As the auto infotainment system expands into a wider range of multimedia applications, its storage subsystem plays an increasingly crucial role. Music and video files must be stored and quickly accessed. Large mapping-data files for 3D GPS systems must be rapidly searched and displayed, and audio files for voice recognition need to be synthesised and stored.
Most auto infotainment systems in use today rely on a ruggedised HDD (hard disk drive) for data storage. These devices typically offer a capacity of 40 to 50Gbytes. As a well-established, proven technology, HDDs offer a number of attractive advantages. On a dollar-per-gigabyte basis, they present designers with a cost-effective solution. In applications that can overcome the HDD’s inherent seek and rotation latencies—where the majority of read accesses are sequential for example—HDDs can stream large amounts of data in a short period of time. In today’s increasingly complex auto infotainment systems, however, other factors play an ever more important role in the selection of a storage subsystem. In many applications, the large amount of data-storage space available on HDDs offers little benefit to auto-infotainment-
system designers as most systems only require 4 to 8 Gbytes of memory to adequately serve today’s multimedia applications. More-over, the automotive industry’s
high expectations for reliability and demanding requirements for ruggedness result in a strong preference for storage subsystems that can offer high levels of shock, vibration, temperature and moisture tolerance.
Compact package
Given these trends, a new generation of industrial-grade, small-form-factor SSDs (solid-state drives) offer designers of auto infotainment systems a highly attractive storage option. These products come in either integrated or discrete versions. Integrated NAND modules, such as the NANDrive product line from SST, combine an integrated ATA controller with one or more NAND Flash die in a single package. These devices offer complete IDE Flash-disk-drive functionality and compatibility and come in a compact BGA package measuring 12x18x1.4mm. The designer simply mounts the BGA on the system motherboard; on boot-up, the system recognises the device via the ATA or IDE interface as a system drive.
As an entirely silicon-based storage solution with no mechanical moving parts, these drives offer the designer better chances to meet the automotive industry’s stringent shock and vibration specifications. From a performance standpoint, small-form-factor SSDs not only eliminate the seek process that a disk-based storage system must perform and that can take 13ms on average; they also offer a write and read performance of up to 30Mbytes/s. The current-generation NANDrive devices are qualified across the industrial temperature range and offer up to 8Gbytes of storage, with higher densities available in the future.
Smaller footprint
Two major advantages of small-form-factor SSDs are their reduced footprint and their high performance. Over the past decade, as automotive manufacturers have introduced a growing number of electronic subsystems into their vehicles, reducing the electronics footprint has become an increasing priority. For example, today’s average automobile integrates anywhere from 30 to 50 microcontroller-based systems. While HDD manufacturers have achieved continual advances in shrinking the footprint of their products, today’s drives still require much more space than silicon alternatives. As an example, a standard size 40Gbyte HDD measures 70x100x9.5mm, against 12x24x1.4mm for the industrial-grade version of the NANDrive. As to weight savings, at a mere 0.8g, a NANDrive’s weight is less than 1/100th that of an HDD.
Boosting data integrity
Data integrity and extended IC endurance are the most crucial storage-subsystem considerations. Today’s small-form-factor SSDs offer a wide array of features that suit these requirements. For example, to compensate for the random read errors that sometimes occur when using NAND Flash, SSDs offer embedded ECC (error-checking-and-correction) circuitry designed to ensure the accuracy of data as it passes in and out of memory. NANDrives, for instance, offer an 8-bit hardware ECC engine.
Bad-block management presents an additional challenge. Unlike NOR Flash, NAND ICs are designed to allow a number of bad blocks. To manage these defects, firmware-based bad-block-management functions, activated when the small-form-factor SSD is initialised, identify the location of these bad blocks and map them out of the memory array. The firmware then directs the controller not to use those specified blocks. As additional bad blocks are identified, the firmware updates the map to ensure that these blocks are not used.
MLC architectures
Write endurance poses another potential obstacle to the use of small-form-factor SSDs in the automotive market. Flash-memory ICs are subject to write-endurance limitations: after repeated erase and write cycles, the memory no longer retains data. The more complex the IC architecture and the smaller the memory-cell size, the lower the IC’s endurance level. For example, SLC (single-level-cell) Flash-memory devices are typically specified at 100,000 cycles; devices using more complex MLC (multi-level cell) architectures, such as those commonly found today in portable consumer devices, are typically specified at 10,000 cycles. Small-form-factor SSD manufacturers are mitigating this risk by using only SLC Flash memory in SSDs designed for the automotive market. In addition, extended endurance is achieved by adopting wear-levelling functions in the device’s firmware. Wear-levelling algorithms track memory usage by block or page by matching an age counter to a map of the logical and physical sectors on the Flash media. With each write and erase action, the age counter is incremented. These complex algorithms automatically balance memory usage by instructing the controller to rotate memory writes to blocks with lower usage. This technology utilises all sectors of the Flash memory to reach their write limit at the same time, thus maximising SSD endurance.
Discrete vs integrated SSDs
Automotive manufacturers opting for small-form-factor SSDs face another decision. They can buy plug-and-play integrated solutions or build their own discrete device using ATA Flash controllers. Many factors will influence this decision. In a discrete solution, the automobile manufacturer or subsystem supplier will purchase controllers and NAND memory ICs from different suppliers and mount the ICs on a board. Each system relies on an embedded Flash file-system block to manage the handshake mechanism between the host and the Flash memory. This approach gives access to a wider variety of suppliers and more flexibility when Flash memory is in short supply; however, it also brings with it a more complex inventory-management challenge. Moreover, as NAND Flash memory technology evolves and the number of suppliers grows, compatibility issues between the controller and the memory present another potential issue. Integrated solutions simplify the procurement and design process by combining an integrated ATA controller with one or more NAND Flash die in a multi-chip package and by optimising the controller for the memory ICs in the drive. These plug-and-play solutions simplify the inventory-management process by sourcing from a single vendor. Moreover, they can take advantage of stacked-packaging techniques to offer substantial space savings. Discrete solutions using ICs from multiple vendors can occupy as much as twice the space of an integrated small-form-factor SSD.
As they fit into a single package, integrated solutions offer an advantage in terms of reliability as well. Since there is only a single chip on the board, an integrated SSD offers fewer points of failure than a discrete solution and is better able to meet the shock and vibration requirements of the automotive environment. As far as NANDrives go, the entire range is extensively tested and qualified across the entire industrial-temperature range of -40 to +85°C.
