M.2

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For other uses, see M2.
Size comparison of mSATA (left) and M.2 (size 2242, right) SSDs

M.2, formerly known as the Next Generation Form Factor (NGFF), is a specification for internally mounted computer expansion cards and associated connectors. It replaces the mSATA standard, which uses the PCI Express Mini Card physical layout. M.2's more flexible physical specification that allows different module widths and lengths, together with more advanced features, makes the M.2 more suitable for solid-state storage applications in general, especially when used in small devices like ultrabooks or tablets.[1]

Computer bus interfaces provided through the M.2 connector, together with supported logical interfaces, are a superset to those defined by the SATA Express interface. Essentially, the M.2 standard is a small form factor implementation of the SATA Express interface (which provides support for PCI Express 3.0 and Serial ATA 3.0), with the addition of an internal USB 3.0 interface. The M.2 connector can have different keying notches that denote various uses of M.2 modules.[1][2]

Features[edit]

A high-level overview of the SATA Express software architecture, which supports both legacy SATA and PCI Express storage devices, with AHCI and NVMe as the logical device interfaces.[3]:4

Multiple functions are supported for add-in cards, including the following device classes: Wi-Fi, Bluetooth, satellite navigation, near field communication (NFC), digital radio, Wireless Gigabit Alliance (WiGig), wireless WAN (WWAN), and solid-state drives (SSDs). Exposed buses are PCI Express 3.0, Serial ATA (SATA) 3.0 and USB 3.0, which is backward compatible with USB 2.0. The SATA revision 3.2 specification, in its gold revision as of August 2013, standardizes the SATA M.2 as a new format for storage devices and specifies its hardware layout.[4][5]

The M.2 specification provides four PCI Express lanes and one SATA 3.0 (6 Gbit/s) port, exposed through the same connector, allowing use of both PCI Express and SATA storage devices in form of M.2 cards. Exposed PCI Express lanes provide a pure PCI Express connection to the storage device, without any additional layers of bus abstraction.[6] PCI-SIG M.2 specification, in its revision 1.0 as of December 2013, provides detailed M.2 specifications.[7]

There are three options available for the logical device interfaces and command sets used for interfacing with M.2 storage devices, provided as features of the SATA Express storage interface:[6][3]

Legacy SATA
Used for SATA SSDs, and interfaced through the AHCI driver and legacy SATA 3.0 (6 Gbit/s) port exposed through the M.2 connector.
SATA Express using AHCI
Used for PCI Express SSDs and interfaced through the AHCI driver and provided PCI Express lanes, providing backward compatibility with widespread SATA support in operating systems at the cost of not delivering optimal performance by using AHCI for accessing PCI Express SSDs. AHCI was developed back at the time when the purpose of a host bus adapter (HBA) in a system was to connect the CPU/memory subsystem with a much slower storage subsystem based on rotating magnetic media; as such, AHCI has some inherent inefficiencies when applied to SSD devices, which behave much more like DRAM than like spinning media.
SATA Express using NVMe
Used for PCI Express SSDs and interfaced through the NVMe driver and provided PCI Express lanes, as a high-performance and scalable host controller interface designed and optimized especially for interfacing with PCI Express SSDs. NVMe has been designed from the ground up, capitalizing on the low latency and parallelism of PCI Express SSDs, and fulfilling the parallelism of contemporary CPUs, platforms and applications. At a high level, the basic advantages of NVMe over AHCI relate to NVMe's ability to exploit parallelism in host hardware and software, based on its design advantages that include data transfers with fewer stages, greater depth of command queues and more efficient interrupts processing.

Form factors and keying[edit]

M.2 connector on a computer motherboard with the key in "M" position, supporting 2260 and 2280 sizes of M.2 cards

The M.2 standard is designed as a revision and improvement to the mSATA standard, allowing for larger printed circuit boards (PCBs) to be manufactured. While mSATA took advantage of the existing PCI Express Mini Card form factor and connector, M.2 has been designed to maximize usage of the card space while minimizing the footprint. As the result of the M.2 standard allowing longer modules and double-sided component population, M.2 devices can provide double the storage capacity within the footprints of mSATA SSD devices.

M.2 modules are rectangular, with an edge connector on one side (75 positions with up to 67 pins, 0.5 mm pitch, pins overlap on different sides of the PCB), and a semicircular mounting hole at the center of the opposite edge. Each pin on the connector is rated for up to 50 V and 0.5 A, while the connector itself is specified to endure up to 60 mating cycles. The M.2 standard allows module widths of 12, 16, 22 and 30 mm, and lengths of 16, 26, 30, 38, 42, 60, 80 and 110 mm. Initial line-up of the commercially available M.2 expansion cards is 22 mm wide, with varying lengths of 30, 42, 60, 80 and 110 mm.[1][2][8][9]

An M.2 module is installed into a mating connector provided by the host's circuit board, and a single mounting screw secures the module into place. Components may be mounted on either side of the module, with the actual module type limiting how thick components can be; maximum allowable thickness of components is 1.5 mm per side. Different host-side connectors are used for single- and double-sided M.2 modules, providing different amounts of space between the M.2 expansion card and the host PCB.[2][8] Host circuit boards usually support multiple lengths of M.2 modules, meaning that the sockets capable of accepting longer M.2 modules usually also accept shorter ones by providing different positions for the mounting screw.[10]

M.2 keying notches in B and M positions; overlapping of the pins on different sides of an M.2 module is also visible.[11]

PCB of an M.2 module provides a 75-position edge connector; depending on the type of module, certain pin positions are removed to present one or more keying notches. Host-side M.2 connectors may populate one or more mating key positions, determining that way the type of modules accepted by the host; as of April 2014, host-side connectors are available with only one mating key position populated (either B or M).[2][8][11] For example, M.2 modules with two notches in B and M positions use up to two PCI Express lanes and provide broader compatibility at the same time, while M.2 modules with only one notch in the M position use up to four PCI Express lanes; both examples also support SATA storage devices. Similar keying applies to M.2 expansion cards that utilize provided USB 3.0 connectivity.[2][11][12]

Various types of M.2 devices are denoted with WWLL-HH-K-K or WWLL-HH-K codes, where WW and LL specify the module width and length, respectively, in millimeters. Part HH specifies, in an encoded form, whether a module is single- or double-sided, and the maximum allowed thickness of mounted components; possible values are listed in the right table above. Module keying is specified by the K-K part, in an encoded form by using key IDs from the left table above; it can also be specified as K only, if a module has only one key.[2][8]

Beside socketed expansion cards, the M.2 standard also includes support for permanently soldered single-sided modules.[8]

See also[edit]

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References[edit]

  1. ^ a b c "SATA M.2 Card". SATA-IO. Retrieved September 14, 2013. 
  2. ^ a b c d e f g h "M.2 Connector (NGFF) Introduction" (PDF). orvem.eu. ATTEND. Retrieved January 17, 2014. 
  3. ^ a b Dave Landsman (August 9, 2013). "AHCI and NVMe as Interfaces for SATA Express Devices – Overview" (PDF). SATA-IO. Retrieved October 2, 2013. 
  4. ^ "Serial ATA Revision 3.2 (Gold Revision)" (PDF). knowledgetek.com. SATA-IO. August 7, 2013. Retrieved March 27, 2014. 
  5. ^ "SATA-IO FAQ" (PDF). SATA-IO. August 8, 2013. p. 2. Retrieved September 14, 2013. 
  6. ^ a b Paul Wassenberg (June 19, 2013). "SATA Express: PCIe Client Storage" (PDF). SATA-IO. Retrieved October 2, 2013. 
  7. ^ "PCI Express M.2 Specification Revision 1.0". PCI-SIG. 2013. Retrieved December 14, 2013. 
  8. ^ a b c d e f g "M.2 (NGFF) Quick Reference Guide" (PDF). Tyco Electronics. Retrieved November 16, 2013. 
  9. ^ "Intel SSD 530 Series Arriving Next Week – Feature NGFF M.2 Interface". WCCF Tech. Retrieved September 14, 2013. 
  10. ^ "M2P4S M.2 (NGFF) PCIe base SSD to PCIe x4 Adapter". hwtools.net. February 14, 2014. Retrieved June 22, 2014. 
  11. ^ a b c Marshall R. (April 7, 2014). "Buying an M.2 SSD? How to tell which is which?". Asus. Retrieved April 28, 2014. 
  12. ^ Les Tokar (November 24, 2013). "Understanding M.2 NGFF SSD standardization (or the lack of)". The SSD Review. Retrieved April 28, 2014. 

External links[edit]

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