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

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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
When ALIGN_COMMA_MSB is TRUE, the PCS places the comma into the most significant byte
(MSB) of RXDATA in the 2-byte mode. Because the PCS is optimized for the 2-byte mode, some
68
Table 2-12.
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Chapter 2: Digital Design Considerations
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004