QspiNAND with ultra-high write speed: a new choice for automotive OTA upgrades

By Anil Gupta, Technology Director, Winbond Electronics Americas

Recently, the BMW Group announced that it will carry out OTA (Over-The-Air Technology) upgrades on more than 750,000 vehicles worldwide, upgrading the BMW 7.0 operating system in the car to the latest version. The upgrade involves modules such as digital services, driver assistance, electrical, chassis and power systems. Nowadays, remote OTA upgrades of cars are no longer the "patent" of new car manufacturers. In the traditional car camp, SAIC-GM is also carrying out OTA upgrades like BMW. It can be seen that OTA upgrades are rapidly developing into a must-have feature for future smart cars. There are two main reasons for this:

First, with the exponential growth of automotive system program codes, cars are increasingly becoming targets of cyber attacks. For cars that use software to implement driver assistance functions, if the system software is hacked or tampered with, then in autonomous driving mode, both the driver and passengers will be in danger. Through OTA updates, the system can quickly install security patches to risky firmware, thereby more effectively ensuring driving safety. Secondly, using OTA upgrades is more cost-effective and convenient. It can not only help car owners save time and trouble, but also help manufacturers and repair centers save update costs. The cost of OTA updates is almost zero, so manufacturers can continue to improve the overall value of the car through more frequent software system upgrades.

Traditionally, SPI NOR flash memory has been a common choice for program code storage due to its high reliability, high data integrity and long-term data retention capabilities. However, in recent electronic engineering research publications and technical conferences, the discussion of considering using serial NAND flash memory to replace SPI NOR has attracted widespread attention.

Figure 1: The Audi A7 "autonomous driving concept," showing the sensor array in the new vehicle design.

(Image source: AUDI AG)

Faster erase and write speeds

OTA updates must be secure, reliable, and most importantly, fast. Speed ​​is critical to users because the car cannot be used while the update is in progress. In addition, the shorter the update process, the lower the chance that the network connection between the car and the original factory data center will be interrupted or disabled due to a failure. For accelerated software updates, new serial NAND flash products have a clear advantage over SPI NOR flash. Fast writes are an inherent advantage of serial NAND flash, even without proprietary acceleration technology.

The OTA update operation is performed in two stages: first, the old program code is erased from the memory, and then the new program code is written. As shown in the data in Figure 2, the erasure speed of serial NAND is about 100 times that of SPI NOR. In addition, the programming speed of Winbond's high-performance QspiNAND (Quad SPI NAND) flash memory is about five times faster than the fastest SPI NOR flash memory on the market. In terms of write processing speed, serial NAND flash memory is more than ten times faster than SPI NOR flash memory.

This means that if a 2Gb software stack for the front camera is stored in a single W25N02JW device consisting of two W25N01JW chips, the programming time is only 35 seconds. If the same program code is stored in an SPI NOR flash array using the W25H02JV, it will take 22 minutes.

Figure 2: Comparison of write speeds of SPI NOR and QspiNAND flash memory devices. (Data source: Winbond)

In addition to the advantages of erase and programming speed, Winbond's automotive-grade QspiNAND flash memory is based on the SLC (Single Level Cell) architecture, which uses a proven and stable 46nm process and has data integrity and durability comparable to SPI NOR flash memory. Winbond's SLC NAND flash memory can provide highly reliable performance throughout the life of the car, thereby ensuring the safety that is critical to automotive applications.

Optimize startup speed

The time taken for OTA upgrades is the key to determining the startup speed of the car. Under normal circumstances, drivers hope that the car system will be ready within a few seconds after pressing "start" - so the startup speed is a key parameter that automakers attach great importance to. However, the size of the car's memory occupied by the code will affect the startup speed and programming speed. Taking ADAS products such as on-board front cameras as an example, when starting up, the program code will be downloaded from flash memory to DRAM such as LPDDR4, and the code of such products needs to occupy more than 512Mb of memory. In order to speed up the OTA process, automakers are encouraged to use serial NAND flash memory to achieve high-speed writing of large codes of more than 512Mb.

As shown in Figure 2, the performance of Winbond's QspiNAND flash products is similar to that of standard SPI NOR flash. Both have a read processing speed of 80MB/s, which can download 1Gb of program code for an ECU in 1.5 seconds, which meets the needs of automotive applications. However, if the ECU contains 2Gb of program code, the download time will increase to 3 seconds, further extending the total startup time of the vehicle. For this part of the demand, the latest NAND flash technology may be a more suitable choice (see Figure 3): Winbond's OctalNAND flash has a better cost per bit than Octal NOR flash and also has an advantage in write processing speed. However, thanks to the x8 interface similar to DRAM, OctalNAND provides a higher read processing speed (240MB/s). Using OctalNAND flash to download the entire 2Gb program code only takes 1 second, which is enough to meet the startup time requirements of any automaker.

Figure 3: Winbond Electronics' OctalNAND flash memory enables fast boot in automotive applications. (Image source: Winbond)

The W35N-JW OctalNAND part is packaged in an industry-standard 8mm x 6mm BGA package, which is footprint-compatible with serial NOR Flash products and is fully pin-compatible with Xccela™ Flash and Octal Flash NOR products currently on the market.

More cost-effective

In general, NOR flash memory occupies a larger chip area per storage bit than NAND flash memory (the area of ​​a serial NAND flash memory cell is 4F2, while that of a NOR flash memory is 10F2). The chip area affects the cost of the packaged parts, but the relationship is not completely linear: when the capacity is less than 512Mb, the proportion of the area on the chip used for control logic and interfaces is higher, so the per-unit cost of NAND has less impact on the total cost. However, when the capacity is greater than 512Mb, the storage array will occupy a large amount of chip area, so compared with SPI NOR flash memory of the same capacity, NAND flash memory with a smaller cell size has a greater cost advantage. For example, in the storage application of 1-2Gb of program code for the front camera of the car, the cost of using serial NAND flash memory is half the cost of the same-level SPI NOR flash memory. In addition, the latest serial serial NAND flash memory technology has a higher write processing speed, making it more advantageous than SPI NOR flash memory.

Compared with the same level of SPI NOR flash memory, using QspiNAND flash memory to store more than 512Mb of automotive program code can save about 50% of the cost. In addition, automakers are now finding that QspiNAND and OctalNAND can significantly reduce write time, which is critical to optimizing the OTA upgrade experience.

More importantly, QspiNAND's shorter write time also helps improve factory efficiency and processing speed when writing large amounts of program code to the ECU. This allows automakers to consider from another perspective the benefits of abandoning traditional SPI NOR flash for code storage in safety-related and mission-critical applications.