Xilinx RocketIO User Manual page 86

Product Not Recommended for New Designs
R
signals an error by asserting RXCRCERR High at the same time RXCHECKINGCRC goes
High. User logic determines the procedure that is invoked when a CRC error occurs.
FIBRE_CHAN
The FIBRE_CHAN CRC is similar to USER_MODE CRC
In FIBRE_CHAN, SOP and EOP are predefined protocol delimiters. Unlike USER_MODE,
FIBRE_CHAN does not need to define the attributes CRC_START_OF_PKT and
CRC_END_OF_PKT. Both USER_MODE and FIBRE_CHAN, however, disregard SOP and
EOP in CRC computation.
RXCHECKINGCRC
Designs should generate only the EOP frame delimiter for a beginning running disparity
(RD) that is negative. (These are the frame delimiters that begin with /K28.5/D21.4/ or
/K28.5/D10.4/.) Never generate the EOP frame delimiter for a beginning RD that is
positive. (These are the frame delimiters that begin with /K28.5/D21.5/ or
/K28.5/D10.5/.) When the RocketIO CRC determines that the running disparity must be
inverted to satisfy Fibre Channel requirements, it will convert the second byte of the EOP
frame delimiter (D21.4 or D10.4) to the value required to invert the running disparity
(D21.5 or D10.5).
Note that CRC generation for EOP requires that the transmitted K28.5 be left-justified in
the MGT's internal two-byte data path. Observing the following restrictions assures
correct alignment of the packet delimiters:
•
•
•
ETHERNET
The Ethernet CRC is more complex
neglected by the CRC. The extension bytes are special "K" characters in special cases. The
extension bytes are untouched by the CRC as are the Trail bits, which are added to
maintain packet length.
86
Note:
Data length must be greater than 20 bytes for USER_MODE CRC generation. For CRC
to operate correctly, at least four gap bytes are required between EOP of one packet and SOP of
the next packet. The gap may contain clock correction sequences, provided that at least 4 bytes
of gap remain after all clock corrections.
SOP
RXCRCERR
Figure 2-24: USER_MODE / FIBRE_CHAN Mode
4-byte data path: K28.5 must appear in TXDATA[31:24] or TXDATA[15:8].
2-byte data path: K28.5 must appear in TXDATA[15:8].
1-byte data path: K28.5 must be strobed into the MGT on rising TXUSRCLK2 only
when TXUSRCLK is High.
Note:
Minimum data length for this mode is 24 bytes, not including the CRC placeholder.
Note:
When CRC_FORMAT=FIBRE_CHAN, TX_CRC_USE must be set to TRUE. Otherwise,
occasional errors will occur in the transmitted data stream. RX_CRC_USE can be either TRUE
or FALSE in this usage.
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Chapter 2: Digital Design Considerations
DATA R R
0 1
(Figure 2-25). The SOP, EOP, and Preamble are
RocketIO™ Transceiver User Guide
(Figure 2-24), with one exception:
R R EOP
2 3
UG024_12_022803
UG024 (v3.0) February 22, 2007