Clock Traces; Coupling; Dc Coupling; Ac Coupling - Xilinx Virtex-4 RocketIO User Manual

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When considering alternate clock sources, the alternate oscillators must meet or exceed the
specifications of the recommended oscillators. As with the regulator, allowances should be
made for both recommended and alternative clock sources to be populated, at least on the
initial board revision.
Depending on the application and its performance goals, it is possible to stray from the
clock source specifications, but in that case the specified performance of the MGT is not
guaranteed.

Clock Traces

As performance of the MGT is directly related to the quality of its reference clock, every
effort must be made to ensure the signal integrity of the clock traces from oscillator to
FPGA. Apply the same techniques for multi-Gb/s trace design in
Transitions"
If more than one clock source is driving a single reference clock differential pair, use a high-
speed switch. When multiple clock sources are bussed together on a single connection, the
signal integrity of the clock is not optimal due to the presence of stubs. Using a high-speed
switch makes every clock path point-to-point with one driver and one receiver.
One example is when an unpopulated oscillator shares the same trace to the clock input
pin as another clock source. The trace segment from the pad to the junction is a stub. Any
signal travelling down the segment is reflected entirely due to the infinite impedance
presented by the open end at the pad.
In applications requiring both the left and right columns of MGTs to be clocked with the
same clock source, use high-speed clock buffers for clock distribution to both left and right
column reference clock inputs. With one source and one sink for each trace, the presence of
branches causing reflections and degrading clock quality is removed. Xilinx recommends
ICS 8543BG as a high-speed clock buffer for clock distribution to both left and right column
reference clock inputs. For more information regarding ICS 8543BG, please visit
www.idt.com.

Coupling

DC Coupling

DC coupling can be used when the common mode ranges of the interconnecting devices
are the same. Any discrepancy in the common mode not only takes margin away from the
differential voltage swing but can also damage the device.

AC Coupling

The RocketIO MGT receivers have built-in DC blocking capacitors with programmable
bypass. AC coupling isolates the common modes of the two devices and is the preferred
configuration in hot-plug applications. The capacitor prevents any DC current from
flowing between connected devices.

External Capacitor Value Selection

If an external DC blocking capacitor is needed, it is important to select an appropriate
value. Series capacitance acts as a high-pass filter. A small value attenuates the lower
frequency components and subharmonics present in a signal. The period of the lowest
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
to these clock traces.
www.xilinx.com
Coupling
Chapter 11, "Design of
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