U.2 Introduction (AN44)

Affected Products

All products

Preface

This Application Note introduces the SFF-8639 (U.2) interface. The goal is to give an overview of the technology and hint on how to design-in this technology for customer carrier boards.

Terminology

Term	Description
NVMe	Non-Volatile Memory Express
SSD	Solid State Drive
SAS	Serial-Attached SCSI
SATA	Serial ATA
SFF	Small Form Factor
OD	Open-Drain

U.2 / SFF-8639 Introduction

Introduction

The U.2 specification (initially SFF-8639), which is released by the PCI-SIG group, describes a hardware interface that supports several SSD interface technologies.

This specification is used mostly for connecting high performance NVMe SSDs. For overview of the NVMe protocol, visit www.congatec.com and view the technote "NVMe support on congatec COMs and SBCs" in the restricted area of the website.

To enable NVMe SSDs, the U.2 pinout features up to four PCIe lanes for inter-connection. Most often, this specification is used for SSDs in 2.5” form factor. This makes implementing the U.2 form factor a good method of scaling up the system speed while still being able to keep the same physical form factor for existing SATA-based SSDs.

In addition to PCIe-based SSDs, the interface also supports SAS and SATA. This enables the connector to work with different SSD technologies. 

This application note is concerned with U.2 specification pinout as well as topologies that can be used to design-in the interface.

Backward Compatibility

One of the advantages of this specification is that it allows SATA, SAS and U.2 NVMe SSDs to be used on the same U.2 connector. This means that systems can be equipped with different types of SSD, depending on the demands of the system. In that way, the system can be scaled to perform different tasks, thereby improving the overall price efficiency of the design.

Advantages

U.2 SSDs are often used in data centers and server-related ecosystems. The SSD housing of a U.2 SSD is bigger and more ruggedized when compared to the bare PCB M.2 SSDs. This also allows U.2 devices to feature a bigger overall capacity.

U.2 devices can support hot-plug and hot-swap features. This capability allows new devices to be connected to the platform (hot-plug) or devices of the same type can be swapped out (hot-swap) while the system is up and running. The following conditions need to be met to support these features:

1. The target SSD needs to support unplanned power loss.
2. The BIOS of the congatec module you want to use needs PCIe hot-plug support.

U.2 SSDs often feature advanced power loss protection. This means that in a surprise power loss event, mandatory data transactions that have been cached can still be completed, making the SSD more robust to corruption in unstable power environments.

Note:

If you are unsure about hot-plug support on your congatec module, contact your FAE.

Connector Overview

This chapter gives a general overview of the U.2 connector. Sub-section 2.1 highlights the overall U.2 connector while sub-section 2.2 lists the pinout of all supported interfaces.

U.2 Connector Overview

General U.2 Connector Pinout

The table below lists the pinout of interfaces that are supported with U.2

Please note that for SATA and SATA Express implementations on U.2, the below signal names do not match the SATA specification exactly.

Pin	Name	Type	PCIe NVMe	SATA	SATA Express	SAS x4
P1	WAKE#	Bi-Dir			Optional
P2	sPCIeRst	-
P3	PWRDIS	Output			Optional
P4	IfDet#	Input	Optional		Optional	Optional
P5	Ground	Ground
P6	Ground	Ground
P7	+5 V	Power
P8	+5 V	Power
P9	+5 V	Power
P10	PRSNT#	Input		Optional	Optional
P11	ACTIVITY#	Input		Optional	Optional
P12	Ground	Ground		Optional	Optional
P13	+12 V	Power
P14	+12 V	Power
P15	+12 V	Power
S1	Ground	Ground
S2	S0T+ (A+)	Diff-Pair
S3	S0T- (A-)	Diff-Pair
S4	Ground	Ground
S5	S0R- (B-)	Diff-Pair
S6	S0R+ (B+)	Diff-Pair
S7	Ground	Ground
S8	Ground	Ground
S9	S1T+	Diff-Pair
S10	S1T-	Diff-Pair
S11	Ground	Ground
S12	S1R-	Diff-Pair
S13	S1R+	Diff-Pair
S14	Ground	Ground
S15	Reserved	-
S16	Ground	Ground
S17	PETp1	Diff-Pair
S18	PETn1	Diff-Pair
S19	Ground	Ground
S20	PERn1	Diff-Pair
S21	PERp1	Diff-Pair
S22	Ground	Ground
S23	PETp2	Diff-Pair
S24	PETn2	Diff-Pair
S25	Ground	Ground
S26	PERn2	Diff-Pair
S27	PERp2	Diff-Pair
S28	Ground	Ground
E1	REFCLKB+	Diff-Pair	DualPort Only
E2	REFCLKB-	Diff-Pair	DualPort Only
E3	+3.3 Vaux	Power	Optional
E4	CLKREQ#/ PERSTB#	CLKREQ# is SSD OD-Output
E5	PERST#	Output
E6	Reserved	-
E7	REFCLK+	Diff-Pair			Optional
E8	REFCLK-	Diff-Pair			Optional
E9	Ground	Ground			Optional
E10	PETp0	Diff-Pair
E11	PETn0	Diff-Pair
E12	Ground	Ground
E13	PERn0	Diff-Pair
E14	PERp0	Diff-Pair
E15	Ground	Ground
E16	Reserved	-
E17	PETp3	Diff-Pair
E18	PETn3	Diff-Pair
E19	Ground	Ground
E20	PERn3	Diff-Pair
E21	PERp3	Diff-Pair
E22	Ground	Ground
E23	SMBCLK	Bi-Dir	Optional
E24	SMBDAT	Bi-Dir	Optional
E25	DualPortEn#	Output	DualPort Only

Topologies

Direct Connect Topologies

Topology 1 (left) connects U.2 module directly to the standard connector. In this topology, SATA, SAS and U.2 NVMe modules can be connected if the carrier board connector supports all U.2 interfaces. Topology 2 (right) uses a PCI Express to U.2 adapter to connect a U.2 NVMe module. This topology can be useful for testing SFF-8639 modules on existing carrier boards that have PCIe slots. The adapters currently on the market feature a PCIe x4 lane configuration to utilize the full capacity of the device.

Topologies with Cabling

Cabling sets on the market right now focus on reuisng existing SFF-8643 cables and connectors. These Mini-SAS HD cables are capable of carrying PCIe signals for operating U.2 NVMe modules. Topology 1 depicts this setup. The carrier baord features the SFF-8643 connector. An SFF-8643 to U.2 cable is used to connect the device. In addition, power is supplied to the U.2 module via a SATA power connector. The second topology depicts a direct connection via U.2 cable.

Note:

Direct-connect U.2 cables are currently in development.

Topologies 3 and 4 use the HD Mini-SAS to U.2 cable as described in the first topology. Both setups can be used to determine the function of your U.2 module in existing carriers by using adapters on existing interfaces. Adapters exist for M.2 as well as PCI Express interfaces.

Note:

Cabling and adapter solutions can negatively impact signal quality.