Xilinx MIcroBlaze Development Spartan-3E 1600E Kit User Manual page 122

Chapter 15: Expansion Connectors
Table 15-2: Differential I/O Pairs (Continued)
Differential
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 100 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 120 . 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.
120
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Signal FPGA
Pair Name Pins
FX2_IP35 D12
15
FX2_IP36 C12
FX2_IP37 A15
16
FX2_IP38
FX2_
CLKIN
17
FX2_
D10
CLKOUT
Table 15-2.
Pads for 100Ω
surface-mount resistor
FPGA
PAD
LxxN_0
LxxP_0
a) External 100W termination resistor
Figure 15-3: Differential Input Termination Options
and Figure 15-5
FPGA Pin Name Direction DIFF_TERM
IP_L07N_0
IP_L07P_0
IP_L02N_0
B15 IP_L02P_0
IO_L11N_0/
E10
GCLK5
IO_L11P_0/
GCLK4
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
MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide
External
Resistor
Designator
Input
Input
Input
Input
I/O Yes
I/O Yes
Figure 15-3a. The
Table 15-2.
Differential termination
(~120Ω)
FPGA
PAD
UG257_15_03_060806
Table 15-2 indicates which resistor is
UG257 (v1.1) December 5, 2007
R
R208
R209
R210
Signal