Abstract:
An intelligent power management system is particularly applicable to accessory devices that can be coupled to a basic device to provide the basic device with an enhanced feature. The accessory device includes a control processor and a power supply unit, wherein the power supply unit supplies electrical energy to the control processor in response to a control signal received from the basic device. The power supply unit includes a power management circuit that maintains the electrical energy supplied to the control processor during fluctuations of the control signal.

Description:
FIELD OF THE INVENTION 
     The invention relates in general to power management systems. More specifically, the invention relates to a power management system that is particularly applicable to an accessory device that can be coupled to a basic device such as a personal digital assistant device. 
     BACKGROUND OF THE INVENTION 
     Personal digital assistant devices, commonly referred to as PDA&#39;s, have recently become popular for use in organizing schedules and personal information. In order to expand the capability of such devices, various additional features have been proposed for use with the PDA&#39;s including, for example, communication devices and digital cameras. While these additional features can be incorporated into the structure of the PDA itself, it is generally preferable to provide accessory devices that can be coupled to a basic PDA via a standard interface to perform the additional functions on an as needed basis. 
     One problem associated with the use of such accessory devices, however, is the mismatch between the power requirements of the basic PDA and the power requirements for the additional features. The basic PDA is required to have a low power drain requirement in order to extend the useful life of its batteries. Devices such as digital cameras, however, usually consume power at a much higher rate than basic PDA&#39;s and other small consumer electronic devices. In order to prevent unnecessary draining of the batteries of the basic PDA device, it is therefore preferable to provide the accessory device with its own batteries. 
     Even with its own power source, however, the power requirements of the accessory device might be unnecessarily drained if the accessory device begins to draw power or is turned on as soon as it is attached to the basic PDA. It would therefore be preferable to provide a mechanism for managing the power requirements of the accessory device to conserve available battery power. 
     In view of the above, it is an object of the present invention to provide an accessory device for a personal digital assistant that includes intelligent power management that prevents undue draining of battery power. 
     SUMMARY OF THE INVENTION 
     The invention provides an intelligent power management system for use in electronic devices. The invention is applicable to accessory devices that can be coupled to a basic device to provide the basic device with an enhanced feature. In a preferred example, the basic device includes a docking interface and the accessory device couples to the docking interface of the basic device. The accessory device includes a control processor and a power supply unit, wherein the power supply unit supplies electrical energy to the control processor in response to a control signal received from the basic device. The power supply unit maintains the electrical energy supplied to the control processor during fluctuations of the control signal with a power management circuit. 
     The power supply unit preferably includes a power management circuit, which receives the control signal from the basic device and a further control signal from the control processor, and a power supply. The power management circuit preferably includes a first switching element that is responsive to the control signal and the further control signal to generate a power activation signal, and a second switching element that is responsive to the power activation signal. The second switching element couples a battery of the accessory device to the power supply in response to the power activation signal. 
     The first switching element is implemented through the use of a bipolar transistor and the second switching element is implemented through the use of a field effect transistor, although other circuit elements may be utilized to perform the same basic function. 
     Alternatively, the power management circuit utilizes a capacitor and resistor network to latch an input of the power supply to a logic level that enables operation. 
     The invention is particularly applicable for implementation in an accessory device that attaches to a personal digital assistant device such as a digital camera, although the invention may also be employed in other types of accessory devices and basic devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to certain preferred embodiments thereof as illustrated in the accompanying drawings, wherein: 
         FIG. 1  illustrates a basic personal digital assistant device and digital camera; 
         FIG. 2  illustrates the digital camera coupled to the basic personal digital assistant device; 
         FIG. 3  is a block diagram of the digital camera illustrated in  FIG. 2 ; 
         FIG. 4  is a schematic diagram of a power unit incorporated into the digital camera illustrated in  FIG. 3 ; 
         FIG. 5  is an operational flow diagram illustrating the operation of the power unit illustrated in  FIG. 4 ; and 
         FIG. 6  is a schematic block diagram of a power supply unit in accordance with a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a PDA  10  that includes a display screen  12 , various user controls  14  and a docking interface  16 . The docking interface  16  is used to couple the PDA  10  to various accessory devices including, as just one example, a digital camera  18  that is supplied with a corresponding mating interface  20 .  FIG. 2  illustrates the digital camera  18  coupled to the PDA  10 . For the purposes of this discussion, it will be understood that the terms “docking interface” and “mating interface” include mechanical and/or electrical components required to mate the digital camera  18  to the PDA  10  and allow the transfer of control and data signals therebetween. It is preferable to utilize a standard communication protocol (for example IEEE RS232) to implement the docking interface  16  and the mating interface  20 , although any other standard or custom communication protocol may also be employed. 
     In the illustrated embodiment, the PDA  10  is provided with an imaging application program that utilizes the display screen  12  of the PDA  10  as a viewfinder for the digital camera  18 . In addition, the imaging application program interprets signals received from the user controls  14  as commands to operate the digital camera  18 . As a result, the digital camera  18  need only be provided with appropriate optics and image capture circuitry, thereby avoiding the necessity of providing duplicate components. Images captured by the digital camera  18  can be stored and downloaded to other devices, such as a personal computer, via the PDA  10 . 
     A detailed schematic block diagram of the digital camera  18  is illustrated in  FIG. 3 . As shown in  FIG. 3 , the digital camera  18  includes a lens system  22  that focuses scene light onto an electronic image sensor  24 . Image data generated by the electronic image sensor  24  is supplied to a programmable logic device  26 , which controls the management and storage of the image data in a memory device  28  in response to control signals supplied by a control processor  30 . A crystal  25  provides a stable reference frequency which is used to generate clock signals in the image sensor  24  and the programmable logic device  26 . The control processor  30  is coupled to a UART  32 , which in turn is coupled to the mating interface  20 . Power is supplied to the various components by a power supply unit  34  that is coupled to ordinary AAA batteries  36 . Alternatively, an integrated rechargeable battery may be employed. A battery voltage sensing circuit  38  is preferably provided to monitor the charge condition of the batteries  36  and provide the status thereof to the control processor  30 . 
     The power supply unit  34  is illustrated in greater detail in  FIG. 4  as including a power management circuit  40  and a conventional switched mode power supply (SMPS)  42 , although other types of power supplies or regulation circuitry could be utilized based on the particular application of interest. The power management circuit  40  includes an NPN transistor Q 2  having its base coupled to a first control signal line, which in the illustrated example is a switched mode power supply enable line (SMPS ENABLE), and to a second control signal line, which in the illustrated example is a CLEAR-TO-SEND control line (CTS) received from the PDA  10  via the docking interface  16  and mating interface  20 , both of which can be utilized to control the operation of the NPN transistor Q 2 . The NPN transistor Q 2 , in turn, is used to control the gate of the power FET Q 1  by supplying a power activation signal thereto. 
     A detailed description of the operation of the power management circuit  40  will now be described with reference to  FIG. 5 . As shown in  FIG. 5 , the CTS control line is low when the PDA  10  is off. When a user turns the PDA  10  on, a decision is made as to whether an imaging application is running on the PDA  10 . If an imaging operation is running on the PDA  10 , the CTS control line is held high, which causes transistor Q 2  to turn on. The activation of transistor Q 2  then causes transistor Q 1  to conduct. As a result, the SMPS  42  is energized from the power supplied from the batteries  36  and the regulated operating voltage for the digital camera  18  is activated. 
     Activation of the power for the digital camera  18  causes the control processor  30  to power up. After the initial power up, the control processor  30  enters a two second idle timer routine. During the idle timer routine, the UART  32  waits to receive command signals from the PDA  10  via the mating interface  20 . If a command signal is received relating to an imaging function, the control processor  30  sets the SMPS ENABLE line high which forces the SMPS  42  to stay on. An imaging operation is then performed under the control of the control processor  30 . 
     After completion of the imaging operation, the control processor returns to the two second idle timer routine. If no user command is detected and the CTS line remains high, the UART  32  continues to wait for receipt of a command signal. If the CTS line is not high, however, it indicates that the PDA  10  has been powered off or the imaging application has been terminated. In such a circumstance, if the idle timer routine being performed by the control processor  30  has also expired, the control processor  30  clears the SMPS ENABLE line and the power for the digital camera  18  is deactivated. 
     The power management circuit  40  insures that the power to the digital camera  18  is activated if the digital camera  18  is attached to the PDA  10  and an imaging application is running. The use of the SMPS ENABLE signal to clamp Q 2  in an on state avoids problems associated with the instability of the CTS signal supplied from the PDA  10 . For example, the CTS signal may toggle at each activation of the user controls  14 . Thus, the CTS signal is high once the PDA  10  enters the imaging application, but when the user activates the user controls  14  to capture an image, the CTS signal may drop low causing the digital camera  18  to inadvertently power down. The SMPS ENABLE signal, however, clamps Q 2  for the duration of the actual imaging operation, thereby avoiding inadvertent power down due to fluctuations or toggling of the CTS signal. 
       FIG. 6  illustrates an alternative embodiment that operates independently from the control processor  30  in which the SMPS ENABLE does not have to be supplied by the control processor  30 . In the embodiment illustrated in  FIG. 6 , the power management circuit  40  of the power supply unit  34  includes a diode  44 , capacitor  46  and a resistor  48  that are coupled to the CTS line and a shutdown input (SHUTDOWN) of the switched mode power supply  42 . As in the previous embodiment, the CTS line goes to a logic high when an imaging operation is running on the PDA  10 , thereby charging the capacitor  46  and pulling the SHUTDOWN input to a logic high to enable operation of the switched mode power supply  42 . If the signal on the CTS line should momentarily drop low during operation, the diode  44  becomes back biased forcing the capacitor  46  to discharge through the resistor  48 . The values of the capacitor  46  and resistor  48  are selected to provide a time constant sufficient to hold a logic high state on the SHUTDOWN input to bridge momentary dropouts of the signal supplied to the CTS line. When the CTS line returns to a logic high, the capacitor  46  is charged in preparation for the next dropout. Once the imaging application is terminated or the PDA  10  is turned off, the signal supplied to the CTS line goes low and the capacitor  46  eventually discharges causing the switched mode power supply  42  to deactivate. 
     The invention has been described with reference to certain preferred embodiments thereof. If will be understood, however, that modifications and variations are possible within the scope of the appended claims. For example, the type of transistors or switching elements employed within the power management circuit may be readily varied based on intended application. Further, the invention is not limited to the use of a CTS signal, but is applicable to any power management application in which an initial power on signal is subject to instability. Still further, although a preferred embodiment of the invention was illustrated using a digital camera, the invention is applicable to any type of accessory device that is coupled to a basic PDA or to any other types of combinations of accessory devices and base components. 
     PARTS LIST 
     
         
         
           
               10  Personal Digital Assistant (PDA) 
               12  Display Screen 
               14  User Controls 
               16  Docking Interface 
               18  Digital Camera 
               20  Mating Interface 
               22  Lens System 
               24  Electronic Image Sensor 
               25  Crystal 
               26  Programmable Logic Device 
               28  Memory 
               30  Control Processor 
               32  UART 
               34  Power Supply Unit 
               36  Batteries 
               38  Voltage Sensing Circuit 
               40  Power Management Circuit 
               42  Switched Mode Power Supply 
               44  Diode 
               46  Capacitor 
               48  Resistor