NAND Flash Memory in Embedded Systems

NAND Flash Memory in Embedded Systems
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New low-cost options are game changers for engineers

You can find NAND flash for your embedded application in a lot of different packages. If you are designing a new product you might look at it as two basic types—chips and modules. These two product types perform the same function, but with different packaging, power and performance. There are a confusing number of NAND storage options around these days. Let’s sort out a few of them.

The chip-type flash devices have SPI serial or 8-bit parallel interfaces and come in NAND or NOR versions. And then there is a choice between SLC and MLC NAND. We’ll focus on cost-effective MLC (two- or three-bit/cell) NAND. A NAND flash array is organized into many blocks. Each byte in one of these blocks can be individually written, but a single block is the smallest erasable portion of the array.

MLC Parallel NAND Flash

The S34ML08G1 by Spansion (Cypress) is a good example of a NAND chip with an x8 parallel I/O. This 8 Gbit, 3.3 V chip comes in 48-pin TSOP 12 x 20 or 63-ball BGA 9 x 11 packages and uses the Open NAND Flash Interface (ONFI) version 1.0. It’s fairly cost effective at less than $9, operates over -40° to 85° C, and has a 25 µs access time. The newer ONFI version 2.2 is the most widely used specification today for this type of device and was ratified back in October 2009. The even-newer ONFI 3 runs at twice the speed of ONFI 2, at 400 M transfers/second. The 3.0 specification was released in March 2011 and uses a 1.8 V supply. There are not a lot of 3.0 chips available; however, one good example is the Micron MT29F128G08CB, a 128 Gbit (16G X 8) ONFI 3.0 device.

  Spansion memory

Figure 1: Spansion memory uses ONFI compliant interface. (Source: www.spansion.com)

eMMC Flash Storage

Since they first started showing up, most smartphones and tablets have used eMMC flash storage. While similar to the NAND flash chips used in SSDs, the eMMC interface focuses on low power—around a half watt in read or write. eMMC devices are “managed NAND” chips that provide data handling and wear management functions that no engineer would want to handle if she/he didn’t have to. Perhaps most important, these chips have ECC and bad block management.

eMMC devices have seen improvements to read and write speeds over the years, going from 104 Mbytes/second (eMMC V4.41) to 200 Mbytes/second (V4.5) to 400 Mbytes/second (V5.0). eMMC is based on an 8-bit parallel interface, and the scaling of its interface performance is nearing its limits. The 16-Gbyte Spansion S40410161B1 eMMC IC is compatible with the JEDEC 4.51 specification. It uses a 3.3 V VCC and a selectable 1.8V/3.3V VCCQ power supply. Package options include the industry-standard 153 ball VFBGA (11.5 x 13 mm, 0.5 mm ball pitch) and the larger 100 ball LBGA (14 x 18 mm, 1.0 mm ball pitch).

Flash Memory in Embedded Systems Toshiba

Figure 2: Toshiba eMMC V5.1 memory. (Source: www.toshiba.com/tai)

Toshiba is now sampling their 16-Gbyte (THGBMBG7D2KBAIL) and 32-Gbyte eMMC Version 5.1 devices, with 64- and 128-Gbyte products to follow. Other Toshiba eMCC chips can be found here. Micron offers the MTFC8GACAANA-4M IT 8-Gbyte eMMC V4.5 chip in a 100-pin TBGA package. This device operates over -40° to 85° C with a supply voltage of 2.7 to 3.6 V. In sleep mode this chip takes only 180 µA.

Universal Flash Storage     

UFS—Universal Flash Storage—is looking to become a faster successor to eMMC. UFS uses a serial PCIe interface. There are two- and four-lane versions, and the technology is being led by Toshiba and Samsung. Two lanes provide an aggregate 5.8 Gbits/second (~725 Mbytes/second) bandwidth, and four lanes double that. UFS also brings full duplex operation (read and write simultaneously) and command queuing. It is a JEDEC-defined standard.

Samsung’s offering will come in 128-, 64- and 32-Gbyte versions. However, after all the talk about UFS, samples in 2013 and production 2014, there is not any general availability. Perhaps these chips are only in use in high-volume mobile devices at this point. Toshiba lists the THGLF2Gxxxxxx 32- and 64-Gbyte devices on their website, but to date, there doesn’t appear to be any availability.

SSD module

Of course you can use standard SSD—which I’m calling the “module” side of embedded storage. The 2.5-inch SATA drives are low-cost and easy to deal with, but the latest in SSDs in a small M.2 format are the ones to look at for embedded. With mSATA and NVME interfaces, these will knock your socks off. The M.2 format allows for three different module widths and lengths, with 22 x 80 mm, with 3.8 mm height, being the most popular. The size is about length of a gum stick. 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 mSATA form factor appeared briefly for a generation of motherboards and notebooks. mSATA SSDs follow the SATA III specification with a maximum performance of 6 Gbits/second and look much like mini-PCI-Express devices, but the two connectors are not inter-compatible. mSATA has been phased out and replaced with the better-designed M.2. There are also SATA Express format cards for motherboards with a faster 10 Gbit/second speed. However, it is unclear whether these will endure. The M.2 format and interconnect will likely endure, though. The M.2 connector can plug in both PCI-Express-based and SATA-based SSDs, but is generally PCI-Express-based only.

PCI Express SSDs with AHCI or NVMe

There are a number of M.2 PCI Express SSDs available. They all have the same M.2 connector with two lanes of PCI Express. Some have the AHCI (Advanced Host Controller Interface) software driver, providing backward compatibility with the widespread SATA support in operating systems. Using AHCI for accessing PCI Express SSDs is an easy upgrade that does not deliver optimal performance of PCIe interface. 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 slower storage subsystem based on a HDD, and as such has some inherent inefficiencies when applied to SSD devices.

Using the same PCI Express physical interface, some SSDs support the NVMe driver—a higher performance and scalable host controller software system that capitalizes on the low-latency and parallelism of PCI Express SSDs.

Some Example M.2 SSDs

Samsung’s SM951 M.2 SSD uses PCIe Gen 3 and comes with either ACHI or NVMe protocol and 512-, 256- and 128-Gbyte capacities. The NVMe version is said to provide very fast 2,260/1,550 Mbytes/second sequential read/write speeds (the 512 Gbyte version) and up to 300 K read IOPS, which is more than twice as fast as its AHCI-based equivalent and more than three times faster than a SATA-based SSD. 4 K random reads happen at 90 KIOPS, which is only a little better than most SSDs. The drive has adopted the L1.2 low-power standby mode, as defined by PCI-SIG, where power is reduced to less than 2 mW. It takes only 50 mW in the L1 low-power operational mode, and full-speed operation takes 6.5 W.

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