Digital Input Signals
Digital input signals have only two condi-tions: high (logical 1) and low (logical 0).
Examples of digital input signals are switch control signals (on/off) and digital sensor signals such as the rotational-speed pulses from Hall-effect and magnetoresistive sen-sors. The microcontroller can process these signals without prior conversion.
Pulse-Shaped Input Signals
The pulse-shaped input signals with infor-mation on rotational speed and reference marks transmitted by inductive sensors are conditioned in special circuitry within the ECU. In this process, interference pulses are suppressed while the actual pulse signals are converted into digital square-wave signals.
Signal Conditioning
Protective circuits limit the voltages of incoming signals to levels suitable for condi-tioning. Most of the superimposed interfer-ence signals are removed from the useful signal by filters. When necessary, the useful signals are then amplified to the input volt-age required by the microcontroller (0...5 V).
Some or all of this initial conditioning can be carried out in the sensor itself, depending on its level of integration.
Signal Processing
The ECU is the switching center governing all of the functions and sequences regulated by the electronic transmission-control sys-tem. The control algorithms are executed by the microcontroller. The input signals from sensors and interfaces linking other systems (e. g. CAN bus) serve as the input parame-ters. The processor runs backup plausibility checks on these data. The ECU program supports calculation of the output signals used to control the actuators.
Electronic Control Unit (ECU) Data Processing 63
ECU structure, using electronic transmission control (GS 8.60) as an example
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UAE0956YMicrocontroller
The microcontroller is the central compo-nent of an electronic control unit (ECU) (Figure 2). It controls the function sequence of the ECU. In addition to the CPU (central processing unit), the microcontroller con-tains not only the input and output chan-nels, but also timer units, RAMs, ROMs, serial interfaces, and other peripheral assem-blies, all of which are integrated on a single microchip. Quartz-controlled timing is used for the microcontroller.
Program and Data Memory
The microcontroller requires a program in order to carry out calculations; this is the software. The software is stored in a pro-gram memory in the form of binary numer-ical values arranged into data records. The CPU reads these values, interprets them as commands, and executes these commands in sequence.
The program is stored in a read-only memory (ROM, EPROM or flash EPROM). This memory also contains variant-specific data (individual data, characteristic curves, and program maps). This is non-variable data which cannot be changed during vehicle operation. It is used to regulate the program’s open- and closed-loop control processes.
The program memory can be integrated in the microcontroller and, depending on the particular application, can be expanded by adding a separate component (e. g., an external EPROM or a flash EPROM).
ROM
Program memories can be in the form of a ROM (read only memory). This is a memory whose contents have been defined perma-nently during manufacture and thereafter remain unalterable. The ROM installed in the microcontroller has a restricted memory capacity, which means that an additional memory is required for complex applications.
64 Electronic Control Unit (ECU) Data Processing
Digital Input signals:
Power supply Actuators
Analog
Pulse-type
Interface with other systems Diagnosis interface
EEPROM Conditioning
of input signals
Micro-controllers Output stages
ECU
Monitoring module
A/D converter
CAN RAM Flash-EPROM Signal processing in the ECU
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UMK1508-3EEPROM
The data on an EPROM (erasable program-mable ROM) can be erased by subjecting the device to UV light. Fresh data can then be en-tered using a programming unit. The EPROM is usually a separate component and is ac-cessed by the CPU via the address/data bus.
Flash EPROM (FEPROM)
The flash EPROM can be erased electrically.
The ECU can therefore be reprogrammed in service repair shops without having to open it. The ECU is connected to the reprogram-ming station using a serial interface.
If the microcontroller is also equipped with a ROM, this contains the programming routines for the flash programming. Flash EPROMs are available which, together with the microcontroller, are integrated on a single microchip.
Its crucial advantages have helped the flash EPROM to largely supersede the conventional EPROM.
Variable-Data or Main Memory A random-access memory is needed in order to store such variable data (variables) such as the computational and signal values.
RAM
All current values are stored in the RAM (random access memory). If complex appli-cations are involved, the memory capacity of the RAM integrated in the microcontroller is insufficient so that an additional RAM is required. It is connected to the microcon-troller via the address/data bus.
When the ECU is disconnected from the power supply, the RAM loses its complete stock of data (volatile memory). However, adaptation values (learned values relating to engine and operating status) must again be available when the ECU is restarted. They must not be deleted when the ignition is turned off. To prevent this from happening, the RAM is permanently supplied with volt-age (continuous power supply). These val-ues are lost, however, when the battery is disconnected.
EEPROM (also known as E2PROM) Data that must be retained when the battery is disconnected (e. g., important adaptation values, fault-memory data) must therefore be permanently stored in a non-volatile memory.The EEPROM is an electrically erasable EPROM in which (in contrast to the flash EPROM) each memory location can be erased individually. This means that it can be used as a non-volatile random-access memory.
Some ECU variants also use separately erasable areas of the flash EPROM as a non-volatile memory.
ASIC
Modern semiconductor technology now enables a whole range of electronic func-tions to be integrated in “ASICs” (applica-tion specific integrated circuits). The ASICs used in the ECUs can be grouped into the following three categories:
Power supply and monitoring,
Signal conditioning, monitoring, and diagnostics and
Power driver stages.
This high degree of integration not only reduces the number of components and with it the amount of space required but also increases reliability.
Monitoring Module
The ECU is equipped with a monitoring module. The microcontroller and the moni-toring module use a “question-and-answer session” to monitor each other. If an error is detected, both devices can initiate appropri-ate substitute functions independently of each other.
Electronic Control Unit (ECU) Data Processing 65
Output Signals
With its output signals, the microcontroller trigger driver stages, which are usually powerful enough to operate the actuators directly. It is also possible for specific driver stages to trigger a relay for particularly large current consumers.
The driver stages are protected against short circuits to ground or battery voltage, as well as against destruction due to electri-cal or thermal overload. The driver stage IC detects such malfunctions and open-circuit lines as an error and reports this error to the microcontroller.
Switching Signals
These are used to switch the actuators on and off (e. g., on/off valves).
PWM Signals
Digital signals can be output in the form of PWM (pulse-width modulated) signals. These signals are constant-frequency square-wave signals with variable on-times (Figure 3) and are used to shift actuators to any desired settings (e. g., PWM valve).
Communication Inside the ECU Microcontrollers and external memories (flash, RAM) exchange data over parallel address/data lines.
A current 32-bit system has a 32-bit data bus and a > 20-bit address bus. These buses are operated at the microcomputer’s cycle (~ 50 MHz).
The SPI bus has established itself as the standard (synchronous, serial 3-wire inter-face, cycle approx. 1 MHz) for communica-tion with the ASICs (slow triggering signals, writing and reading of diagnostic informa-tion) or the external E2PROM.
EOL Programming
The extensive variety of vehicle variants with differing control programs and data records makes it imperative to adopt a system which reduces the number of ECU types needed by a given manufacturer. To this end, the flash EPROM’s complete memory area can be programmed at the end of production with the program and the variant-specific data record. This is referred to as EOL or end of line programming.
A further possibility for reducing the variety of variants is to have a number of data variants available (e.g. engine variants), which can then be selected by special coding at the end of the line. This coding is stored in an EEPROM.
66 Electronic Control Unit (ECU) Data Processing
Fig. 3
a Period duration (fixed or variable) b Variable on-time
Time
Signal voltage
a b
a b PWM Signals
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UAE0738EElectronic Control Unit (ECU) Very severe demands are made on the ECU. 67
Very severe demands are made on the ECU
Basically, an ECU in a motor vehicle functions in the same way as a conventional PC. Data are entered from which output signals are calculated.
As in a PC, the heart of an ECU is the printed-circuit board (PCB) with microcontroller using high-precision microelectronic techniques. How-ever, there are a number of other requirements which an ECU must also fulfill:
Real-Time Compatibility
Systems for the engine and the transmission de-mand a very fast control response and the ECU must therefore be “real-time-compatible”. This means that the control’s reaction must keep pace with the actual physical process being controlled.
It must be certain that the real-time system responds within a fixed period of time to the demands made upon it. This necessitates appro-priate computer architecture and very high computer power.
Integrated Design and Construction The weight of equipment and the installation space it requires inside the vehicle are becoming increasingly decisive. The following technologies, and others, are used to make the ECU as small and light as possible:
Multilayer: The printed-circuit conductors are between 0.035 and 0.07 mm thick and are
“stacked” on top of each other in layers.
SMD components: These are very small and flat and have no wire connections through holes in the PCB. They are soldered or glued to the PCB or hybrid substrate, hence SMD (Surface Mounted Devices).
ASIC: Specifically designed integrated component (Application Specific Integrated Circuit) which can combine a large number of different functions.
Operational Reliability
Very high levels of resistance to failure are -provided by integrated diagnosis and redundant computing processes (additional processes, usu-ally running in parallel on other program paths).
Environmental Influences
Notwithstanding the wide range of environmental influences to which it is subjected, the ECU must always operate reliably.
Temperature: Depending on the area of appli-cation, the ECUs installed in motor vehicles must perform faultlessly during continual opera-tion at temperatures ranging between –40°C and + 60 ... 140°C. In fact, due to the heat radi-ated from the electronic components, the tem-perature at some areas of the substrate is con-siderably higher. The temperature change in-volved in starting from cold and then running up to hot full-load operation is particularly severe.
EMC: The vehicle’s electronic circuitry has to undergo severe electromagnetic-compatibility testing. That is, the ECU must remain com-pletely unaffected by electromagnetic interfer-ence emanating from such sources as electro-mechanical actuators, or radiated by radio transmitters or cellular phones. In turn, the ECU itself must not negatively affect other electronic equipment.
Resistance to vibration: ECUs which are mounted in the transmission must be able to withstand vibrations of up to 30 g (i.e., 30 times the acceleration due to gravity!).
Sealing and resistance to operating mediums: Depending on its installation posi-tion, the ECU must be able to withstand damp, moisture, chemicals (e.g. oils), and salt spray.
The above factors and other demands mean that the Bosch development engineers are continually faced with new challenges in their endeavors to economically realize the ever-increasing range of functions.
Hybrid substrate of an ECU