Feature: Thermal management


Table 1: Data retention in NAND Flash cells dramatically declines as temperature rises


Figure 3: The DataRefresh function increases the frequency of recharge operations as NAND Flash cells age


Equally, heat accelerates electron leakage, so data retention also


declines faster as temperature rises, as shown in Table 1. Tis is the problem with NAND Flash-based storage devices


for automotive applications like infotainment systems: in a car’s central console, an infotainment ECU might be required to operate at temperatures as high as 85°C. But, data loss is not acceptable in infotainment applications such as mapping and navigation. And the AEC-Q100 standard mandates a defect rate of zero when tested at temperatures up to 85°C (for Grade 3 qualification). As Table 1 shows, the acceleration of electron leakage from NAND


Flash memory cells at high temperatures dramatically shortens data retention, down to as little as two days at 85°C for an MLC that has undergone its rated maximum number of P/E cycles. It is a problem that a Silicon Motion storage controller and its firmware can solve.


Controller technology extends data retention A NAND Flash-based storage system consists of two basic elements: • NAND Flash array; • NAND Flash controller IC. Te basic role of the controller is to be the bridge between the NAND


Flash cells and the host processor that writes to and reads from memory. Te controller manages the mapping of bits to cell addresses. Te Silicon Motion solution is to monitor the voltage at every cell in


an array, to detect those in which leakage has progressed to a critical level and then re-program those at risk. Te Ferri product feature which implements this technology, IntelligentScan + DataRefresh, is capable of extending NAND Flash array lifetimes far beyond the nominal P/E cycle lifetime specified by the Flash manufacturer; see Figure 3. Te intelligence in the IntelligentScan feature also responds automatically to temperature, and scans more frequently when operating at high temperature. When a cell’s oxide layer has degraded so much that it can no longer


be sufficiently recharged, the IntelligentScan function will repair it where possible, or retire it, thus avoiding any risk to data integrity. Te controller in Ferri products also implements advanced global


wear levelling, so that P/E operations, and thus wear, are evenly distributed across an entire array.


32 September 2022 www.electronicsworld.co.uk


Configuring NAND Flash cells Te controller embedded in Silicon Motion AEC-Q100-qualified storage devices also allows the user to configure operation of TLC NAND Flash to extend data integrity. Te SLCMode function virtually groups together TLCs or MLCs into a one-bit SLC-like configuration. Tis is ideal for use where the nominal storage capacity (in TLC mode) is over-specified. Operation with the SLCMode feature turned on reduces the nominal capacity, but gives SLC-like data retention and data integrity performance. Ferri products offer MLC mode operation as well as SLC


mode, giving users the ability to choose the ideal balance between data integrity and capacity for their application. In addition, they provide the SSDLifeGuard feature that automatically monitors the health of the SSD and triggers the controller to implement data protection and error correction operations when blocks or pages are at risk.


Extending cell operation AEC-Q100 qualification for automotive storage devices calls for zero-defect performance at high temperature – a very challenging requirement for NAND Flash-based devices, because of the acceleration in data leakage from NAND cells at temperatures of 85°C and above. Te fact that Silicon Motion’s Ferri products are qualified for automotive use at temperatures up to 105°C is a testament to the effectiveness of unique technologies like IntelligentScan + DataRefresh, which Silicon Motion has developed for managing NAND Flash arrays. Te optimisation of storage products for automotive


applications results from Silicon Motion’s commitment to the automotive market – a commitment that is also demonstrated by its compliance with the full range of automotive industry standards, including IATF 16949, ASPICE and VDE 6.3. Now, automotive manufacturers can also embed robust solid-


state storage into infotainment and ADAS ECUs with confidence that Flash products’ lifetime, data integrity and data retention will meet users’ expectations for vehicle lifetimes of ten years and above.


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