Serdes Alignment; Overview; Serializer; Deserializer - Xilinx RocketIO User Manual

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SERDES Alignment

SERDES Alignment

Overview

Serializer

The multi-gigabit transceiver multiplies the reference frequency provided on the reference
clock input (REFCLK) by 20, or by 10 if half-rate operation is selected. Data is converted
from parallel to serial format and transmitted on the TXP and TXN differential outputs.
The electrical polarity of TXP and TXN can be interchanged through the TXPOLARITY
port. This option can either be programmed or controlled by an input at the FPGA core TX
interface. This facilitates recovery from situations where printed circuit board traces have
been reversed.

Deserializer

The RocketIO transceiver core accepts serial differential data on its RXP and RXN inputs.
The clock/data recovery circuit extracts clock phase and frequency from the incoming data
stream and re-times incoming data to this clock. The recovered clock is presented on
output RXRECCLK at 1/20 of the received serial data rate.
The receiver is capable of handling either transition-rich 8B/10B streams or scrambled
streams, and can withstand a string of up to 75 non-transitioning bits without an error.
Word alignment is dependent on the state of comma detect bits. If comma detect is
enabled, the transceiver recognizes up to two 10-bit preprogrammed characters. Upon
detection of the character or characters, RXCOMMADET is driven High and the data is
synchronously aligned. If a comma is detected and the data is aligned, no further
alignment alteration takes place. If a comma is received and realignment is necessary, the
data is realigned and RXREALIGN is asserted. The realignment indicator is a distinct
output. The transceiver continuously monitors the data for the presence of the 10-bit
character(s). Upon each occurrence of the 10-bit character, the data is checked for word
alignment. If comma detect is disabled, the data is not aligned to any particular pattern.
The programmable option allows a user to align data on plus-comma, minus-comma, both,
or a unique user-defined and programmed sequence.
The electrical polarity of RXP and RXN can be interchanged through the RXPOLARITY
port. This can be useful in the event that printed circuit board traces have been reversed.

Ports and Attributes

Comma definition can be accomplished using the attributes discussed below. This method
of definition makes the MGT extremely flexible in implementing different protocols.

ALIGN_COMMA_MSB

This attribute determines where the commas will reside in the parallel received data. The
comma indicates to the deserializer how to parallelize the data. However, with the
multiple data path widths available, the PCS portion must determine where to place the
comma in the parallel data bytes.
When ALIGN_COMMA_MSB is FALSE, the PCS may place the comma in any of the
RXDATA bytes. In the 1-byte mode, of course, there is only one location in which the
comma can be placed. In the 2-byte and 4-byte paths, some uncertainty exists as to which
byte will contain the comma, as shown in
RocketIO™ Transceiver User Guide
UG024 (v3.0) February 22, 2007
Table 2-12.
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