Currently, the most widely-used storage device is the Hard Disk Drive (HDD), but its popularity is rapidly declining.
In an HDD, the media that stores the binary data consists of rotating discs or platters made of glass-coated aluminum alloy with a magnetic material on both sides or surfaces.
Given the nature of the technology, designing an HDD involves several disciplines, including mechanics, acoustics, aeronautics, and electronics. Still, the HDD has not changed that much in the past 10 years or so. The last major enhancement to increase the storage density occurred about seven years ago; this involved the use of perpendicular recording, a rather ingenuous endeavor that posed significant engineering challenges. Heat-Assisted Magnetic Recording (HAMR) promises new gains in density, but has been plagued by delays, and is finally due out in 2018.
The HDD controller is a jewel of ingenuity. It combines a digital System-on-Chip (SoC) supported by a large amount of firmware that is the brain of the controller along with an analog/digital mixed-signal block that interfaces to the spinning disc (see the block diagram in Figure 2).
Figure 2. The hard disk drive includes an SoC, memories, and an analog/digital mixed-signal block that interfaces with and drives the spinning disc (Click Here for a larger image. Source: Mentor Graphics)
The SoC includes a CPU/RISC processor group. ARM cores seem to be the most popular, but Tensilica and ARC (Synopsys) cores are also used. It comprises the host interface, built on either SATA/SAS or USB 3.1 standards, and the digital interface to the analog/digital mixed-signal block.
The SoC also encompasses a few additional functions, such as an encryption/decryption engine, a compression engine, power management, a DDR memory controller, and a mechanism to track bad sectors.
The mixed-signal functional block is the most critical element in an HDD. A conventional motor controller drives the spinning disc, while a servo operates the arm and reading/writing head. The Fly Height Control encompasses the aeronautics used to fly the head only a few nanometers from the surface of the media without touching it.
Recently, an HDD designer drew an analogy to explain the level of precision required. It is like a Boeing 747 flying 1/32nd of an inch (for non-US reader, that's about 0.8 mm) off the ground, without ever touching the ground, at nearly the speed of sound. Statistically, the head touches the media every 20 seconds or so, whose friction causes a thermal event with potential loss of data. The loss is taken care by a "go-around" of the head to re-read or re-write the data at that point.
The level of analog signal processing (ASP) required is increasing constantly with time. Signal-to-noise ratios continue to diminish, and algorithms like Partial Response, Maximum Likelihood (PRML) are used to reconstruct a bitstream. This is performed over a long stream of bits with elaborate mathematics in a short processing window.
TMR, or Tunnel Magneto-Resistance, and now PMR, for Perpendicular Magnetic Recording, allowed massive increases in bit-density.
The firmware controls all HDD operations. It is the volume and complexity of the firmware that sets an HDD design apart from most other embedded systems.
HDD design is anything but simple. The most complex and challenging blocks are developed by third-party intellectual property (IP) vendors, chiefly Marvell and Broadcom. They sell the IP blocks to the three remaining HDD manufactures -- Seagate, Toshiba, and Western Digital -- who integrate them into their HDD systems.