Fail-safe Storage is Not Optional in the Autonomous Vehicle Era for Determining Where Fault Lies | 赛普拉斯半导体
Fail-safe Storage is Not Optional in the Autonomous Vehicle Era for Determining Where Fault Lies
The following post contains excerpts from an article published in EE Times in December 2018.
The typical car owner probably doesn’t even know that within their vehicle is something called the Event Data Recorder (EDR). The EDR is essentially the automotive black box for safety and monitoring purposes.
The essential functional requirement of a vehicle’s EDR is to capture all specified data inputs — from sensing devices such as visible light and infrared cameras, and radar and LiDAR sensors, and from input devices such as the throttle, brake pedal and steering wheel. It should capture these inputs before and during a safety event such as a collision with another vehicle, providing crash investigators with a complete picture of the vehicle’s movement before an accident takes place.
Traditional approaches to the storage required in an EDR relied on technologies that could be implemented easily and relatively cost-effectively. However, the risk of losing or not recording critical data have caused concerns as we move into the autonomous, self-driving era where 100 percent fail-safe data collection and storage is necessary.
EDR is set to become more critical as more semi-autonomous and fully autonomous vehicles go on public roads for testing and, eventually, as production models for the masses. In these vehicles, the camera and sensor data stored in the EDR as part of the ADAS system will play a crucial role in:
- Establishing fault and liability for damages suffered in road accidents by collecting last-second information
- Providing data that help vehicle makers improve the design of autonomous driving systems when an incident occurs
- Informing legislators’ approach to the setting of regulations for autonomous vehicles on public roads based statistical event data collected
As a result, the specification and operation of EDRs is of great interest to the Automotive OEMs, the insurance industry, and governments. The EDR needs to record data on an ongoing basis, but only needs to capture the last event before an accident. This requires fast write times and the ability to retain the last event data under harsh environments. Reliability and performance are characteristics that have taken on new urgency in the AV era.
F-RAM meets the challenges of the AV era
The risk of losing data in certain circumstances is leading automotive manufacturers to take a new interest in a different, and proven, non-volatile memory technology such as ferroelectric RAM, or F-RAM. The attributes of F-RAM — its instant write capability, infinite endurance, high data integrity and harmony with functional safety requirements — are ideally suited to new and more reliable EDR applications.
F-RAM non-volatile memory for automotive EDRs has sparked interest because its use addresses the shortcomings of traditional storage solutions. It offers instant write capability, unlimited endurance, and near-zero soft error rates to support compliance with functional safety standards, including ISO 26262.
The operation of a ferroelectric memory is completely different from that of a traditional writable non-volatile memory derived from floating gate technologies, which work by storing charge in a bit cell. Flash or EEPROM memories use charge pumps to generate high voltages on-chip (10 V or more) and force charge carriers through the gate oxide. This produces long write delays and requires high write power, which is destructive to the memory cell.
By contrast, the write speed for F-RAM is effectively instant — a matter of a few picoseconds. Because of its short duration, this write operation can be powered by the inherent capacitance of an F-RAM memory chip. This means that once data is presented to the device’s pins, it is guaranteed to be stored even if the system power supply fails — and with no need for a capacitor or any other external power source. The instant write speed also means that there is no need for a high-speed buffer SRAM or DRAM memory on the board (see Figure 1).
An effective automotive EDR implementation will pair an unlimited-endurance F-RAM device, which is available in densities of 2 Mb or 4 Mb, with high-density flash. The memory will typically be configured to continuously store the most recent 1 to 5 s of data, while a flash memory array is used for bulk storage of older data. In the case of the Excelon-Auto device, there is a serial peripheral interface, and it uses standard non-volatile commands for configuration and read and write operations.
The availability of automotive-qualified F-RAM memory devices today for use in EDRs is providing the fast operation and long endurance required to store a vehicle’s sensor data up to the very last moment before an incident with no need for expensive buffer memories and backup power supplies. The use of F-RAM in automotive EDRs produces a simpler, smaller and better-performing design than the conventional flash- or EEPROM-based approaches do. This will be critical as we move further along the path to mainstream use of AVs.