The life cycle of NAND flash memory has come under the spotlight after a recall of 158,000 Tesla’s electric vehicles.
The Office of Defects Investigation (ODI) of the National Highway Traffic Safety Administration (NHTSA) in the US is investigating a potential safety-related problem with the media control unit (MCU) in the vehicles. The failure of the embedded NAND memory results in the loss of rearview camera and other safety-related vehicle functions in the Model S and Model X built before 2018. The issue will affect vehicles shipped across the world.
Certain of the vehicles use an Nvidia Tegra 3 processor with an integrated 8GB eMMC NAND flash memory module. Part of the module is used each time the vehicle is started but the eMMC NAND cell hardware fails when the storage capacity is reached, resulting in failure of the MCU.
As the MCU controls certain features, the cars have to be recalled, says the NHTSA.
Specifically, failure of the MCU results in loss of the rearview/backup camera and loss of HVAC for defogging and defrosting the windscreen. The failure also has an adverse impact on the Autopilot advanced driver assistance system (ADAS), says the ODI.
The ODI found that the expected usage life rating for the 8GB eMMC NAND flash memory device is approximately 3,000 Program-Erase cycles, after which the eMMC NAND flash memory device would become fully ‘consumed’ and no longer be operational. As part of this is used at startup with a daily cycle usage rate of 1.4 per block, accumulation of 3,000 P/E cycles would take only five to six years.
The ODI says this life expectancy for a component integral to providing the driver with safety functions is insufficient. Tesla confirmed that all the MAC units will fail given the memory device’s finite storage capacity. Data provided by Tesla showed MCU failure rates between 14.2 – 17.3 percent for the 2012-2015 Model S vehicles, and failures rates between 1.9 – 4.1 percent for the newer 2016-2018 Model S and Model X vehicles, although the ODI expects that rate to increase over time.
The issue is significant, as Tesla used the same MCU with the Tegra 3 processor in approximately 158,000 vehicles up until 2018. The company shipped just under 500,000 vehicles in 2020.
It is also not restricted to Tesla. Problems with the rearview camera systems have also been seen in vehicles from BMW, Honda, Jaguar Land Rover and Subaru, but not necessarly from the falsh memory. It also highlights potential reliability issues with the complex controllers in autonomous, self-driving vehicles.
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