44 Technology focus | Data storage code execution, and high reliability for applications such as boot code and program code. This is especially critical for scenarios such as reliably booting up a vehicle’s operating system as soon as the vehicle starts, as code failure may result in cars being ‘bricked’ or rendered inoperable. NAND flash Developers are now looking to store mass data from sensors and AI frameworks in embedded solid-state drives (SSDs) that are soldered to the printed circuit boards to minimise the impact of vibration. These NAND SSDs store multiple bits in each memory cell to create drives with capacities up to 1 TB. The current generation of NAND memory chips have 286 layers, while the next generation with 300 layers and then 430 layers is set to increase the capacity of the drives to 8 TB and 16 TB. However, all these memory subsystems must be designed to the ISO 26262 functional safety requirements that establish the automotive safety integrity level (ASIL) standard. The range of A to D specifies the requisite safety level of the various systems within the vehicle, with A the lowest and D the highest. The ASIL qualification requires devices to be free from defects, deficiencies and significant variations, which for billions of memory cells is a major manufacturing challenge. It also requires absence from risk due to hazards caused by the failure of the electronic systems in a vehicle during operation to improve safety, detect faults and control failure – in essence to remove or eliminate uncertainty. This detect and control function can be at the supplier level or system integrator level. The essential technology is error correction codes (ECC), which includes information on the consistency of the data being transferred. There are other techniques, including the built-in selftest (BIST), regular monitoring of memory devices, and even using dual banks with the same data to provide redundancy. The devices must have an operating temperature range of -40 C to 105 C, which is compliant with Grade 2 of the AEC-Q100 automotive quality standard. eMMC vs UFS Standards such as Embedded Multimedia Card (eMMC) and universal flash storage (UFS) are evolving for memory storage in automotive applications. UFS is the successor to the eMMC storage standard. Both eMMC and UFS are JEDEC-standard specifications, with a key feature being the integration of flash memory and a controller in a single package. In 2013, eMMC devices started shipping for automotive use, with UFS following in 2017. Today, eMMC, which became a standard in 2007, is still used in many consumer, industrial and automotive devices. However, the transfer speed of eMMC is limited to 400 Mbit/s due to its parallel interface, which has increasingly led to eMMC being unable to keep up with high-end smartphones and other devices requiring faster operating speeds. In 2011, the UFS standard was introduced, which uses a serial interface. Because the interface supports fullduplex communication, the host device can read and write data to the storage medium simultaneously. This has enabled transfer speeds to be increased significantly over those of eMMC, with UFS 3.1 (released in 2020) topping out at 2320 Mbit/s, and the most recent UFS 4.0 version achieving maximum transfer speeds of 4640 Mbit/s. Limits of eMMC NAND-based SSDs are finding adoption at the top-end of data-storage capacities over 1 TB in applications, such as ADAS L5 logging and sensorfusion applications. At the lower end (32-256 GB) of the storage spectrum, eMMC has been the traditional medium. However, eMMC can no longer keep pace with the steeply evolving requirements of the latest systems. A removable disk drive for sensor development (Image courtesy of Seagate) A two SSD rack for high data capacity (Image courtesy of Seagate) June/July 2024 | Uncrewed Systems Technology
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