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Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
Chapter 7:
GTP Receiver (RX)
R
Configurable Termination Impedance
To minimize distortion due to reflections, the termination impedance of each receiver must
be matched as closely as possible to the impedance of the serial trace to which it is
connected. See
Chapter 10, “GTP-to-Board Interface,”
for more information about how the
termination impedance is calibrated.
Configurable Termination Voltage
As shown in
, the line termination circuit connects the RXP and RXN pads
through the calibrated line termination impedances to one of three termination voltages:
GND, MGTAVTTRX, or 2/3 MGTAVTTRX (from the GTP receiver). At any given time,
only one of these connections is active.
The appropriate RX termination voltage is selected based on the type of coupling used, the
TX termination voltage, and the requirements of the protocol being used.
summarizes the termination options available in the GTP receiver.
shows how to
activate the different RX termination options.
Table 7-3:
Recommended RX Termination Settings
Coupling Type
Valid RX
Termination
Settings
Notes
DC coupled
2/3 MGTAVTTRX,
MGTAVTTRX
The TX termination (and TX termination voltage)
must match the selected RX termination (and RX
termination voltage) to prevent DC currents.
2/3 MGTAVTTRX is optimal for the receiver.
External AC
coupling only
2/3 MGTAVTTRX
Recommended configuration because it permits
connections to the widest variety of transceivers
including hotswap applications.
Built-in AC
coupling only
MGTAVTTRX,
2/3 MGTAVTTRX,
GND
The RX termination (and RX termination voltage)
must match the TX termination (and TX termination
voltage) to prevent DC currents.
The built-in AC-coupling capacitors do not block a DC
current path between the RX termination of the
receiver and the TX termination of the transmitter.
Only an external AC-coupling capacitor will prevent a
DC current path between the transmitter’s TX
termination and the receiver’s RX termination.
Built-in AC
and external
AC coupling
MGTAVTTRX,
2/3 MGTAVTTRX,
GND
Use GND to support the PCI Express TXDETECTRX
feature. See
“PCI Express Receive Detect Support,”
.
Table 7-4:
RX Termination Attribute Settings
RX Termination Voltage
RCV_TERM_GND
RCV_TERM_VTTRX
MGTAVTTRX
FALSE
TRUE
2/3 MGTAVTTRX
FALSE
FALSE
GND
TRUE
FALSE