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A customer recently asked me if the SF3700, our latest flash controller, supports SATA Express and fired away with a bunch of other questions about the standard. The depth of his curiosity suggested a broader need for education on the basics of the standard.

To help me with the following overview of SATA Express, I recruited Sumit Puri, Sr. Director of Strategic Marketing for the Flash Components Division at LSI (SandForce). Sumit is a longtime contributor to many storage standards bodies and has been working with SATA- IO – the group responsible for SATA Express – for many years. He has first-hand knowledge of SATA- IO’s work.

Here are his insights into some of the fundamentals of SATA Express.

What is SATA Express?
Sumit: There’s quite a bit of confusion in the industry about what SATA Express defines. In simple terms, SATA Express is a specification for a new connector type that enables the routing of both PCIe® and SATA signals. SATA Express is not a command or signaling protocol. It should really be thought of as a connector that mates with legacy SATA cables and new PCIe cables.

Sumit Puri 

Why was SATA Express created?
Sumit: SATA Express was developed to help smooth the transition from the legacy SATA interface to the new PCIe interface. SATA Express gives system vendors a common connector that supports both traditional SATA and PCIe signaling and helps OEMs streamline connector inventory and reduce related costs.

What is the protocol used in SATA Express?
Sumit: One of the misconceptions about SATA Express is that it’s a protocol specification. Rather, as I mentioned, it’s a mechanical specification for a connector and the matching cabling. Protocols that support SATA Express include SATA, AHCI and NVME.

What are the form factors for SATA Express?
Sumit: SATA Express defines connectors for both a 2.5” drive and the host system. SATA Express connects the drive and system using SATA cables or the newly defined PCIe cables.

What connector configuration is used for SATA Express?
Sumit: Because SATA Express supports both SATA and PCIe signaling as well as the legacy SATA connectors, there are multiple configuration options available to motherboard and device manufacturers. The image below shows plug (a) which is built for attaching to a PCIe device. Socket (b) would be part of a cable assembly for receiving plug (a) or a standard SATA plug, and Socket (c) would mount to a backplane or motherboard fir receiving plug (a) or a standard SATA plug. The last two connectors are a mating pair designed to enable cabling (e) to connect to motherboards (d).

When will hosts begin supporting SATA Express?

Sumit: We expect systems to begin using SATA Express connectors early this year. They will primarily be deployed in desktop environments, which require cabling. In contrast, we expect limited use of SATA Express in notebook and other portable systems that are moving to cableless card-edge connector designs like the recently minted M.2 form factor. We also expect to see scant use of SATA Express in enterprise backplanes. Enterprise customers will likely transition to other connectors that support higher speed PCIe signaling like the SFF-8639, a new connector that was originally included in the SATA Express specification but has since been removed.

Will LSI support SATA Express?
Sumit: Absolutely. Our SF3700 flash controller will be fully compatible with the newly defined SATA Express connector and support either SATA or PCIe. Our current SF-2000 SATA flash controllers support SATA cabling used on SATA Express, but not PCIe.

Will LSI also support SRIS?
Sumit: PCIe devices enabled with SRIS (Separate Refclk Independent SSC) can self-clock so need no reference clock from the host, allowing system builders to use lower cost PCIe cables. SRIS is an important cost-saving feature for cabling that supports PCIe signaling. It doesn’t support card-edge connector designs. Today the SF3700 supports PCIe connectivity, and LSI will support SRIS in future releases of SF3700 and other products.

Why is it called SATA Express?
Sumit: SATA Express blends the names of the two connectors and captures the hybridization of the physical interconnects. The name reflects the ability of legacy SATA connectors to support higher PCIe data rates to simplify the transition to PCIe devices. SATA Express can pull double duty, supporting both PCIe and SATA signaling in the same motherboard socket. The same SATA Express socket accepts both traditional SATA and new PCIe cables and links to either a legacy SATA or SATA Express device connector.

How fast can SATA Express run?
Sumit: The PCIe interface defines the top SATA Express speed. A PCIe Gen2 x2 device supports up to 900 MB/s of throughput, a PCIe Gen3 x2 device up to 1800 MB/s of throughput – both significantly higher than 550 Mb/s speed ceiling of today’s SATA devices.

Is SATA Express similar to M.2?
Sumit: There are two key similarities. Both support SATA and PCIe on the same host connector, and both are designed to help transition from SATA to PCIe over time.

SATA Express delivers the future of connector speeds today
SATA Express was born of the stuff of all great inventions. Necessity. The challenge SATA-IO faced in doubling SATA 6 Gb/s speeds was herculean. The undertaking would have been too time-consuming to support the next-generation connection speeds that PCIe answers. It would have been too involved, requiring an overhaul of the SATA standard. Even in the brightest scenario, the effort would have produced a power guzzler at a time when greater power efficiency is a must for system builders. SATA-IO found a better path, an elegant bridge to PCIe speeds in the form of SATA Express.

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The term ”form factor” is used in the computer industry to describe the shape and size of computer components, like drives, motherboards and power supplies. When hard disk drives (HDDs) initially made their way into microprocessor-based computers, they used magnetic platters up to 8 inches in diameter. Because that was the largest single component inside the HDD, it defined the minimum width of the HDD housing—the metal box around the guts of the drive.

The height was dictated by the number of platters stacked on the motor (about 14 for the largest configurations). Over time the standard size of the magnetic patter diameter shrank, which allowed the HDD width to decrease as well. The computer industry used the platter diameter dimensions to describe the HDD form factors, and those contours shrank over the years. Those 8” HDDs for datacenter storage and desktop PCs shed size to 5” to today’s 3.5”, and laptop HDDs, starting at 2.5”, are now as small as 1.8”.

What defines an SSD form factor?
When solid state drives (SSDs) first started replacing HDDs, they had to fit into computer chassis or laptop drive bays (mounting location) built for HDDs, so they had to conform to HDD dimensions. The two SSDs shown below are form factor identical twins—without the outer casing—to 1.8” and 2.5” HDDs. The SSDs also use standard SATA connectors, but note that the SATA connector for 1.8” devices is narrower than the 2.5” devices to accommodate the smaller width.

Internal circuit board of 1.8″ and 2.5″ SSD without the case

 

However, there’s no requirement for the SSD to match the shape of a typical HDD form factor. In fact some of the early SSDs slid into the high-speed PCIe slots inside the computer chassis, not into the drive bays.  A PCIe® SSD card solution resembles an add-in graphic card and installs the same way in the PCIe slot since the physical interface is PCIe.

SSD manufactured as a PCIe expansion card

 

The largest component of an SSD is a flash memory chip so, depending on how many flash chips are used, manufacturers have virtually limitless options in defining dimensions. JEDEC (Joint Electronic Device Engineering Council) defines technical standards for the electronic industry including SSD form factors. JEDEC defined the MO-297 standard, which establishes parameters for the layout, connector locations and dimensions of 54mm x 39mm Serial ATA (SATA) SSDs, so they can use the same connector as standard 2.5” HDDs, but fit into a much smaller space.

MO-297 with standard width (2.5”) SATA connector

 

The most important element of an SSD form factor is the interface connector, the conduit to the host computer. In the early days of SSDs, that connector was typically the same SATA connector used with HDDs. But over time the width of some SSDs became smaller than the SATA connector itself, driving the need for new connectors.

Standard SATA connector is too large for small form factors

Card edge connectors – the part of a computer board that plugs into a computer – emerged to enable smaller designs and to further reduce manufacturing and component costs by requiring the installation of only a single female socket on the host as a receptor for the edge of the SSD’s printed circuit board. (The original 2.5” and 1.8” SSD SATA connector required both a male and female plastic connector to mate the SSD to the computer).

With standardization of these connectors critical to ensuring interoperability among different manufacturers, a few organizations have defined standards for these new connectors. JEDEC defined the MO-300 (50.8mm x 29.85mm), which uses a mini-SATA (mSATA) connector, the same physical connector as mini PCI Express, although the two are not electrically compatible. SSD manufacturers have used that same mSATA edge connector and board width, but customized the length to accommodate more flash chips for higher capacity SSDs.

MO-300 mSATA (left) and custom-length mSATA (right)

 

In 2012 a new, even smaller form factor was introduced as Next Generation Form Factor (NGFF), but was later renamed to M.2. The M.2 standard defines a long list of optional board sizes, and the connector supports both SATA and PCIe electrical interfaces. The keyways or notches on the connector can help determine the interface and number of PCIe lanes possible to the board. However, that gets into more details than we have space to cover here, so we will save that for a future blog.

M.2 form factor supports PCIe and SATA, with many board size options

Apple® MacBook Air® and some MacBook Pro systems use an SSD with a connector and dimensions that closely resemble those of the M.2 form factor. In fact Apple MacBook systems have used a number of different connectors and interfaces for its SSD over the years. Apple used a custom connector with SATA signals from 2010 through 2012 and in 2013 switched to a custom connector with PCIe signals. 

 

MacBook Air SSDs with custom SATA and PCIe connectors

 

 

In some cases, standard SSD form factor configurations are not an option, so SSD manufacturers have taken it upon themselves to create custom board and interface configurations that meet those less typical needs.

Various custom SSD form factors and connectors

And finally there’s the ubiquitous USB-based connection. While USB flash drives have been around for nearly a decade, many people do not realize the performance of these devices can vary by 10 to 20 times. Typically a USB flash drive is used to make data portable—replacing the old floppy disk. In those cases the speed of the device is not critical since it is used infrequently.

Now with the high speed USB 3 interface, a SATA-to-USB 3 bridge chip, and a high performance flash controller like the LSI® SandForce® controller, these external devices can operate as a primary system SSDs, performing as fast as a standard SSD inside the system. The primary advantages of these SSDs are removability and transportability while providing high-speed operation.

USB host interface containing high-speed SSDs

If there’s one constant in life, it’s demand for ever smaller storage form factors that prompt changes in circuit layout, connector position and, of course, dimensions. New connectors proposed for future generations of storage devices like the SFF-8639 specification will enable multiple interfaces and data path channels on the same connector. While the SFF-8639 does not technically define the device to which it connects, the connector itself is rather large, so the form factor of the SSD will need to be big enough to hold the connector. That’s why the primary SFF-8639 market is datacenters that use back-plane connectors and racks of storage devices. A similar connector – like SFF-8639, very large and built to support multiple data paths – is the SATA Express connector. I will save the details of that connector for an upcoming blog.

The sky’s the limit for SSD shapes and sizes. Without a spinning platter inside a box, designers can let their imaginations run wild. Creative people in the industry will continue to find new applications for SSDs that were previously restricted by the internal components of HDDs. That creativity and flexibility will take on growing importance as we continue to press datacenters and consumer electronics to do more with less, reminding us that size does in fact matter.

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