Bigger role for 16-bit chips

Jim Sibigtroth, Applications Manager
Jim Williams, 16-Bit Applications Section Leader
Motorola Inc.
Austin, Texas
www.motorola.com
by Sherri Koucky

Eight-bit microcontrollers have historically handled simple tasks on vehicles such as controlling window lift motors. Though these components worked well in the past, they run out of steam for some of today's more sophisticated automotive uses. Increasingly, complex applications in automotive body-control applications demand the higher performance of 16-bit microcontrollers.

Traditionally mechanical systems such as doors, seating, interior lighting, and ventilation are becoming electronically defined. Moreover, these electronic systems increasingly interact with each other in some way. This has brought a need for gateway interfaces between systems to allow information sharing and interaction. Sixteen-bit MCUs can handle these tasks by managing multiple serial interfaces such as Controller-area networks (CAN), Local-interconnect network (LIN) buses, and Serial-peripheral interfaces (SPI). These processes also have the horsepower to handle large control programs written in high-level languages.

Even seemingly simple control nodes can require a fair amount of processing power. An outside rearview mirror serves as an example. Aside from the actual control functions, mirror circuits must be able to withstand wide temperature swings and serious power problems like jump-start (up to 40 V) and reverse battery.

A full-featured mirror can have three operational motors (vertical tilt, horizontal tilt, and automatic folding), a heater, a sensor to know when to use the heater, a turn-signal lamp, and a puddle lamp. In addition, the mirror needs information from systems elsewhere in the car. For one thing, it needs data from the turn-signal lever to control the turn lamp. It also needs to talk with the door-open switch and possibly an ambient light sensor to determine whether or not to illuminate the puddle light.

Such a control node probably would be handled by an eight-bit MCU. It would connect through a LIN bus to a more powerful MCU in the door or a central body controller. One reason an eight-bit chip could handle such tasks is that the LIN bus has a fairly simple protocol. This leaves a lot of processing time available for managing the mirror subsystem.

Oscilloscope plot showing the inrush current of an incandescent headlamp. Scale is 1 A/division vertical and 20 msec/division horizontal.
Oscilloscope plot showing the PWM protection mechanism of the MC33888. Scale is 1 A/division vertical and 50 msec/division horizontal. This is similar to the other figure, but one of two headlights was shorted to simulate an overload condition. The inrush current from the working headlight is still evident at, and immediately after, the initial rising edge. The overload limit is disabled by the MC33993 for a short time after turn-on to allow for this inrush current to subside. In this case, the current is still too high after this delay so the protection circuit switches the load in PWM fashion in order to avoid damaging the driver.

Understanding the power system
Power is one of the most important subsystems in a vehicle, yet is often overlooked until the design is almost complete. Overall power consumption, reverse-battery protection, vehicle jump-start, vehicle noise, and vehicle stand-by power all are key factors.

An example of how ICs for automotive uses handle such concerns can be found in the System Basis Chip (SBC) MC33989 from Motorola. It holds two voltage regulators for powering an MCU and peripherals. A 1-Mbaud CAN interface, four high-voltage wake-up inputs, and system-protection circuitry are also part of the silicon. This integrated circuit provides all necessary system voltages and has an internal low-noise 200-mA regulator to power the MCU subsystem. There's a provision to control an external-pass transistor to let a secondary supply handle power-intensive applications. The secondary supply lets select peripherals power down on command, reducing power consumption.

Input power for the device comes directly from the vehicle battery. The only additional component is an external diode for reverse-battery protection. The SBC protects against all over-voltage conditions, including load dump (40 V) and jump-start (27 V). DMOS power MOSFET technology lets the device operate with battery voltages down to 4.5 V, with battery fail detection to 3 V. Thermal-shutdown circuitry kicks at 160*C and there's a software thermal prewarning flag at 130*C. Low-power standby reduces system current consumption to as little as 40*A.

The SBC's CAN transceiver detects protocol time-outs and protects against overheating and short circuits on the CAN inputs. CAN inputs are also internally protected for jump-start, reverse battery, and shorts to battery or ground.

Today's automotive applications need highly sophisticated input-sensing circuits to detect switch closures. That's the reason for a multiple switch-detect interface (MSDI) IC. The MC33993 serves as a switch interface while offering special contact-wetting currents and circuit fault detection absent from MCUs.

The MSDI can detect the closing and opening of up to 22 switch contacts. Switch status transfers to the MCU through a high-speed serial link. Adding a static-discharge capacitor on the inputs protects against transients. All inputs are also protected against reverse battery, jump-start, and load-dump conditions.

The MSDI can use wetting currents if need be to detect closure of metallic switches. The device contains current sources, which lets it serve as a power supply for small loads like sensors, LEDs, or MOSFET gates. This brings design flexibility by letting inputs not needed for switch detection serve another purpose.

A 22:1 analog multiplexer within the MSDI selects channels according to instructions from the MCU sent via a high-speed serial link. Besides making it possible to diagnose switch problems, this feature lets the chip work with analog sensors and resistive ladder interfaces.

Understanding output systems

Many automotive loads, such as motors and lamps cannot work directly from the MCU or other low-current devices. The usual approach is to switch higher currents with relays or mechanical switches. Low cost and known design parameters will probably continue to make electromechanical components the preferred approach. But electromechanical systems create problems. For example, relay contacts possess inertia upon opening or closing and can bounce before coming to rest. This bounce dictates the maximum operating frequency. Electromechanical systems also have no diagnostic capabilities.

One example of a solid-state device designed to drive such electromechanical loads is the MC33888 Quad High Side and Octal Low Side Switch. It can directly control four high-side loads of up to 60 W each and up to eight low-side, low-current (2.5 W) loads, respectively. It also can handle the inrush currents associated with incandescent lighting.

Inrush currents for incandescent lamps can be 10 to 15 times their nominal operating current. Anything driving these lamps must be designed to accommodate these large transients. The MC33888 addresses this through a turn-on timer that lets the lamp warm up before enabling the overcurrent protection circuit. Once it's on, the protection circuit works by detecting the overcurrent and pulse-width modulating the output driver to a level the device can sustain. Integral reverse-battery protection, load-dump protection, and low-power operation help reduce the need for external components. Diagnostic capabilities include open-load, short-circuit, and overtemperature detection. The IC includes a built-in watchdog timer to shut things down if communication to the MCU is interrupted. The MCU controls the device through a high-speed serial interface.

Finally, electric-motor control can make use of the MC33887 Motor Driver. This is a full H-bridge circuit featuring greater than 5-A continuous-drive capability for lock motors, antenna motors, or washer pumps. It also carries high-side current-sense feedback for modifying the frequency and duty cycle of the motor drive based on real-time motor-current feedback. Like lamp drivers, it automatically pulse-width modulates the output if it detects an over-current condition. The MC33887 also allows full control of the H bridge, enabling direction, free wheel, and load-braking control.

Embedded flash memory
Modern automotive MCUs include on-chip flash memory to store the main operating program. The preferred method today is to program it once it's part of a completed electronic-control module, after final assembly. This eliminates the risks and delays associated with extra handling necessary if programming took place prior to module assembly. Parts come directly from semiconductor manufacturers in sealed packages with their fine-pitch leads clean and straight. After module assembly, usually during final testing, the main program is programmed into the MCU though a simple serial interface. Some manufacturers use inexpensive stand-alone programmers for this, while others incorporate the programming operations into their end-of-line test equipment.

The MCU used as an example in this article provides a single-wire background debug interface for flash programming, calibration, and general debugging operations. During normal operation, the serial-communication pin is pegged at the reset level so the background system remains disabled and inactive. For debugging, the pin is held low to force the MCU into active-background mode rather than starting the application program. It's also possible to connect a host system to the target MCU during normal operation to monitor the contents of memory or register without disturbing the running application.

Traditional in-circuit emulators may require as many as 30 to 40 connections into the target system. In contrast, this background-debug interface requires only four connections at most. The simple interface lets automotive developers debug the MCU in an electronic-control module installed in a normal running vehicle. This is advantageous because many problems can only be debugged while the vehicle is operating normally. Using the single-wire background debug interface, the target MCU can theoretically receive a word of information in about 27 msec, just slightly slower than the time needed to program the flash memory on the device.