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Eight-bit MCUs claim half the automobile

Posted: 16 Jan 2006 ?? ?Print Version ?Bookmark and Share

Keywords:adam prengler? david stone? nec? nec electronics america? automotive applications?

Dozens of 8bit mcus inhabit the average new automobile, which says something about both 8bit MCUs and automobiles. Today's automobiles are nothing like their predecessors. While automobile engineers/inventors were originally concerned with creating a means of transportation, today's engineers are now perfecting how our vehicles transport us, thanks to the microchip. The intelligence built into today's vehicles enables a safer, more comfortable and enjoyable experience than vehicles have ever offered. This evolution has created a demand for electronic systems unlike any other industry.

Leveraging the choices
When the time comes to choose a device, system designers have to consider all of their requirementsmeeting system requirements from the customer, the form-factor specs from mechanical designers, memory size from software developers and other specs for function, flexibility, efficiency and cost. When comparing devices and features, having a firm grasp of the choices available and the knowledge of how they apply to application requirements is critical.

Because 8bit MCUs integrate many features, they offer the overall advantage of design simplicity. With high-speed internal clocks reaching into the multimegahertz range, it's possible to use an MCU with a single resistor as the only external component. While an internal oscillator doesn't have the timing tolerance necessary for driving a CAN interface, the clock provides enough stability for LIN. Thus, an MCU with barely any additional components can act as a full-capability networked slave device.

Internal clocks also offer the ability for lower current consumption and faster wake-up time. Take an application such as a vehicle's wireless receiver. This module must go to a low-power sleep mode and wake up periodically to retrieve data from wireless sources, such as the tire-pressure sending or remote keyless entry units. An internal oscillator allows the MCU to enjoy low standby current while allowing the MCU to wake up quickly to perform the necessary tasks. Additionally, because some MCUs can wake in just a few microseconds, it's practical to service many interrupts from a sleeping start.

In addition to the internal high-speed clocks, 8bit MCUs also integrate high-speed CAN modules and LIN-enhanced UARTs for networked applications. The advantage of these on-chip modules is that they can reduce the CPU load by performing many of the basic functions required by the networking protocol. This gives designers more freedom to integrate functionality into the MCU, since bandwidth is not lost on network handling.

For security and reliability, watchdog timers are critical for automotive applications, as they help prevent software runaways. A new addition to some of these timers is a windowing feature that provides even greater protection. You can set the timer to function like a traditional watchdog with a 100 percent open window or you can select a window of 75 percent, 50 percent, or 25 percent of the time, at which point the watchdog timer accepts a clearance only during the open period. This feature makes it far less likely that a looping program can periodically clear the watchdog timer and keep looping forever.

Road ahead
The main reason for the recent upsurge in 8bit MCU growth is due to low-cost networking standards in automobiles. These networks have simply moved 8bit MCUs from their role as main processors to that of subprocessors used for handling a huge range of individual functions.

Certainly, a single, high-powered controller is ill-equipped to handle large numbers of applications. The necessary number of I/Os, high-current wires and timing issues suggests that individual distributed MCUs are more realistic.

Individual control modules based on 8bit MCUs also make it easy to differentiate car models. To upgrade a model, simply add several automatic features by plugging in the appropriate modules. Additionally, the modules are easy to replace in the event of failure, simplifying service.

They also turn out to be suitable platforms for adding new components to an automobile. Want to implement a new personal comfort feature? Control it with an 8bit MCU.

Still, some degree of integration makes sense and can be expected in the years to come. Since the purpose of integration is to pull features from what were previously subsystem controllers into a single main controller, the opportunities for 8bit MCUs will change. In an integrated system, the satellite modules will have a narrowed function and thus require fewer features than previously needed. This will create an opportunity for new products based on 8bit MCUs.

As the means for enabling both the distributed and integrated approaches, networking standards continue to evolve. Because the LIN standard requires only a single wire compared to CAN's two, LIN is becoming the low-speed, low-cost network of choice for automotive body applications that can encompass the entire vehicle. A general hierarchy of networks is emerging, with actuators and sensors at the periphery, MCUs interfacing to them via CAN or LIN, and gateway devices routing messages over different domains and networks.

Whether integrated or distributed architectures prevail, both trends generally point toward greater use of 8bit MCUs. Versatility, capability and low cost make these devices a good match for automotive applications.

Adam Prengler, Technical Applications Engineer
David Stone, Technical Marketing Manager
Automotive Strategic Business Unit
NEC Electronics America Inc.

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