H8SX Series Microcomputers Are Used to Implement Airbag Designs that Are Becoming Increasingly Complex and Sophisticated

The airbag control system samples data at high speed from multiple impact sensors located at the front, sides, and elsewhere on the vehicle. In a crash, the system uses this data to determine which airbags to inflate and how to inflate them. The time available for this sequence of steps is only a few tens of milliseconds. "One of the challenges of designing airbag control systems is finding ways that the electronics can identify those cases in which the airbag does NOT have to be triggered. That's because it is important to prevent airbags from being set off by minor collisions. The solution is to have the airbag control microcomputer analyze the sampled data from various sensors at high-speed using proprietary collision-identification algorithms," explained Mr. Ogusu.

Denso has been using the H8 family of 8-bit and 16-bit microcomputers for airbag control since around 1990. Since 2005 they have been using the H8SX family of 32-bit microcomputers to handle the growing complexity of the control tasks required by the airbags themselves. The company rates Renesas microcomputers highly for their quality of design and production, and for their cost competitiveness, which is achieved through the use of advanced process technology. "We have continued to use Renesas microcomputers because of their high-performance cores and because they have supplied us with models that match our requirements. We want to maintain the ongoing exchange of information that is essential to ensuring that Renesas can continue to supply us with microcomputers suited for our use in airbags," Mr. Ogusu said.

"Part of the background to Denso's use of H8SX microcomputers was our need to cope with the larger numbers of sensors that are now used to enable airbags to respond effectively to a more types of collisions. A high-speed CPU is required to handle important new safety functions. One of these is two-stage initiation control, which inflates the airbag in stages rather than fully inflating it as soon as the collision occurs. Another is a function that controls airbag inflation based on factors such as seat position and whether or not the person is wearing their seatbelt," Mr. Ogusu explained.

Because airbags can't be checked after they have been fitted, close attention must be paid to quality control. There is a strong requirement for rigorous measures to improve microcomputer yield and to eliminate potential problems such as bugs in hardware functions or the compiler. "Because Denso has to ensure the absolute quality of the software we use in the microcomputer, we perform detailed checks on microcomputer operation. The tool we use for this task is the full ICE( In-Circuit Emulator) supplied by Renesas," Mr. Ogusu said.
As airbags become ever more diverse and adopt still more complex control techniques, the relative importance of software development is growing. This is increasing the need for solid and effective software development tools.