BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to electronic control units and, and more particularly, to vehicular electronic control units with power loss compensation. 
         [0003]    2. Background of the Invention 
         [0004]    Automobiles are increasingly using electronic control units (ECUs) to control vehicle equipment based on sensor data. A forward looking camera may, for example, detect preceding and oncoming traffic and control the vehicle&#39;s headlights in response thereto. More specifically, the camera may automatically control high headlights beams to turn on only if no other vehicles will be subjected to undue glare. A camera may also detect lane markings and warn the driver in case of accidental lane departures. A radar sensor may alert the driver to objects in the driver&#39;s blind spot. 
         [0005]    The increasing sophistication of vehicle features requires calculating complex algorithms and/or processing large amounts of sensor data. Especially driver assistance systems employing radar sensors or vision sensors have to process large amounts of raw sensor data, and possibly combine data from several sensors to control vehicle equipment. This requires electronic processors capable of computing intensive tasks in real time, which causes increased processor power consumption. 
         [0006]    Electronic control units may include two or more electronic processors. Such multi-processor architectures are common in vehicular driver assistance systems, e.g. camera, radar or lidar systems. In multi-processor configurations one processor may be a microprocessor dedicated to interfacing with the vehicle while other electronic processors, e.g. digital signal processors or programmable gate arrays, may be used for computing intensive tasks such as image processing or the analysis of radar echoes. 
         [0007]    A problem in vehicles is that large electric consumers, e.g. steering motors, can cause momentary vehicle battery voltage drops. This subjects electronic control units to short periods of total or partial power loss. Momentary power loss is often considered unavoidable by the vehicle manufacturer, and has to be compensated by electronic control units to avoid vehicle equipment malfunction. Vehicle manufacturers often require microprocessors in electronic control units to not reset during momentary power loss up to a specified duration, typically between 10 and 100 milliseconds. A microprocessor reset must be avoided, since it would cause the electronic control unit to enter a default state upon recovery from the power loss. This could lead to vehicle equipment being temporarily switched on/off during a power loss induced microprocessor reset. For example, an automatically activated high beam headlight might be temporarily switched off during a reset and back on after the microprocessor recovers from its reset. This may cause undesirable flickering and must be avoided. 
         [0008]    Traditional power supplies in electronic control units include hold capacitors as energy storage devices to compensate for momentary external power loss. During momentary drops of the external supply voltage the power supply inside the electronic control unit maintains a constant voltage to the microprocessor and other components by discharging the hold capacitor. The hold capacitor is dimensioned such that the electronic control unit can survive battery power loss up to the anticipated maximum duration, typically between 10 and 100 milliseconds, without microprocessor reset. 
         [0009]    The conventional approach of using a hold capacitor sufficiently large enough to keep the electronic control unit operational during momentary battery power loss is, however, limited when it comes to control units with high power demand. One disadvantage with the conventional approach is the increase in cost to provide sufficient capacitance for the entire unit to operate during periods of external power loss. Another is the relatively large size of capacitors with sufficient capacitance. Size is of particular concern, if the electronic control unit is mounted in a visible location, as is the case e.g. with a front camera mounted to the vehicle&#39;s windshield. 
         [0010]    Therefore, in light of the problems associated with existing approaches, there is a need for improved electronic control units that can compensate momentary supply power loss without the cost and size increase associated with large hold capacitors. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with the described embodiments vehicular electronic control units and their operating methods are described. In an exemplary embodiment the electronic control unit reduces its power consumption during periods of external power loss. External power loss is detected by the electronic control unit&#39;s electronic processor utilizing a low voltage detection circuit. The electronic processor thereupon disables one or more components within the electronic control unit so that the electronic control unit operates with limited functionality for the duration of external power loss. The electronic control unit uses internal energy storage, e.g. a hold capacitor, to sustain its limited functionality operation during the momentary external power loss. Upon recovery from the external power loss the electronic control unit resumes full operation. 
         [0012]    In another exemplary embodiment the electronic control unit comprises two or more electronic processors, which are communicating with each other. A first processor interfaces with external vehicle equipment, e.g. the vehicle headlamps, an electric blower motor, a relay, or visual, audible, or tactile driver warning equipment. The first electronic processor may control the vehicle equipment directly, e.g. by changing the state of one of its outputs, or indirectly, e.g. by sending a message through a serial communication system to another electronic control unit. A second electronic processor performs computing intensive tasks, e.g. analyzing the video stream from an image sensor or analyzing the echo data from a radar receiver. During normal full operation the first electronic processor controls the state of the vehicle equipment in response to information processed by the second electronic processor. 
         [0013]    Both electronic processors are powered by an internal power supply, which is connected to the vehicle battery. Vehicle battery voltage is monitored by a low voltage detection circuit. If the vehicle battery voltage falls below a predetermined value a low-voltage signal is generated. The internal power supply comprises a hold capacitor, which is discharged during external power loss. To maximize the time that can be compensated by the limited energy stored in the hold capacitor, the electronic control unit&#39;s power consumption is reduced during external power loss. Power consumption is reduced by switching the second processor into a low current consumption state, such as by turning off the second processor&#39;s supply voltage, reducing the second processor&#39;s operating frequency, or ordering the second processor into a sleep, deep sleep, or hibernation mode. 
         [0014]    Abruptly turning off the second processor&#39;s supply voltage may cause undesirable memory loss, and should be avoided. Therefore the second processor may be turned off with a delay after a low-voltage condition is detected. The low-voltage signal is communicated to the second processor, which responsive thereto prepares for an imminent power loss by saving settings into keep-alive memory. Power to the second processor may be removed with a predetermined delay time sufficiently long for the second processor to save its settings. Alternatively, the second processor may signal that it is ready to shut down. 
         [0015]    While the second processor is in low power consumption mode it no longer performs its computing intensive tasks, and communication with the first processor may be lost. The electronic control unit operates in limited function mode. The first processor maintains the vehicle equipment state unchanged while the second processor is unavailable. Once the external battery voltage recovers the second processor resumes normal operation. The first processor may once again update the state of the vehicle equipment based on information provided by the second microprocessor. 
         [0016]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a block diagram of an exemplary electronic control module with external power loss compensation. 
           [0018]      FIG. 2  is a block diagram of another exemplary electronic control module that is suitable for use in connection with the described embodiments. 
           [0019]      FIG. 3  is a graph that is useful in understanding the operating environment of the described embodiments. 
           [0020]      FIG. 4  is a flow diagram that describes steps in a method in accordance with one of the described embodiments. 
           [0021]      FIG. 5  is a flow diagram expanding on the method illustrated in  FIG. 4  and describes steps in a method in accordance with one of the described embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Referring to  FIG. 1 , a block diagram of an exemplary electronic control unit  100  in which the principles of the present invention may be advantageously practiced is illustrated generally. Electronic control unit  100  illustrates building blocks of a forward looking automotive camera. The camera may e.g. be used as part of a Lane Departure Warning System, a High Beam Control System and/or an Object Detection and classification System. Electronic control unit  100  includes an image sensor  114 , which is operatively connected to digital signal processor  112 . A stream of digital video is transmitted from image sensor  114  to digital signal processor  112 . Image sensor control information is sent in the opposite direction from digital signal processor  112  to image sensor  114 . Digital signal processor  112  analyzes the video stream provided by image sensor  114  and derives the desired vehicle feature, e.g. a decision to warn the driver of an accidental lane departure, or a decision to turn on high-beam headlights. 
         [0023]    The interface between electronic control unit  100  and vehicle equipment  104 , e.g. the headlamps or a warning device, is controlled by microprocessor  110 . During normal operation microprocessor  110  communicates with digital signal processor  112  and determines the desired state of vehicle equipment  104  based on the result of the sensor information processed by digital signal processor  112 . Microprocessor  110  may control external vehicle equipment  104  directly by selecting the state of output driver  118 , or indirectly by communicating with other electronic control units connected to a serial data communication system, here illustrated by CAN transceiver  116 . 
         [0024]    Power to all components in electronic control unit  100  is provided by power supply  106 , which is connected to vehicle battery  102 . Power supply  106  comprises an electric energy storage component, e.g. a hold capacitor or a backup battery. During momentary external power loss the electric energy storage component is discharged in order to keep electronic control unit  100  operational with a least limited functionality. To maintain limited functionality supply  106  maintains a constant internal supply voltage to at least microprocessor  110 , CAN transceiver  116  and output driver  118  in the depicted embodiment. Keeping those components powered enables electronic control unit  100  to maintain the current state of any vehicle equipment  104  that is controlled through either CAN messages or direct outputs through momentary external power losses. 
         [0025]    External battery voltage VBAT is monitored through battery voltage monitoring circuit  108  by microprocessor  110 . In case of a low external battery voltage microprocessor  110  switches digital signal processor  112  into a low power mode. Reducing the power consumption in digital signal processor  112  causes the internal energy storage component inside power supply  106  to be discharged at a slower rate. This extends the time without external power that can be compensated without causing microprocessor  110  to reset. For automotive applications external power loss lasting up to between 10 and 100 milliseconds must typically be sustained without affecting the interface between electronic control unit  100  and other vehicle equipment  104 . 
         [0026]    While a specific example has been shown in  FIG. 1  it will be appreciated that many equivalent alternatives for each component exist. Controller area network (CAN) interface  116  may for example be any other data communication interface, among them LIN, Class  2 , MOST, USB, Firewire, and Flexray. Microprocessor  110  and Digital Signal Processor  112  may be any other electronic processor, among them microprocessor, microcontroller, flexible programmable gate array or application specific integrated circuit. Image sensor  114  may be any form of electronic sensor, e.g. a radar sensor, ultrasonic sensor, radio frequency receiver, inertia sensor, or lidar sensor. 
         [0027]      FIG. 2  further illustrates an exemplary electronic control unit in accordance with one embodiment of the invention. Here, microprocessor  110  is powered by a 5V regulated voltage which is provided by power supply  106 . Digital signal processor  112  and image sensor  114  are powered by 3.3V regulated voltage provided by power supply  106 . Power supply  106  comprises two step-down converters  232 ,  234  to generate the internal 5V and 3.3V supply voltages. The step-down converters may for example be commonly used L5973 type step down monolithic power converters manufactured by ST microelectronics. The output voltage of converter  232  is filtered using the low pass characteristics of inductor  214  and capacitor  220 . Diode  212  servers as a free-wheeling diode when the output of converter  232  is switched off. Resistors  216  and  218  form a voltage divider to establish the required feedback voltage to regulate converter  232 . Similarly the output voltage of converter  234  is filtered using the low pass characteristics of inductor  224  and capacitor  230 . Diode  222  servers as a free-wheeling diode when the output of converter  234  is switched off. Resistors  226  and  228  serve as a voltage divider, providing the required feedback voltage to regulate converter  234 . Capacitors  236 ,  238  and resistor  240  provide a compensation circuit and are connected to the error amplifier output of converter  232 . Capacitors  242 , 244  and resistor  246  serve the same purpose at converter  234 . Converters  232 , 234  are connected to the vehicle battery  102  through a low battery voltage protection diode  250 . 
         [0028]    Battery voltage VBAT is monitored using low voltage detection circuit  108 . Low voltage is detected by dividing VBAT through voltage divider resistors  202 , 204  which are connected to analog input  206  of microprocessor  110 . Digital output  208  of microprocessor  110  is connected to inhibit input  210  of converter  234 . If low battery voltage VBAT is detected microprocessor  110  can set its output  208  to high, causing converter  234  to turn off the 3.3V supply to digital signal processor  112  and image sensor  114 . 
         [0029]      FIG. 3  illustrates characteristic voltage curves that may be experienced in the circuit illustrated in  FIG. 2 . During normal driving conditions before time t 0  vehicle battery voltage VBAT, represented by line  300 , is around 13.5 Volts. VCC, the voltage at regulators  232 ,  234  and hold capacitor  248 , is around 13.2 V, corresponding to a 0.3 Volt drop over low battery protection diode  250 . VCC is illustrated by line  302 . Regulator  232  generates a constant 5V output illustrated by line  304 , regulator  234  a constant 3.3V output illustrated by line  306 . 
         [0030]    Activation of large electric consumers in the vehicle, e.g. large electric motors such as electric steering actuators, may momentarily cause battery voltage VBAT to drop below its nominal value. This is illustrated in  FIG. 3  by a drop of VBAT to 0 Volt beginning at time t 0 . After t 0  voltage VCC at hold capacitor  248  is higher than VBAT, which causes diode  250  to block. Regulators  232 , 234  are effectively decoupled from vehicle battery  102  and powered from the energy stored in hold capacitor  248 . This causes hold capacitor  248  to be rapidly discharged, as illustrated by a fast decline in VCC between t 0  and t 1  in curve  302 . The low battery voltage condition is sensed by microprocessor  110  through its analog input  206 , which is connected to voltage divider resistors  202 ,  204 . 
         [0031]    After a low battery voltage occurs at time t 0  microprocessor  110  communicates the low voltage condition to signal processor  112 . Signal processor  112  prepares for an imminent power loss by saving critical data to memory not affected by a loss of the 3.3V supply voltage. This may for example be EEPROM or Flash memory, RAM memory not powered by the 3.3V power supply, or memory within microprocessor  110 . After all critical memory is saved, digital signal processor  112  communicates its readiness for shutdown to microprocessor  110 . 
         [0032]    At time t 1 , responsive to receiving a shutdown readiness notice from digital signal processor  112 , microprocessor  110  turns its digital output  208  to high, causing inhibit input  210  at converter  234  to go high, which turns converter  234  off. Curve  306  illustrates the 3.3V output of converter  234  going to zero as converter  234  is turned off at time t 1 . With digital signal processor  112  and image sensor  114  being powerless after t 1  the overall power consumption in the electronic control unit is substantially decreased. Therefore hold capacitor  248  is discharged at a slower rate, shown by a slower gradient of VCC between t 1  and t 2  in line  302 . The slower discharge rate allows regulator  232  to maintain a constant output voltage up to t 2 , at which point VCC reaches about 5.98 Volts, the minimum input voltage required to generate a constant 5V output. 
         [0033]    As illustrated VBAT recovers after t 2 , which allows the electronic control unit to resume normal operation and reactivate the 3.3V power supply  234  to digital signal processor  112  and image sensor  114 . 
         [0034]      FIG. 4  is a flow chart illustrating an exemplary method of operating an electronic control unit during momentary power loss. The electronic control unit after powering up in step  400  periodically monitors external battery voltage. If in step  402  external battery voltage is found to be sufficiently high the electronic control unit operates in full functionality mode  406 . If in step  402  a low external battery voltage is detected the electronic control unit operates in limited functionality mode  404  with reduced power consumption. 
         [0035]      FIG. 5  is a more detailed flow chart expanding on the method of  FIG. 4 . The method illustrated in  FIG. 5  is applicable for example for automotive sensor electronic control units such as a forward looking cameras or radar sensors. After power on step  400  the electronic control unit cyclically monitors for low voltage conditions. If in step  402  a sufficiently high external supply voltage is detected the electronic control unit operates in full functionality mode  406 . Full functionality comprises collecting sensor data step  500 , processing sensor data step  502  and controlling vehicle equipment based on the processed sensor data step  504 . 
         [0036]    If low external supply voltage is detected in step  402  the electronic control unit prepares to reduce its power consumption. Components that can not be abruptly disabled are informed about an imminent power loss in step  506 . Once it is determined in step  508  that the unit is ready to enter low power mode, i.e. affected components have indicated their readiness to shut down or enter a sleep mode, the electronic control unit enters limited functionality mode  404 . In limited functionality mode sensor data collection step  510  may be paused, e.g. by removing power from a sensor component, e.g. an image sensor or radar transceiver. Correspondingly sensor data processing step  512  is paused, e.g. by removing power from a digital signal processor or switching a digital signal processor into sleep mode. The interface between electronic control unit and external vehicle equipment in step  514  is no longer updated. 
         [0037]    The effect of operating the electronic control unit in limited functionality mode  404 , especially maintaining the last know state of vehicle equipment in step  514 , may take various forms, depending on the vehicle function controlled by the electronic control unit. An automatic high beam control system may e.g. maintain the state of high beam activation in lieu of new sensor data, i.e. not react to new vehicles or vehicles leaving the field of view of the camera during momentary power losses. A lane departure warning system may not issue new warnings when crossing a lane marking, but may choose to let warnings issued before entering limited functionality mode  404  expire based on a predetermined latency. In this case maintaining the last state of vehicle equipment step  404  consists of not preventing the default expiration of a warning and turning off e.g. a warning light, buzzer or vibration actuator. 
         [0038]    The methods illustrated in  FIG. 4  and  FIG. 5  may be executed cyclically, e.g. by reading and evaluating external voltage in an A/D converter in microprocessor  110  in a fixed cycle time. Since the power consumption in the electronic control unit has to be reduced very quickly after a loss of external power, typically within less than a few milliseconds, the cycle time for monitoring external power supply voltage has to be very fast, e.g. at least once every millisecond. Such fast cycle times may be incompatible with the software architecture in microprocessor  110 , which may be designed around cycle times around 20-100 milliseconds. An alternative embodiment may overcome this limitation by utilizing a low voltage detection circuit with digital output, that is connected by an external interrupt input to microprocessor  110 . The low voltage detection circuit is designed to cause a processor interrupt when the external battery supply voltage falls below a predetermined value, e.g. around 9 Volts. Microprocessor  110  can therefore detect low external voltage without cycle time dependent latency. 
         [0039]    While the present invention has been described with reference to exemplary embodiments, it will be readily apparent to those skilled in the art that the invention is not limited to the disclosed or illustrated embodiments but, on the contrary, is intended to cover numerous other modifications, substitutions, variations and broad equivalent arrangements that are included within the spirit and scope of the following claims.
In accordance with the described embodiments vehicular electronic control units and their operating methods are described which cost effectively compensate momentary external power loss by reducing the unit's power consumption while external power is lost. In an exemplary embodiment external power loss is detected by the electronic control unit's microprocessor. The microprocessor thereupon disables some components within the electronic control unit and operates with limited functionality for the duration of external power loss. The electronic control unit uses internal energy storage, e.g. A hold capacitor, to sustain its limited functionality operation. Upon recovery from the external power loss the electronic control unit resumes full operation.