7–18
Chapter 7: High-Speed Differential Interfaces in the Cyclone III Device Family
Software Overview
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Longer traces have more inductance and capacitance. These traces must be as
short as possible to limit signal integrity issues.
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Place termination resistors as close to receiver input pins as possible.
Use surface mount components.
Avoid 90° corners on board traces.
Use high-performance connectors.
Design backplane and card traces so that trace impedance matches the impedance
of the connector and termination.
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Keep an equal number of vias for both signal traces.
Create equal trace lengths to avoid skew between signals. Unequal trace lengths
result in misplaced crossing points and decrease system margins as the
transmitter-channel-to-channel skew (TCCS) value increases.
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Limit vias because they cause discontinuities.
Keep switching transistor-to-transistor logic (TTL) signals away from differential
signals to avoid possible noise coupling.
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Do not route TTL clock signals to areas under or above the differential signals.
Analyze system-level signals.
f
For more information about PCB layout guidelines, refer to the High-Speed Board
Layout Guidelines and Guidelines for Designing High-Speed FPGA PCBs application
notes.
Software Overview
Cyclone III device family high-speed I/O system interfaces are created in core logic
by a Quartus II software megafunction because they do not have a dedicated circuit
for the SERDES. The Cyclone III device family uses the I/O registers and LE registers
to improve the timing performance and support the SERDES. Altera Quartus II
software allows you to design your high-speed interfaces using the ALTLVDS
megafunction. This megafunction implements either a high-speed deserializer
receiver or a high-speed serializer transmitter. There is a list of parameters in the
ALTLVDS megafunction that you can set to customize your SERDES based on your
design requirements. The megafunction is optimized to use Cyclone III device family
resources to create high-speed I/O interfaces in the most effective manner.
1
When you are using the Cyclone III device family with the ALTLVDS megafunction,
the interface always sends the MSB of your parallel data first.
f
For more information about designing your high-speed I/O systems interfaces using
the ALTLVDS megafunction, refer to the LVDS SERDES Transmitter/Receiver
(ALTLVDS_TX amd ALTLVDS_RX) Megafunction User Guide and the Quartus II
Handbook.
Cyclone III Device Handbook
Volume 1
December 2011 Altera Corporation
ALTERA [ ALTERA CORPORATION ]
