FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to electronic devices and more specifically to battery-powered electronic devices with data storage capabilities.  
         BACKGROUND OF THE INVENTION  
         [0002]    One problem that arises with battery-powered devices is the sudden loss of power due to battery failure. This problem is especially serious in a portable device that stores data such as a digital camera, hand-held computer, or cellular phone because the file system can become corrupted due to the unexpected loss of power.  
           [0003]    One way to deal with the problem is to disable operation of the device when the battery charge drops to a predetermined percentage of its capacity. For example, the device may warn the user to cease operation, or the device may prevent further operation, when the battery drops to 20 percent of its capacity. Although this approach prevents sudden, catastrophic power loss, it wastes significant battery capacity and shortens the time between recharges. Moreover, some battery types such as nickel cadmium batteries eventually lose capacity if they are not substantially depleted before being recharged. Therefore, the method of holding battery charge in reserve can shorten the usable life of such batteries.  
           [0004]    A more sophisticated approach involves predicting the remaining life of specific batteries or battery types. This approach takes advantage of differences in quality among batteries by allowing the device to operate longer without recharging when a high-quality battery is present. This approach increases development costs because extensive testing and profiling of the wide variety of available batteries is required. Also, the device&#39;s software or firmware must be configured to implement multiple power management profiles, increasing complexity.  
           [0005]    It is thus apparent that there is a need in the art for an improved power management method for battery-operated devices.  
         SUMMARY OF THE INVENTION  
         [0006]    A method is provided for starting up a battery-powered device after a battery failure. An apparatus is also provided to carry out the method.  
           [0007]    Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIGS. 1A and 1B are flowcharts showing exemplary prior-art methods for providing a robust file system used by the invention.  
         [0009]    [0009]FIG. 2 is a block diagram of a digital camera in accordance with an exemplary embodiment of the invention.  
         [0010]    [0010]FIG. 3 is a block diagram of a method for shutting down the digital camera shown in FIG. 2 in accordance with an exemplary embodiment of the invention.  
         [0011]    FIGS.  4 A- 4 C are flowcharts of a method for starting up the digital camera shown in FIG. 2 in accordance with an exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Useable battery life in a battery-operated device may be extended by allowing a device to operate until a battery failure occurs. Throughout this detailed description, a “battery failure” denotes a condition in which the battery charge drops below the level required to operate the device. Since most batteries, once the load has been removed, recover sufficiently after a few moments to allow brief operation of the device, that brief period may be used to shut down the device properly until the battery has been replaced or recharged. Once the battery has been replaced or recharged, the device may take steps to correct any anomalies in its stored data that may have been caused by the sudden loss of power. For example, the device may ensure the integrity of its file system, in particular its file allocation table (FAT), by deleting any incomplete FAT entries that were not properly written during the battery failure.  
         [0013]    The approach just described assumes an underlying robust file system that is designed to prevent the loss of more than one data object (e.g., the most recently created data object) due to an unexpected loss of power. Although such robust file systems are well known in the art, two simple examples are provided in FIGS. 1A and 1B. Both examples include a FAT, although not all file systems use a FAT, as those skilled in the art will recognize. In either case, the file system includes a method for keeping track of files and where they are located. In FIG. 1A, a need to update the FAT at  105  causes the FAT to be copied from non-volatile storage such as flash memory to random access memory (RAM) at  110 . The update is made to the copy at  115 . If power is suddenly lost due to a battery failure, the original FAT is still stored in non-volatile memory, and only the newest entry in the copy is lost. Under normal conditions, the updated FAT created from the copy is saved to non-volatile memory at  120 , and the original FAT is deleted from non-volatile memory at  125 . In FIG. 1B, a new entry is appended to the FAT in non-volatile memory at  125 , eliminating the need to copy and rewrite the entire FAT each time an update is made. Periodically, when suitable criteria are satisfied at  130  (e.g., when the battery is well charged and the device is turned on but idle), the FAT is rewritten to eliminate out-of-date entries at  135 . The methods shown in FIGS. 1A and 1B are merely examples. Any file system robust against unexpected power loss, including those not employing a FAT, may be used in implementing the invention.  
         [0014]    The invention may be applied to a wide variety of battery-operated devices such as digital cameras, hand-held computers, personal digital assistants (PDAs), electronic organizers, or cellular phones. FIG. 2 is a block diagram of a digital camera  200  in accordance with an exemplary embodiment of the invention. In FIG. 2, controller  205  communicates over data bus  210  with imaging device  215 , internal memory  220 , battery status circuit  223 , power button  225 , display  230 , and nonvolatile memory  235 . Imaging device  215  converts optical images received from optical system  240  to digital images. In a typical implementation, imaging device  215  comprises a charge-coupled device (CCD), an analog-to-digital converter (A/D), a gain control, and a digital signal processor (DSP) (not shown in FIG. 2). Internal memory  220  may comprise both RAM and non-volatile memory such as flash memory. The RAM is used to hold digital images temporarily until they can be transferred to non-volatile storage. A shutdown bit  245  and a disable bit  250 , which will be explained in a later portion of this description, may be stored in the nonvolatile portion of internal memory  220 . Non-volatile memory  235 , which is typically of the removable type, is used for long-term storage of digital images produced by imaging device  215 . Non-volatile memory  235  includes a file system  255 , of which one key component is FAT  260 . Digital camera  200  may be powered by a battery  265 . When digital camera  200  is off, power may be removed from all components except controller  205  so that controller  205  can detect changes in state of power button  225 .  
         [0015]    Controller  205 , via battery status circuit  223 , may determine the charge status of battery  265 . For example, battery status circuit  223  may measure the voltage of battery  265 . Alternatively, battery status circuit  223  may comprise an electromechanical mechanism that detects the removal and insertion of battery  265 . Techniques for measuring battery voltage and detecting battery insertion and removal are well known in the art. Throughout this detailed description, the charge status of battery  265  will be considered “favorable” whenever battery  265  is capable of powering digital camera  200  for a period longer than a brief shutdown sequence performed whenever digital camera  200  is powered off. Otherwise, the charge status of battery  265  will be considered “unfavorable.” 
         [0016]    In an exemplary embodiment of the invention, shutdown bit  245  is used to determine whether digital camera  200  was shut down properly. Shutdown bit  245  is nominally cleared (logic “0”) when digital camera  200  is off. When digital camera  200  is turned on, shutdown bit  245  is set (logic “1”) to correspond to the power-on state. When digital camera is shut down normally, shutdown bit  245  is cleared. The normal shutdown procedure is summarized in the flowchart of FIG. 3. If a request to turn off digital camera  200  is received at  305 , a shutdown sequence is performed at  310 . The shutdown sequence may comprise such tasks as removing power from all components of digital camera  200  except controller  205 , placing control lines in a high-impedance state, removing charge from a strobe (flash unit), and retracting a lens included in optical system  240 . Controller  205  remains powered to sense subsequent input from power button  225  after digital camera  300  has been turned off. At  315 , shutdown bit  245  is cleared to indicate a normal shutdown, and the process ends at  320 .  
         [0017]    If during operation of digital camera  200  power is suddenly lost due to a battery failure, the normal shutdown sequence is not performed, and shutdown bit  245  is not cleared. The next time digital camera  200  is started up, controller  205  can detect that digital camera  200  was not shut down properly, indicating the possibility of a battery failure. Disable bit  250  may be used to indicate that a battery failure has occurred and that further operation of digital camera  200  should be disabled until battery  265  is recharged or replaced. For example, if shutdown bit  245  is found to be set upon start up, indicating a possible battery failure, the status of battery  265  may be determined via battery status circuit  223  as explained previously. If the status of battery  265  is unfavorable, the residual charge on battery  265  may be used to perform the shutdown sequence and to set disable bit  250 , indicating that further operation of the device is to be prevented until battery  265  is recharged or replaced. Further attempts to start up digital camera  200  will fail until battery  265  is recharged or replaced, at which time digital camera  200  may be started up normally, disable bit  250  cleared, and the integrity of file system  255  checked.  
         [0018]    FIGS.  4 A- 4 C are flowcharts of a method for starting up digital camera  200  in accordance with an exemplary embodiment of the invention. The method shown in FIGS.  4 A- 4 C may be performed whenever digital camera  200  is off and power button  225  is pressed to activate the device. If power button  225  is pressed at  405 , the status of shutdown bit  245  is polled at  410 . If shutdown bit  245  is cleared (logic “0”), the status of disable bit  250  is polled at  415 . If disable bit  250  is also cleared (logic “0”), digital camera  200  is started up normally at  420 . Shutdown bit  245  is then set (logic “1”) at  425 , and the start-up process terminates at  430 . If shutdown bit  245  is set (logic “1”) at  410 , controller  205  can determine that digital camera was not shut down properly, possibly due to a battery failure. In that case, control proceeds to FIG. 4B.  
         [0019]    In FIG. 4B, the status of battery  265  is obtained from battery status circuit  223  at  435 . If the status is favorable at  440  (e.g., battery  265  has been recharged or replaced since the battery failure), digital camera is started up at  420 . If the status of battery  265  is unfavorable at  440 , the shutdown sequence is performed at  310 . Following the shutdown sequence, shutdown bit  245  is cleared and disable bit  250  is set at  450 , after which the process terminates at  430 . Disable bit  250  is set at  450  to indicate that digital camera  200  may not be operated again until battery  265  is either replaced or recharged. If the status of battery  265  was favorable at  440  and digital camera  200  started up successfully at  420 , file system  255  may have been corrupted by the failure of battery  265 . In this particular exemplary embodiment, it is assumed that new FAT entries are appended as explained in connection with FIG. 1B. At  455 , controller  205  checks for an incomplete FAT entry. If one is found, it may be deleted at  460 , and an error message may be output to display  230  at  465  to warn a user that the last image captured has been lost. In other embodiments, the FAT may be copied before being modified as in FIG. 1A. In that case, an incompletely saved FAT may have to be deleted, not just a single entry. In either case, however, at most one FAT entry (the most recent one) is lost.  
         [0020]    If shutdown bit  245  is cleared (logic “0”) at  410  in FIG. 4A and disable bit  250  is set (logic “1”) at  415 , control proceeds to  435  in FIG. 4C, where the status of battery  265  is determined from battery status circuit  223 . In this case, the steps shown in FIG. 4B have already been performed. That is, a battery failure has occurred, and digital camera has been shut down using the residual power of battery  265 . Any further attempt to turn on digital camera  200  leads to  435  in FIG. 4C. If the battery status is unfavorable at  440  in FIG. 4C, the process ends at  430  without digital camera  200  being started up because disable bit  250  is set. If the status of battery  265  is favorable at  440 , digital camera  200  is started up at  420 , after which shutdown bit  245  is set and disable bit  250  is cleared at  470 . Processing then proceeds as indicated in FIG. 4B (steps  455 ,  460 , and  465 ).  
         [0021]    The foregoing description of the present invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.
A power management method and associated apparatus allows a device to make maximum use of its battery before replacement or recharging. After a battery failure, the device may be shut down properly and disabled until the battery is replaced or recharged. An exemplary embodiment is described in the context of a digital camera.