The speed of NAND flash memory has made a rapid progress. The ONFI v4.0 that is under development will use the 1.2VDDR3 transmission interface technology. Single channel flash transmission rate can reach the speed of 800 MB/s. As a result, solid state drive (SSD) products adopting SATAIII (6Gbps) as the transmission interface are about to face an insurmountable bottleneck.
Instead, the PCIe interface technology is promising. The introduction of the new M.2, SATA Express, PCIe x16 add-in card, and SFF-8639 are all new generation SSD products that the storage device industry are banking on to transcend the performance limitations.
SATA 6G reaching a bottleneck
Cc Wu , Innodisk's vice president of Embedded Flash Div. , discussed at a recent forum the application of the flash storage devices used by the industrial control sector as well as the evolution of the storage transmission interface specifications for IDE, SATA, and other interfaces. Wu indicated that the NAND flash manufacturing process has continued to progress each year from 60nm in 2006, 50nm in 2007, 42nm in 2008, 34nm in 2009, 27nm in 2010, 24nm in 2011, 21nm in 2012, and 19nm in 2013 and early 2014.
The current industrial mainstream storage devices still adopt the rewritable and high tolerance SLC made using processes between 42nm and 21nm. MLC has also begun to penetrate into the industrial application sector since 2012, manufactured at the 21nm to 19nm nodes. Because TLC has a smaller number of rewritable times and the price difference between TLC and MLC is not as great as that between SLC and MLC, TLC applications are still mostly for consumer electronics, and the industrial market has yet to adopt them. When TLC will find its way into the industrial market remains to be seen.
Judging from the evolution of flash performance, ONFI and Toggle mode have both become the mainstream flash interfaces. Under ONFI v3.1/Toggle mode 2.0, flash adopts the DDR2 interface transfer technology with transmission rates reaching 400MB/s (ONFI v3.1) or 533MB/s (ONFI v3.2), and the operating voltage reduced to merely 1.8V(SSTL_18). ONFI v4.0, which is being developed, will adopt the DDR3 transmission interface with the operating voltage dropping to 1.2V and the highest single-channel transmission rate reaching up to 800MB/s.
In terms of performance, if an ONFI 3.2 NV-DDR2 goes with a 16KB Page MLC and Cache Read/Cache Program, the read speeds of single-channel 1-die, 2-die, 4-die, and 8-die stacks can reach as high as 533MB/s, and the respective write speeds can reach 20MB/s, 41MB/s, 82MB/s, and 164 MB/s. As for 16KB Page SLC chips, the write speeds can reach 73MB/s, 146MB/s, 291MB/s, and 533MB/s (in full load) under the same Cache Read/Cache Program conditions. But now the 600MB/s rate of SATAIII (SATA 6G) is about to become a transmission bottleneck for flash chips. To develop SSDs with higher transmission speeds, we must turn to the next generation of higher-speed transmission interface as early as possible.
SATA remains as mainstream industrial interface, but set to give way to Mini PCIe/M.2
In terms of flash applications in the industrial sector, a significant number of SLC has been used in embedded devices, acting as the embedded operating systems' boot storage with capacity mostly under 2GB. When more than 32GB in storage capacity is needed, MLC is a more cost-efficient alternative.
When the storage capacity is less than 2GB and demand for storage is high, SLC can be used as an embedded OS hard disk to provide a higher degree of reliability and extend the service life for such products as industrial PCs, gaming machines, automation equipment, and military equipment. When the device needs storage of more than 32GB, MLC can be used as a data disks for storing application programs, enabling higher SSD capacity and providing better cost-efficiency for product segments including surveillance, POS, networking and digital signage.
Take an industrial motherboard for an example. The designs mentioned above allow for numerous connection interfaces to connect the storage devices. They include: PATA CF/IDE DOM (Disk on Module) that can connect to the conventional 40pin IDE interface; SSD with 1.8 inch SATA or 2.5 inch SATA; SATA DOM, mSATA, SATA Slim, Cfast, or CF-SATA memory cards that connect to the SATA interface; USB modules that connect to a 10-pin USB connector; SD/micro SD memory cards that can be read through a card reader; and the eMMC and uSSD single-chip disks that are built into the motherboards.
These various interfaces must have at least five years of product life in order to serve industrial purposes. In the past, newly designed motherboards would often keep the old transmission connection interfaces in use for several more years to give clients more time to transition to new interfaces. At the same time, multiple interfaces can cover a wide range of differentiated applications. Small-size connection interfaces are also required due to system space constraints. SATA remains as the mainstream transmission interface for industrial applications.
PCIe to become the ultimate solution
Cc Wu provided a chronicle of the SSD evolution in terms of form factors and transmission interfaces. In 1995, the CompactFlash (CF) had a transmission rate of only 8.3 MByte/s (PIO mode 2). In 1999, the Secure Digital (SD) specification was jointly developed by SanDisk, Matsushita and Toshiba. The SATA Rev 1.0a specification was launched in January 2003, which increased the transmission rate to 150MB/s (1.5Gbps). In 2003, the PCI-SIG organization premiered the PCIe 1.0a single plane (1x plane) achieving the transmission rate of 250MB/s (2.5Gbps). The SATA Rev 2.0 was unveiled in 2006, which improved the transmission rate to 300MB/s (3Gbps). In January 2007, PCI-SIG released the PCIe 2.0 specification whereby the single plane (1x plane) with a transmission rate up to 500MB/s (5Gbps).
In June 2008, the SATA Rev 3.0 interface boosted the transmission rate to 600MB/s (6Gbps). In 2009, JEDEC developed SATA Slim (MO-297A). In September 2009, the SATA-IO organization introduced the mSATA interface. In 2010, the PCIe Gen3 enabling single-plane 1GB/s (8GT/s) was launched. The SATA Express interface appeared in 2011 and Intel released the M.2 (NGFF) interface in 2012. The high-speed transmission interface evolution continues.
The PCI Express (PCIe) interface evolution started with PCIe 1.0,which was based on the 8b/10b encoding scheme with x1 transmission rate reaching 2.5GT/s (250MB/s) and the x16 transmission rate at 4GB/s. Then came PCIe 2.0, which was based on the 8b/10b encoding scheme boosting the x1 transmission rate to 5GT/s (500MB/s) and the x16 transmission rate to 8GB/s. PCIe 3.0 adopted the 128b/130b encoding principles, raising the x1 transmission rate to reach 8GT/s (1GB/s), and the x16 transmission rate to 16GB/s. PCIe 4.0 is expected to have an x1 transmission rate exceeding 16GT/s (2GB/s), and the transmission rate can reach 32GB/s in the x16 mode.
PCIe interface form factors and market trends
PCIe has diverse interface form factors. A Mini PCIe connector has 52 pins and works well with cloud computing and data centers; PCIe x16 is used in graphic cards and PCIe SSD emphasizes high IOPS; M.2(NGFF) launched by Intel in 2012 features three sizes - 2242, 2260 and 2280 - that are currently used in Ultrabooks.
The 2.5-inch SSD has adopted the SATAIII (600MB/s) interface. The SATA Express combines the conventional SATA and PCIe interfaces and provides x2 dual planes. PCIe 2.0/3.0 x2 can achieve 1GB/s and 2GB/s. Asustek has showcased motherboards adopting SATA Express, and their interfaces are compatible with conventional SATA cables and PCIe connectors.
Finally, the SFF-8639 interface is currently used in the back panels of the enterprise-grade 2.5-inch storage devices. It can connect to PCIe, SATA as well as SAS storage devices, and is compatible with SATA Express. SFF-8639 offers six PCLe lanes, but only a maximum of four can be used at a time. PCIe 2.0/3.0 can raise the transmission rate to 2GB/s and 4GB/s, respectively.
In terms of the industry support for SFF-8639, Dell's PowerEdge R820/R720 1U Servers are connected through PCIe HBA adapters at the backplane connection and expansion slots, and it is adopted by Apple's MacBook Air as well as some MacBook Pro models. However, it is still rarely seen in the industrial computers/motherboards. Micron has premiered the P320h 2.5-inch PCIe SSD that adopts the SFF-8639 connection interface. It supports the x4 PCIe Gen2 specification and its sequential read and write speeds can reach 1.75GB/s and 1.1GB/s, respectively.
SSDs developed with the PCIe interface in the form of x16 add-on cards include: OCZ's Z-Drive R4(PCIe-SATA+SATA SSD); Micron's P420m; Fusion IO's ioDrive; Seagate's X8 (Virident); and OCZ's Z-Drive R5 native PCIe SSD cards. They provide high IOPS that is most needed by servers. Their transmission rates can reach up to 1.5GB/s to 3GB/s, or even up to 6.5GB/s.
Industrial motherboards have miniPCIe slots of the 50.8mm (L) x 29.8mm (W) x 4.4mm (H) size. In the past, these slots were slated to accept Intel's Wi-Fi wireless LAN modules. Manufacturers have also modified them into mSATA slots with similar pin counts. M.2 (NGFF) can serve as a new generation of slot/interface specifications compatible with miniPCIe, USB, SDIO, UART, PCM, I2C, and SATA(mSATA) add-on cards. There are Socket 1 Type 1630, 2230, 3030; Socket 2 Type 2242, 3042, which support PCIe x2/SATA; and Socket 3 Type 2260, 2280, 22110, which support PCIe x4/SATA.
PCIe is poised to replace the existing SATAIII to become the future mainstream specification for storage device. Judging from the history of storage devices where a transition would usually take three to five years, Cc Wu expects that PCIe SSDs will become the mainstream by 2015. In addition, judging from the fact that Apple's MacBook Air and MacBook Pro have already adopted M.2 and SFF-8639, the transition of the SSD industry will continue to accelerate. Various SSD products adopting PCIe will start to appear in the market in the second half of 2014.
Cc Wu, Innodisk's vice president of Embedded Flash Div.