Xilinx RocketIO User Manual page 92

Product Not Recommended for New Designs
R
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
Value
92
Table 2-23
Input
Mode
Normal Mode
00
Internal Parallel
01
Mode
Internal Serial
10
Mode
www.xilinx.com
Chapter 2: Digital Design Considerations
shows the three loopback modes.
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.
Internal Parallel Mode allows testing the transmit and
receive interface logic PCS without having to go into the
PMA section of the transceiver, or to another transceiver.
See Figure 2-28.
Internal Serial Mode is used to check that the entire
transceiver is working properly, including testing of 8B/10B
encoding/decoding. This emulates what another
transceiver would receive as data from this specific
transceiver design.
Since the TXP/TXN pins are still being driven during this
loopback mode, PCB traces on these pins should be
terminated to remove reflections; otherwise, loopback bit
errors could result. Termination can be accomplished by any
of a variety of methods. Two examples:
• Connect SMA terminators on the TXP/TXN SMA
connectors (if applicable), or simply use 50Ω resistors on
the transmitter backplane pins.
• Connect the unterminated TXP/TXN to the RXP/RXN of
another instantiated transceiver, allowing its receiver
inputs to terminate the transmitter outputs.
RocketIO™ Transceiver User Guide
UG024 (v3.0) February 22, 2007