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.

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.

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.