Rxpolarity Txinhibit; Tx_Diff_Ctrl Pre_Emphasis; Loopback - Xilinx RocketIO User Manual

Miscellaneous Signals
TXPOLARITY,
RXPOLARITY,
TXINHIBIT
A differential pair has a positive-designated and a negative-designated component. If for some
reason the polarity of these components is switched between two transceivers, the data will not be
passed properly. If this occurs, TXPOLARITY will invert the definition of the TXN and TXP pins.
On the receiver side of the MGT, the RXPOLARITY port can invert the definition of RXN and RXP.
For some protocols, the MGT must turn off the TXN/TXP pins. The TXINHIBT port shuts off the
transmit pins and forces them to a constant value (TXN = 0, TXP = 1). Asserting TXINHIBIT also
disables internal serial loopback.
TX_DIFF_CTRL,
PRE_EMPHASIS
These two attributes control analog functionality of the MGT.
The TX_DIFF_CTRL attribute is used to compensate for signal attenuation in the link between
transceivers. It has five possible values of 400, 500, 600, 700, and 800 mV. These values represent
the peak-to-peak amplitude of one component of the differential pair; the full differential peak-to-
peak amplitude is two times these values.
The PRE_EMPHASIS attribute has four values—10%, 20%, 25%, and 33%—which are designated
by 0, 1, 2, and 3 respectively. Pre-emphasis is discussed in greater detail in
Design Considerations."

LOOPBACK

To facilitate testing without the requirement to apply patterns or measure data at gigahertz rates, two
programmable loopback features are available.
One option, serial loopback, places the gigabit transceiver into a state where transmit data is directly
fed back to the receiver. An important point to note is that the feedback path is at the output pads of
the transmitter. This tests the entirety of the transmitter and receiver.
The second loopback path is a parallel path that checks only the digital circuitry. When the parallel
option is enabled, the serial loopback path is disabled. However, the transmitter outputs remain
active and data is transmitted over the serial link. If TXINHIBIT is asserted, TXN is forced High and
TXP is forced Low until TXINHIBIT is de-asserted.
LOOPBACK allows the user to send the data that is being transmitted directly to the receiver of the
transceiver.
Table 2-23: LOOPBACK Modes
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004
Table 2-23 shows the three loopback modes.
Input
Mode
Value
Normal Mode
00
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Description
Normal Mode is selected during normal operation. The
transmitted data is sent out the differential transmit ports (TXN,
TXP) and are sent to another transceiver without being sent to its
own receiver logic. During normal operation, LOOPBACK
should be set to 00.
R
Chapter 3, "Analog
91