Using Differential Outputs; Differential Trace Layout Considerations - Xilinx Spartan-3A User Manual

Chapter 15: Expansion Connectors
behaves like an internal termination resistor of approximately 100Ω. On-chip differential
termination is only available on full I/O pairs, not on Input-only pairs.
Differential inputs are powered by the V
Differential inputs are available in any I/O bank.

Using Differential Outputs

Differential inputs are supported within any I/O bank. However, with Spartan-3A and
Spartan-3AN FPGAs, differential outputs are only supported on I/O Bank 0 or 2.
Differential outputs are powered by the respective I/O bank output voltage, V
Spartan-3A/3AN Starter Kit board, I/O Banks 0, 1, and 2 are all powered by a 3.3V supply.
I

Differential Trace Layout Considerations

Figure 15-6
differential I/O signal traces. These traces were routed for optimal signal integrity.
•
•
•
•
•
128
Pads for 100Ω
surface-mount resistor
FPGA
IBUFDS or
PAD
LxxP_0
BUFGDS
LxxN_0
a) External 100Ω termination resistor
(not provided on the Starter Kit)
Figure 15-4: Differential Input Termination Options
shows board layout extracted from the Starter Kit board that highlights the
All differential pairs are routed with matched 100Ω impedance on the top board layer
for maximum performance.
The traces were routed to avoid via where possible.
The trace lengths for differential pairs routed to a specific header (either the "Receive"
or "Transmit" header) were matched to within 0.25 inches.
The differential signals connections on the FPGA use the outer two ball rings to avoid
breakout congestion.
The "Receive" differential clock pair, highlighted in blue in
differential global clock input pair, GCLK7 and GCLK8. Using these global clock
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supply, which is 3.3V by default.
CCAUX
LxxP_0
Signal
LxxN_0
b) On-chip differential input termination
FPGA
PAD
OBUFDS
Signal
UG330_c12_06_072706
Figure 15-5: Differential Outputs
Spartan-3A/3AN Starter Kit Board User Guide
Differential termination
(~100Ω)
FPGA
IBUFDS or
PAD
BUFGDS
Signal
UG334_c15_04_052407
CCO
LxxP_2
LxxN_2
Figure 15-6, connects to a
UG334 (v1.0) May 28, 2007
R
. On the