Xilinx Virtex-6 FPGA User Manual

256 www.xilinx.com Virtex-6 FPGA GTX Transceivers User Guide
UG366 (v2.5) January 17, 2011
Chapter 4: Receiver
RX Gearbox
Functional Description
The RX gearbox uses output pins RXDATA[31:0] and RXHEADER[2:0] for receiving data.
Similar to TX Gearbox, page 146, the RX gearbox operates with the PMA using a single
clock. Because of this, occasionally, the output data is invalid. Output pins
RXHEADERVALID and RXDATAVALID determine if the appropriate header and data are
valid. The RX gearbox only supports 2-byte and 4-byte interfaces. A 1-byte interface is not
supported.
The data out of the RX gearbox is not necessarily aligned. Alignment is done in the FPGA
logic. The RXGEARBOXSLIP port can be used to slip the data from the gearbox cycle-by-
cycle until the correct alignment is reached. It takes a specified amount of cycles before the
bitslip operation is processed and the output data is stable.
Descrambling of the data and block synchronization is done in the FPGA logic.
Ports and Attributes
Table 4-50 defines the RX gearbox ports.
Table 4-51 defines the RX gearbox attributes.
Table 4-50: RX Gearbox Ports
Port Dir Clock Domain Description
RXDATAVALID Out RXUSRCLK2 Status output when Gearbox 64/66 or 64/67 is used, which indicates that
the data appearing on RXDATA is valid. For example, during 64B/66B
encoding, this signal is deasserted every 32 cycles for the 4-byte interface
and every 64 cycles for the 2-byte interface.
RXGEARBOXSLIP In RXUSRCLK2 When High, this port causes the gearbox contents to slip by one bit. It is
used to achieve alignment with the FPGA logic. Asserting this port for
one RXUSRCLK2 cycle changes the data alignment coming out of the
gearbox.
When RXGEARBOXSLIP is asserted for more than one cycle, the gearbox
realigns the data once for each RXUSRCLK2 cycle that it is held High.
RXHEADER[2:0] Out RXUSRCLK2 Header outputs for 64/66 (1:0) and 64/67 (2:0).
RXHEADERVALID Out RXUSRCLK2 Indicates that the RXHEADER is valid when using the gearbox.
RXSTARTOFSEQ Out RXUSRCLK2 When Gearbox 64/66 or 64/67 is enabled, this output indicates when the
sequence counter is 0 for the present RXDATA outputs.
Table 4-51: RX Gearbox Attributes
Attribute Type Description
GEARBOX_ENDEC 3-bit Binary Gearbox Modes:
000: 64B/67B using external sequence counter
001: 64B/66B using external sequence counter
010: 64B/67B using internal sequence counter
011: 64B/66B using internal sequence counter
RXGEARBOX_USE Boolean When TRUE, this attribute enables the RX gearbox.
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Xilinx Virtex-6 FPGA Specifications

General

Technology	40nm
Clock Data Recovery (CDR)	Integrated
Logic Cells	Up to 760, 000
I/O Pins	Up to 1200
Transceiver Features	Pre-emphasis, equalization
Transceiver Protocol Support	PCIe, SATA, Ethernet, CPRI, OBSAI, Serial RapidIO
Power Consumption	Varies by model and configuration
Transceiver Type	Multi-Gigabit Transceivers (RocketIO GTP/GTX)

Summary

Preface: About This Guide

Guide Contents

Lists the chapters and appendices included in this manual.

Additional Documentation

Provides links to other Xilinx documents for further information.

Chapter 1: Transceiver and Tool Overview

Overview

Introduces the Virtex-6 FPGA GTX transceiver and its features.

Port and Attribute Summary

Summarizes GTX ports and attributes, grouped by functionality.

Simulation

Explains prerequisites and setup for simulating GTX transceiver designs.

Implementation

Details mapping GTX transceivers to device resources and UCF creation.

Chapter 2: Shared Transceiver Features

Reference Clock Input Structure

Describes the structure and ports for reference clock inputs.

Reference Clock Selection

Explains how to select and route reference clocks for GTX transceivers.

PLL

Details the Phase-Locked Loop (PLL) architecture and its settings.

Power Down

Describes the various power-down modes and capabilities of the GTX transceiver.

Loopback

Explains loopback modes for testing the transceiver datapath.

ACJTAG

Covers the ACJTAG interface support for GTX transceivers.

Dynamic Reconfiguration Port

Explains the DRP for dynamic parameter changes in GTXE1 primitive.

Chapter 3: Transmitter

TX Overview

Introduces the functional blocks and key elements of the GTX transmitter.

FPGA TX Interface

Describes the gateway for transmitting data to the GTX transceiver.

TX Initialization

Details the procedures for resetting and initializing the GTX TX.

TX 8B/10B Encoder

Explains the 8B/10B encoding scheme used for outgoing data.

TX Gearbox

Describes support for 64B/66B and 64B/67B encoding for high-speed protocols.

TX Buffer

Explains the TX buffer's role in resolving phase differences between domains.

TX Buffer Bypass

Covers the advanced feature of bypassing the TX buffer for reduced latency.

TX Pattern Generator

Details the PRBS and other patterns for testing signal integrity.

TX Oversampling

Explains the built-in 5X oversampling feature for serial rates.

TX Polarity Control

Describes the function to invert outgoing data polarity before transmission.

TX Fabric Clock Output Control

Details the serial and parallel clock divider control for TX fabric clocks.

TX Configurable Driver

Explains the high-speed current-mode differential output buffer features.

TX Receiver Detect Support for PCI Express Designs

Describes the feature for detecting receiver presence on a link.

TX Out-of-Band Signaling

Covers support for SATA/SAS OOB sequences and PCI Express beaconing.

Chapter 4: Receiver

RX Overview

Introduces the functional blocks and key elements of the GTX receiver.

RX Analog Front End

Describes the high-speed current-mode input differential buffer.

RX Out-of-Band Signaling

Covers support for decoding SATA/SAS OOB sequences and PCI Express beacons.

RX Equalizer

Explains the circuit for compensating high-frequency losses in the channel.

RX CDR

Details the Clock Data Recovery circuit for extracting clock and data.

RX Fabric Clock Output Control

Covers serial and parallel clock divider control for RX fabric clocks.

RX Margin Analysis

Discusses methods for determining link quality via eye diagrams.

RX Polarity Control

Describes the function to invert incoming data polarity.

RX Oversampling

Explains the built-in 5X oversampling for low serial rates.

RX Pattern Checker

Details the built-in PRBS checker for testing channel signal integrity.

RX Byte and Word Alignment

Explains the process of aligning serial data to byte boundaries.

RX Loss-of-Sync State Machine

Describes the state machine for detecting channel malfunction.

RX 8B/10B Decoder

Explains the decoder for RX data, indicating errors and control sequences.

RX Buffer Bypass

Covers the advanced feature of bypassing the RX elastic buffer for low latency.

RX Elastic Buffer

Explains the buffer for resolving clock domain differences.

RX Clock Correction

Details the circuit for tolerating frequency differences between clock domains.

RX Channel Bonding

Describes using the RX elastic buffer to cancel skew between lanes.

RX Gearbox

Describes support for 64B/66B and 64B/67B encoding for high-speed protocols.

RX Initialization

Details the procedures for resetting and initializing the GTX RX.

FPGA RX Interface

Describes the interface for receiving RX data from the GTX RX.

Chapter 5: Board Design Guidelines

Overview

Discusses implementing GTX transceivers on a PCB for optimal performance.

Pin Description and Design Guidelines

Describes GTX transceiver pins and provides design guidelines.

Termination Resistor Calibration Circuit

Explains the circuit for calibrating termination resistors.

Analog Power Supply Pins

Details the MGTAVCC and MGTAVTT analog power supply pins.

Reference Clock

Focuses on the selection criteria for reference clock sources.

Power Supply Distribution Network

Discusses issues regarding power supply implementation on the PCB.

Crosstalk

Explains how crosstalk degrades GTX transceiver performance and how to avoid it.

SelectIO Usage Guidelines

Provides guidelines for SelectIO interface usage to minimize GTX impact.