Tiny hard disks and massive flash memory devices give designers a spicy array of storage choices.
"We want more storage!" it's the hue and cry heard from designers of portable devices. Flash memory and compact hard drives help meet that need with ever-larger capacities.
Flash memory is the low-power winner, but hard drives have the edge when it comes to capacity. Both have changed the way mobile devices are designed. They also have driven the growth of scores of devices, from cell phones to digital cameras to MP3 players.
Removable flash memory comes in a range of form factors, like those from SanDisk (Fig. 1). Commonly available products include Compact Flash, MultiMedia Card, Memory Stick, SD (secure digital), miniSD, SmartMedia, and xD-Picture.
Many cell phones have the Subscriber Identity Module (SIM). Although SIM cards typically use EEPROM, they're turning to flash as storage requirements grow. Kingston's DataTraveler II+ is an example of the USB flash memory drive (Fig. 2). PCMCIA storage cards are also available. In the next few years, however, ExpressCard likely will replace PCMCIA and give USB flash-memory drives a run for their money (see "ExpressCard Replaces PCMCIA," p. 69).
Non-removable storage uses the same technology as its removable counterpart. Non-removable storage tends to be found in devices like cell phones and MP3 players, where size is critical and a removable device slot takes up too much space.
Not all flash memory is alike. Flash arrays come with and without controllers. For example, Compact Flash contains a circuitry interface between the flash array and the outside world, unlike SmartMedia. The other choice is between NAND or NOR flash-memory technologies.
NOR flash started out in front because of its memory-style interface. It also had a faster read access time than NAND flash. This made NOR ideal for applications, because they could be executed in place (XIP). But beware of NOR's downsides. For instance, it takes longer to reprogram NOR flash than NAND. This isn't as important for applications that are rarely written, but it becomes an issue for data storage that changes more often. Also, when it comes to the number of erase cycles, NOR flash has a lower limit compared to NAND by a factor of 10. NOR erase cycles are on the order of 10k to 100k, which is more than enough for many applications, but definitely not for all.
NAND implementations required the application code to be copied to RAM first. Single-chip solutions like M-Systems' DiskOnChip combine NAND flash and SRAM to get the capacity of NAND and the flexibility of NOR (Fig. 3). This is one reason why NAND is gaining steam in new applications.
The DiskOnChip also adds an intelligent front end that hides the internal mapping of logical blocks to physical flash blocks to implement a wear-leveling algorithm. This is important because typically a flash chip is useless if one of its blocks wears out. For instance, if periodically changing data is stored in the first logical block 10 times as often as the other blocks change, then the life of the chip will be based on the use of the first block. Wear leveling essentially employs the least-used block for the next update. It takes a little juggling to move blocks around and keep track of what will be used next, but this is transparent to the host.
NAND also can erase smaller blocks, but this tends to be less of an issue for block-oriented storage devices. Byte-level erasures are usually more important in flash microcontrollers that utilize flash for nonvolatile data storage or for making minor changes to an application. It can make a difference in storage applications if data is packed in very small blocks.
Both NAND and NOR technologies encounter the problem of bit flipping, where data changes inadvertently. It's not all that common, but it occurs more with NAND. Error-detection/error-correction (EDC/ECC) bits enable access algorithms to detect errors, which also is more likely to happen with NAND devices. It can be implemented by the host as well, but intelligent flash memory can make this feature operate transparently.
Large arrays of anything tend to raise the probability of problems with one or more blocks. A host can handle bad block remapping or, again, an intelligent flash device can handle this transparently. A hard disk typically works this way, too.
The importance of handling errors is usually based on the type of data. Application code needs very reliable storage, whereas a few errors in multimedia data often will go unnoticed as the problems occur for only a fraction of a second.
In addition, NAND holds the edge when it comes to price and capacity. The 2-Gbyte USB flash drives are NAND-based. NOR is smaller by a factor of eight. NAND also costs less, even when adding in front-end logic to make the NAND flash easier to use.
Developers tend to prefer single-chip solutions whenever possible. At this point, high-end NOR flash capacities are on the order of 512 Mbytes. NOR also dominates many handheld devices like cell phones. Likewise, NOR developers such as Intel, AMD, and Fujitsu are pushing NOR capacity with multiple bits per cell and low-power, 1.8-V operation. Intel's StrataFlash Multi-Level Cell technology delivers two bits per cell.
Embedded developers need to talk with a range of vendors to find the best fit for their application. Flash vendors tend to use one technology, and their solution won't always match your design requirements. In many instances, rotating magnetic media may better meet capacity requirements.
DOING IT THE HARD WAY
In the last couple of years, tiny hard drives have moved from novelty to mainstay. IBM and Hitachi started by putting a sub-1-in. hard disk into the Compact Flash form-factor MicroDrive. IBM has since left the hard-drive market. Now, there are more vendors with compact solutions than for their larger hard-disk cousins, such as Cornice, Western Digital, and Hitachi Global Storage Technologies.
USB flash drives were quick to catch up and exceed the initial capacities of 1-in. hard drives. But while the upper limit of flash is about 2 Gbytes, hard-drive vendors have pushed their capacity into the 5-Gbyte region.
Seagate's ST-1 is one of the first entrants in this arena (Fig. 4). It's the basis for one of Seagate's consumer products, the USB 2.0 Pocket Hard Disk (Fig. 5). Like a USB flash drive, it draws all of its power from the USB connection. But hard disks tend to draw more power than flash. Even so, this type of hard drive has led the way for high-end multimedia players like Apple's iPod (see "Inside iPod," electronic design, Jan. 20, 2005, p. 30, ED Online 9500).
These general-availability, low-cost compact hard drives force vendors to provide unique features to differentiate their products. SoniqCast's Aireo 2 sports a 1.8-in., 20-Gbyte drive, but its claim to fame is a built-in 802.11b wireless link for remote synchronization (Fig. 6). The FM transmitter and receiver don't hurt either.
Even with rising capacities and even more rugged drives, designers still must contend with problems. Most hard-disk-based MP3 players have a significant amount of RAM for two reasons. First, it provides skip-free operation by minimizing the time that the hard disk is running. Second, it minimizes the power required to drive the hard disk.
Cornice, like most hard-disk vendors, is addressing the problems with its Crash Guard technology. For example, the preferred head position when the drive is idle is no longer over an unused track, as with other hard disks. Instead, the area is away from the drive's platter so the heads don't crash if the drive is dropped. Likewise, movement detection and crash prevention techniques are helping to minimize damage. This adds to the controller's complexity, but it also significantly increases the disk's reliability and lifetime.
As expected, the tiny drives' hard-disk capacity is smaller than the capacities of their larger siblings. Unfortunately, so are their speed and throughput. Performance tends to be limited by power, which most designs attempt to reduce. Rotational speed is on the order of 3600 rpm versus 15,000 rpm for high-end hard drives. The throughput of a new Serial ATA drive is on the order of gigabits per second, leaving the tiny drives in the dust. Such small drives are more than sufficient for streaming audio and video, but they're a poor replacement for a desktop hard drive.
Heat remains a problem that's not much of an issue for flash alternatives. Cutting down on runtime helps, but heat dissipation is now part of a portable device's design process.
Tiny hard-disk interfaces have essentially been proprietary due to the drive's compact size. To achieve that small size, the number of connections had to be minimized. Unfortunately, this makes alternate source a problem. It also increases the complexity of the system design. This was less of an issue when the drives first came out because they tended to be custom-made for a small number of designs.
The forthcoming CE-ATA standard should alleviate the compatibility problem (see "CE-ATA: Tiny Hard-Disk Standard," below). Although based on the MultiMedia Card (MMC) interface, it uses a subset of the ATA commands because MMC was initially defined for flash memory. The CE-ATA interface will require its own device driver, but software programmers will have familiarity with the hard-disk interface.
It should be interesting to see if a standard electrical interface will emerge. Most interfaces at this point are custom and use a flexible cable. There are no surface-mount drives, and most removable drives will employ USB or an existing, intelligent form factor such as Compact Flash or ExpressCard.
TECHNOLOGIES LYING IN WAIT
Flash memory and hard drives aren't the only high-capacity, nonvolatile storage technologies. Ferroelectric RAM (FRAM) and magnetic RAM (MRAM) are two alternatives that have bounced on the horizon for many years. They offer better performance, lower cost, and higher capacity. The big problem is that they aren't here yet. They will have to build or fit into the existing infrastructure. That takes time, of course, so don't count on them in the near term.
Until then, developers will have enough choices to improve existing products and come up with new products that weren't possible without high-capacity, nonvolatile storage. Hard-disk capacities and performance are expected to rise keeping them ahead of flash memory in terms of capacity and price per gigabyte. It will be up to designers to work the tradeoffs.
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