Packaging trends | Noise reduction | Power consumption in Microcontroller

Packaging trends
It can no longer be assumed that automotive microcontrollers packaging requirements are conventional PCB mounted plastic / ceramic units. Processing is becoming distributed around the vehicle as dictated by the locations of sensors and actuators, and packaging requirements are changing accordingly. A mechatronic approach is now being taken for microcontroller packaging.

Mechatronics is a discipline, which has arisen from a need to look at systems as a whole, rather than component parts, such as electrical/electronic engineering and mechanical engineering. It aims to bring about completely optimized systems by integrating the individual components of design into a process, rather than the traditional approach, which often yields a collection of electrical, mechanical and hydraulic subsystems interfaced together with a control unit. There is much opportunity for mechatronic technology to improve today's automobile systems.

Figure 6 shows two approaches to an electronic motor controller. On the left, the electronic control circuit uses a number of different interconnect technologies including rivets, solder, surface mount devices and wire bonding to a silicon die. The diagram on the right is an example of a mechatronic solution where an electronic microsystem has been constructed using a multi-chip module and 'connectorized' for robust and efficient interfacing with a motor housing. This type of mechatronic system would usually have many less interconnections than the traditional solution.

Both motor control circuits are connected to a bus via a plug. On a modern automobile, this interface is likely to be a communications bus, linking several motor control circuits (such as window lifters, seat positioners and mirrors).

Figure 6 - Mechatronic motor solution

Noise reduction
ECUs and all electronic components contained within are thoroughly tested to measure Electromagnetic Compatibility (EMC) performance. EMC is becoming a bigger issue for automobile manufacturers as the operating speed of electronic components is becoming faster (higher frequencies lead to increased electromagnetic emissions) and the number of ECUs, which could potentially affect each others operation is increasing.

Worst-case, the result of an ECUs operation being corrupted by radio frequency emissions (from either another ECU or external source from the vehicle) could be a potential compromise on safety. Electromagnetic compatibility can be optimized by careful design of the integrated circuit and the printed circuit board. A system is considered electromagnetically compatible if it satisfied three criteria:

· It does not cause interference with other systems
· It is not susceptible to emissions from other systems
· It does not generate interference with itself

In recent times, the automobile manufacturers have been putting more pressure on the ECU suppliers to produce units with better EMC performance - this pressure has, in turn, been put on the semiconductor suppliers to produce more robust microcontrollers. At the integrated circuit design level, there are many considerations that can enhance the EMC performance of the design: Using less clocks and turning off clocks when not in use, reducing output power buffer drive, using multiple power and ground pins and reducing internal trace impedance on these pins, eliminating integrated charge pump circuitry and positioning high frequency signals next to a ground bus are all steps which are now taken to improve EMC.

Power consumption
Until only recently, low power consumption was never considered a priority requirement for automotive ECUs. This has changed on account of the number of systems, which are required to operate when the ignition is switched off. These systems would quickly drain the battery unless special attention was given to their power consumption. The door modules are a good example. These ECUs need to exist in a 'ready' mode permanently in order to recognize a signal from a 'Remote Keyless Entry' device.

All automotive MCUs are now optimized for power consumption. This is mainly done by switching off the chip's internal clock source, when the chip is inactive. This also results in reduced noise emission. Power consumption is also a consideration for Airbag ECUs. Airbag ECUs need to function in the event that a crash situation disconnects the electrical power supply for the ECU. A large capacitor is normally used to ensure that under these circumstances, there is enough energy available to fire the bag(s). By careful design attention to power consumption requirements, the size of this capacitor may be reduced, thus reducing ECU cost. Airbag microcontrollers are often selected primarily by measuring maximum MCU performance while operating at a speed defined by a given power consumption limit.