
UG230_c12_04_022406
UG230 (v1.0) March 9, 2006
R
Signal