Other Important Design Notes - Xilinx RocketIO User Manual

R
Table 2-23: LOOPBACK Modes

Other Important Design Notes

Receive Data Path 32-bit Alignment
The RocketIO transceiver uses the attribute ALIGN_COMMA_MSB to align protocol delimiters
with the use of comma characters (special K-characters K28.5, K28.1, and K28.7 for most
protocols). Setting ALIGN_COMMA_MSB to TRUE/FALSE determines where the comma
characters appear on the RXDATA bus. When ALIGN_COMMA_MSB is set to FALSE, the comma
can appear in any byte lane of RXDATA in the 2- and 4-byte primitives. When
92
Input
Mode
Value
Internal Parallel
01
Mode
Internal Serial
10
Mode
TX PCS
TXDATA
RX PCS
RXDATA
Figure 2-28: Serial and Parallel Loopback Logic
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Chapter 2: Digital Design Considerations
Description
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
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.
TX SERIALIZER
PARALLEL
LOOPBACK = 01
MUX
RX DESERIALIZER
Figure 2-28.
SERIAL
LOOPBACK = 10
MUX
UG024_25_110503
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004
TXP/TXN
RXP/RXN