Tiny Disk Drives: A New Spin on Embedded Data Storage

It has never been easy to think of disk drives as components for embedded systems. Granted, disk drives have been going into large, industrial-type embedded systems for years, and several million small drives have gone into portable music players, such as Apple's iPod, in the last couple of years. But despite the smashing success of the iPod and its copycats, there is still a popular perception that disk drives are too bulky, too fragile, and too power hungry for embedded applications in general.

The 1-inch disk drive is changing that notion, however. Small as a matchbook, easy on power, and more rugged than you might think, the 1-inch disk unit (Figure 1) is thriving in the rough-and-tumble world of portable music players. Strapped to the arm of a jogger, it keeps working. With 1 to 5 Gbytes of storage, it holds hundreds or even thousands of songs, and its small size helps make some disk-based music players barely larger than flash-memory models. And applications for the small drives are increasing. By the end of the year, you can expect to see them in cell phones, with other embedded applications soon to follow.

Figure 1:  1-inch disk units are about the size of a matchbook and weigh well under an ounce

Surprisingly, perhaps, 1-inch disk units easily tolerate the jostling they undergo on the arm or hip of a jogger. When a Microdrive from Hitachi Global Storage Technologies first went into a music player, Hitachi application engineer Bill Heybruck provided six of the players to aerobics instructors for an acid test, and none failed. "Jogging is not that much of a shock," says Heybruck. "It's dropping it where you have the problem."

If jogging isn't a big problem, though, it's at least partly because disk makers have taken measures to accommodate it. In the development of Cornice's 1-inch Storage Element, for example, engineers created what they call a jog profile--characterizations of accelerations and vibrations typically encountered during jogging. Then, says Cornice executive vice president Scott Holt, "We did 2X of that to create a guardband, and we made sure that our servo system was stiff enough to withstand the kinds of vibrations we're going to have."

Seagate Technology, a new entrant in the 1-inch drive market, provides still more jogging protection with its RunOn technology. RunOn, using a motion sensor, identifies harmonic frequencies introduced into Seagate's ST1 drive by jogging or other physical activities. Because these harmonic frequencies can induce unwanted motion of the read and write heads, RunOn predicts wayward motion and compensates in the drive's servo to help keep the heads on track.

Taking the Hits Hard impacts are a different matter, however. Small disk drives typically can survive a 1-meter drop onto bare concrete while powered off, but that's assuming that they're embedded in a music player or other application with adequate shock mounting. Disk manufacturers, therefore, often work with OEM customers to help ensure that shock mounting is up to snuff.

For cell phones, some of which will soon contain 1-inch disk drives as storage for the digital cameras and music players that they include, falls can be especially hazardous to a drive. For one thing, cell phones are hand-held much more often than portable music players are and thus are more likely to be dropped. Also, a cell phone dropped from ear level by a standing person usually falls more than a meter, and that's risky. "If you're going to be in cell phones," says Cornice's Holt, "you better be a meter and a half, at least, onto concrete." Holt says Cornice drives will soon be well in excess of the current 1-meter drop tolerance.

Disk drives are also more vulnerable to drop damage when they're operating. A nonoperating 1-inch drive, for example, might survive a 2000-G impact, whereas an operating drive can be vulnerable at 200 to 400 Gs. Because of this vulnerability, designers who embed the drives in portable devices try to ensure that the disks need to be powered up and spinning as little as possible.

In portable music players, fortunately, disk drives can be powered off almost all of the time. A disk needs to spin for only a few seconds in order to read several minutes of music, and once the music goes into a memory buffer, the disk drive can turn off. As it does, it parks the read and write heads on a ramp away from the disk platter, and a solenoid latches them in place. Once latched, the heads are safe from most impacts.

Nevertheless, a long fall and a resulting severe jolt can still result in damage, because a very hard impact can unlatch the read and write heads and send them flying out over the disk platter. Then, if the super-smooth surface of a head comes into contact with the super-smooth disk surface, a process called "stiction" comes into play, and the head gets stuck. "It's like a sheet of paper falling on a nicely waxed floor," says Hitachi's Heybruck. "You try to slide that sheet of paper, and you can't." The disk drive thus becomes inoperable.

Perhaps surprisingly, the extremely thin, glass disk platter in a small disk drive seldom breaks in a fall. What can happen, though, is that the impact can cause the glass platter to shift slightly in the clamp that attaches it to its motor spindle. As a result, the tracks of a spinning disk platter no longer appear circular to the read and write heads, but elliptical. The disk's servo can do a fairly good job of keeping the heads on imperfect tracks, but it does have a limit, says Heybruck, that's typically five to ten times the specified track tolerance for new drives. "When the major and minor axes of an ellipse are so different that the heads can't move back and forth fast enough to stay on track," Heybruck says, "that's when your disk shift has exceeded what you can do."

And makers of small disk drives have to guard against more than just physical damage. If an impact causes heads to go off track while a write operation is occurring, the resulting corrupted data can be a problem. Hitachi's Microdrive, therefore, has a shock sensor that senses an impact and invokes an immediate suspension of writing. If the writing were to continue, it's possible that data would be written not to the correct track, but to an adjacent track. The Microdrive, however, simply stops writing, waits one disk revolution, and then resumes writing when the appropriate disk sector comes around again.

Reducing Power Consumption
Besides ruggedness, the other main goal for 1-inch disk drives is low power consumption, and that's best achieved by keeping the disk powered down most of the time. In fact, reducing power consumption in portable music players is the main reason for powering down the disk. The fact that a powered-down disk is less prone to damage when it's dropped is merely an additional benefit.

Fortunately, it's a simple matter to power down the disk drive in a portable music player, because you can read enough music from the disk in just a few seconds to play for several minutes. Seagate's 2.5- and 5-Gbyte ST1 drives, for example, contain a 2-Mbyte memory buffer that can hold from one to two minutes of high-quality music. Hitachi's Microdrive, with capacities from 340 Mbytes to 4 Gbytes, has a smaller cache, and Cornice's 1- and 2-Gbyte Storage Element has none, but music players contain their own buffers that store music that has been read from the disk. Typically, says Hitachi's Heybruck, players hold about a minute of music in a buffer. Cornice's Holt notes that the buffers in some music players can hold 20 minutes of music.

With disk read rates around 4 to 5 Mbps, it takes very little time to fill a memory buffer with music data. For Hitachi, says Heybruck, "We can power up, read a minute or more of audio, and power down in less than two seconds. You're running less than two seconds every minute." Holt, citing the larger memory buffers sometimes used with Cornice's Storage Element, notes that a music player can spin the disk up in two seconds, load memory for eight to ten seconds, and then play music for 20 minutes before needing to read more data.

The bottom line for power consumption, of course, is battery life, and today's portable music players are increasing battery life considerably. The first disk-based music players spun the disk constantly, and battery life was only about two hours. Now, battery life ranges from eight to 16 hours, even with fairly small batteries. The difference, as noted by Cornice's Holt, comes from turning the disk off most of the time. The maximum current draw for Cornice's Storage Element, Holt says, is 280 mA, which occurs during write operations. In shutdown mode, current draw is less than 10 µA.

Of the 1-inch disk units available, you'll find two basic types. Those that are available for consumer purchase come with CompactFlash Type II packaging and interface (Figure 2) and are removable. Versions made for OEM use typically connect with a flexible ATA interface (Figure 3) and are not removable. Some of the OEM models are also now going into pocketable portable data-storage devices that have a USB interface.

Figure 2:  1-inch disks available for consumer purchase comply with CompactFlash Type II package and interface requirements

Figure 3:  Seagate's ST1 disk drive is available in both an OEM version, shown here with a flex interface, and a CompactFlash version

Among 1-inch OEM disk units, the greatest difference is between those that are modified consumer versions and those that are designed specifically for OEM use. Hitachi and Seagate sell both consumer and OEM versions, with the only differences being the interface and the packaging. Cornice, on the other hand, designs its Storage Element exclusively for OEM use, stripping away as many components as possible from the disk unit itself and shifting their functions to the application device that the disk resides in. Cornice, in fact, doesn't refer to its Storage Element as a disk drive, because it lacks many of the components that make up a self-contained drive.

Cost reduction was Cornice's primary motivation in designing the Storage Element as a stripped-down model for OEM use. "We wanted to get retail products on the shelf at about $199," says Cornice's Holt, and that meant providing disk storage for $60 to $70. That was possible only by eliminating components found in conventional disk drives and shifting the functions of those components elsewhere.

For example, the Storage Element contains no microprocessor or cache memory, instead relying on the processor and memory that are already in the host device to perform their disk-related functions. Cornice reduced other components from 110 to 30, putting the eliminated components into an ASIC that installs on a host device's circuit board. Eventually, the ASIC's functions will be incorporated directly in media processors. Six companies, according to Holt, are already in the process of developing such a device.

Competing with Flash
Flash memory, of course, is the competition for 1-inch disk units, and that competition primarily pits the inherent ruggedness of flash against the lower per-Gigabyte cost of disk. Disks have gotten more rugged, but, as mechanical devices, they still can't match flash's solid-state ruggedness. Similarly, flash is dropping in price, but the price is always proportional to storage capacity. For capacities less than about 512 Mbytes, therefore, flash costs less. At 1 Gbyte and above, disk wins hands down.

And capacities for 1-inch disks are continuing to increase. Seagate's recently introduced ST1 now tops the list at 5 Gbytes, and all three makers of 1-inch units say that increases will be ongoing. Cornice predicts density increases of 50% per year, with cost reductions that keep per-Gigabyte prices comfortably lower than for flash.

Increasing capacities will also bring new applications. Rob Pait, Seagate's director of consumer electronics marketing, says that capacities of 8 to 10 Gbytes will enable 1-inch drives to go in handheld video players, embedded digital video cameras, and possibly some sort of a hybrid between a PDA and a laptop. "Once you get beyond 10 to 20 Gbytes," Pait says, "you can start to look at a world where the computer itself starts to change form factor in a real inflection-point type of shift."

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