XC2365 Microcontroller
in Electronic Power Steering

【Author】
Robert Weiss, Dipl. Ing. (FH), Munich
Senior Staff Application Engineer, Automotive
Microcontroller, Infineon Technologies

EPS is an electromechanical power steering system which uses an electronicallycontrolled electric motor in theplace of conventional hydraulic steeringassistance. An EPS system occupiesless space in the engine compartment,is easier to assemble, and reducesfuel consumption. In addition, iteliminates the need for toxic hydraulicoil in the steering system. In smallervehicles, the electric motor is connectedto the steering column via a gearbox,whereas in mid-range automobiles,it is flange-mounted crosswise orlengthwise onto the gear rack and operates through a gearbox. When the driver turns the steering wheel, the electric motor applies power assistance to the steering.

Overview:
An EPS system consists of a controlunit, a number of sensors, and an actuator- in this case, an electric motor.The control unit controls the systemand supplies the information needed bythe electric motor. The control systemreceives information from sensors thatmeasure things like the steering angle,driving speed and torque. Sensors thatdetect the motor position and motorcurrent ensure that the motor is workingat its optimum operating point. Figure1 shows the key components in anext-generation electromagnetic powersteering system with system monitoring.The XC2365 operates as themaster processor, controlling the servomotor and other components. A devicewith less flash memory from theXC2300 family can be used as a securitycontroller.

Figure 1 Key components of an electromechanical power steering system with integrated system monitoring

The actuator:
As a rule, the actuator is a threephasesynchronous or asynchronousbrushless motor. The motor's rotatingfield is generated electronically. Apulse width modulated signal affectsthe speed and the torque. The signalfrequency is around 20 kHz.

Sensors:
Rotary encoders or magnetic sensors- so-called Giant Magnetic Resistors(GMRs) - provide the data identifyingthe rotor position. The phase currentsare detected by shunts or Hall sensors.The overall costs are reduced by measuringthe current in the DC link. Theabove mentioned sensors have analogoutputs that need to be amplified forprocessing. The forces acting on the steeringcolumn and, thus, the amountof assistance required from the motorare measured by the torque sensor.Signal processing is carried out in thecontrol unit. The wheel sensor suppliesinformation on the speed of travel andthe steering angle sensor delivers informationon the current position of thesteering wheel. Other control unitsprocess these signals. The data istransmitted over the CAN. Some of the evaluation logic can be integrated intothe sensors, depending on the types ofsensors chosen for the system. This improvesprecision and reduces the susceptibilityto faults.

The control unit:
The control unit consists of a numberof voltage regulators, CAN transceivers,signal processing circuitry,bridge drivers, power switches, andmicrocontrollers.The voltage regulators supply the differentvoltages needed by the sensors,microcontrollers, and ASICs. TheCAN transceivers act as a bridge betweenthe CAN and the microcontroller.Sensor data signal processing isanalog or digital. Because microcontrollersare not able to control the powerswitches (B6 bridge) directly, abridge driver is needed. The bridgedriver generates the gate voltage andthe related currents needed to switchthe transistors rapidly. Intelligent driversalso include diagnostic interfaceswhich can detect a variety of problems,including half-bridge short circuits, lowphase voltages, or high componenttemperatures. The microcontrollercontrols and monitors the motor andthe entire system. It also has to performdiagnostics and communicatewith the network. An additional controlleris used to detect faults and to activateemergency operating modes.

The control algorithm:
EPS systems use field oriented controlbecause of their high demands interms of motor dynamics and constanttorque. This type of control acts directlyon the motor's rotor field and requiresconsiderable processing powerbecause it involves computing multiplecoordinate transformations (Clark/Parktransformations) and the regulation ofboth phase currents at 50_s intervals. APWM signal is needed to control themotor using the space vector method.Because the CPU is also used to performdiagnostics and to communicatewith the network, the task can only beperformed by a powerful 16/32-bit microcontroller.

System monitoring:
Electromechanical power steering isa safety-critical application, and precisemonitoring of the entire system istherefore essential. Monitoring involvesa combination of hardware andsoftware. The minimum hardware requiredis an independent watchdogwith its own clock generation logic andvoltage monitoring. Depending on thenumber of tests needed and on the degreeof robustness required, an additionalmicrocontroller (8/16-bit) may beessential. Using fully autonomous microcontrollersincreases the controlunit's functional reliability.The software performs microcontrollerand system diagnostics separately.Parts of the microcontrollerhardware, including flash memory errorcorrection, the opcode decoder, interruptarbitration, and the contextswitch, are tested when the systempowers up. Random values are writtento the data segment and read out atregular intervals. To enhance reliability,sensitive data is verified with acyclic redundancy check (CRC), whichinvolves writing data twice and comparingit. The entire system is testedby injecting faulty stimuli and checkingthe control unit's response. An additionalmethod to ensure that the systemis functioning correctly involvesimplementing an algorithm in two separateways and comparing the results.Figure 2 illustrates how system monitoringworks with the XC2300 microcontrollerfamily. Today's more demandingrequirements in terms of securityand reliability as well as realtimeprocessing performance pose a seriouschallenge for the processor. TheXC2300 family from Infineon offers aprocessor architecture that addressesthe issue of system monitoring in detail,is capable of processing demandingcontrol algorithms fast, and has extensivebuilt-in hardware support. Itprovides the capabilities to handle complextasks rapidly and efficiently. Alook at how the XC2300 works revealsthe possibilities it affords.

Figure 3 The XC2365 microcontroller

The XC2300 family:
The XC2365 microcontroller is amember of Infineon's XC2300 microcontrollerfamily. It is based on an onwarddevelopment of the C166 core(now an industry standard) and offersoutstanding control and DSP capabilities.In contrast to the C166 architecture,however, it can execute instructionsin a single clock cycle thanks toits high-performance pipeline. TheXC2365 offers close to twice the processingpower at the same clock speed.The XC2365 also has a multiply accumulateunit that enables matrix operationsor finite impulse response (FIR)filter functions to be implemented easily.This means that a 16x16-bit multiplicationwith a cumulative 32-bit additionor subtraction can be executed in a single clock cycle. The rapid processingof matrix operations (Clark/Parktransformations) and the implementationof powerful PI controllers bothplay an important part in an EPS system.The XC2365 supports up to 128 interruptsources on 16 interrupt levels.Besides classic interrupt handling, theprocessor also features a DMA transferoption in the form of a peripheralevent controller (PEC), which enableslarge data blocks to be moved orcopied easily within the 16 Mbyte addressspace. The program memory is64 bits wide and currently supports upto 574 Kbytes of embedded flash. Theflash memory is separated physicallyinto multiple blocks and incorporateserror correction and monitoring forgreater operational reliability. Flashmemory areas can be individuallyread- and write-protected with a password.There are also 50 Kbytes of embeddedSRAM that can be used tomanage data. This data is protectedwith a parity mechanism. An additionalprotection mechanism can also beused to block unauthorized access toimportant CPU registers. For greateroperational reliability, protective mechanismsare also triggered when restrictedinstructions are executed orthe CPU stack is overwritten. Infineonis currently planning a memory protectionunit, which will be implementedin the next chip. This will also offer additionalbenefits for developers of safetysystems, because it supports the integrationof third-party software (e.g.,AUTOSAR).With on-chip debug support (OCDSLevel 1), the microcontroller providesthe means for effective and inexpensivesystem emulation with break points,memory/register access, and singlestepexecution. The XC2365's peripheralmodules include flexible timer units,three USIC modules (to support differentsynchronous and asynchronous serialinterfaces), capture/compare (CAPCOM)modules, a real-time clock andwatchdog functionality, two independenthigh-speed 10-bit A/D converters(with conversion times >= 1.2 _s), andthree CAN controllers. Two independentADC units are required to controlthe phase currents. The quality of controlachieved by measurements conductedwith one ADC unit in the DClink and with sequential measurementis lower. The CAPCOM6 capture/compareunit was developed specially tosupport CPU-independent control ofmultiple-phase motors. Two independent16-bit wide timers clocked at theCPU frequency serve as a time base.The CAPCOM6 module can generateup to seven separate pulse width modulatedsignals or store input signal durationsand duty cycles. It has a resolutionof 12.5 ns with a system clock of 80MHz. An 8-bit timer supports deadtimegeneration. The signal can be assignedindividually to every channel.Thanks to the synchronous design andthe use of shadow registers, registerupdates are exactly defined. The CAPCOM6unit supports synchronous andasynchronous pulse width modulation.In the event of an error, each outputcan be placed immediately in an inactivestate. There are numerous triggersources that can trigger synchronouscurrent measurements with correspondingdelays. Up to two motors canbe controlled in one system using thetwo CAPCOM6 units. All peripheralmodules are connected to the CPU overa high-speed 16-bit peripheral bus. Theperipheral modules in the XC2300 familyare more highly integrated than inother microcontroller architectures.This enables the CPU to concentrate oncomputing and diagnostic tasks. Figure3 shows the key function blocks in theXC2365 microcontroller. (Figure 3)In conclusion, it is also worth notingthat the XC2300 family, with its exceptionallypowerful MAC unit, CAPCOM6unit and double ADC, is optimallytailored to the needs of EPS systems.In addition, features like the CRCunit and comprehensive diagnostic capabilitiesenhance the reliability of programexecution.

Development tools:
Given today's increasingly rapid developmentcycles, mature, powerfultools like emulators, debuggers, compilers,assemblers and evaluation boards,as well as CASE tools for software engineeringare of crucial importance, especiallyin the automotive industry.Thanks to the broad acceptance of theC166 architecture, developers candraw on a comprehensive and provenportfolio of development tools. Starterkits are available.