Xilinx Virtex-5 RocketIO GTP, User Manual

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Virtex-5 RocketIO GTP Transceiver User Guide www.xilinx.com 209
UG196 (v1.3) May 25, 2007
REFCLK Guidelines
R
GTP Reference Clock Checklist
The following criteria must be met when choosing an oscillator for a design with GTP
transceivers:
•
Provide AC coupling between the oscillator output pins and the Virtex-5 dedicated
GTP_DUAL clock input pins.
•
Ensure that the differential voltage swing of the reference clock is the range as
specified in the
Virtex-5 Data Sheet (the nominal range is 200 mV-2000 mV, and the
nominal typical value is 1200 mV).
•
Meet or exceed the reference clock characteristics as specified in the Virtex-5 Data
Sheet .
•
Meet or exceed the reference clock characteristics as specified in the standard for
which the GTP transceiver provides physical layer support.
•
Fulfill the oscillator vendor’s requirement regarding power supply, board layout, and
noise specification.
•
Provide a dedicated point-to-point connection between the oscillator and GTP_DUAL
clock input pins.
•
Keep impedance discontinuities on the differential transmission lines to a minimum
(impedance discontinuities generate jitter).
Description
Oscillator Selection
Selecting an oscillator and designing a clock distribution system requires a careful
selection of components as well as a proper board layout to ensure that overall system
requirements are met.
When designing a clocking system for a design with a GTP transceiver, the requirements of
the implemented standard (Ethernet, OC-48, SDI, etc.) as well as the requirements given in
the GTP Transceiver section of the
Virtex-5 Data Sheet must be fulfilled. However, the
requirements for the GTP transceiver reference clock as specified in the Virtex-5 Data Sheet
must be met or exceeded. Under these conditions, the GTP transceiver was characterized
as specified in the
Virtex-5 Data Sheet .
The differential clock input of the GTP_DUAL primitive requires AC coupling capacitors
between the oscillator output pins and the dedicated clock input pins of the Virtex-5
device.
Sourcing More Than One Differential Clock Input Pair from One Oscillator
If a clock needs to be shared between more than seven GTP_DUAL primitives of a Virtex-5
device, more than one differential clock input pair is required. Either an oscillator with
multiple outputs or a single output oscillator and a multi-output clock buffer is required.
The connection between the dedicated clock input pin pair of a GTP_DUAL primitive and
the outputs of the oscillator or buffer MUST be a point-to-point connection. No bifurcated
transmission lines, “T-stubs”, branches, or daisy-chaining are permitted!