Abstract:
An aspect of the invention provides an image processing apparatus that processes image data on the basis of received data, the image processing apparatus comprising: a CPU operatable on multiple CPU clocks; a selecting unit configured to select a first CPU clock on the basis of an operation mode of the image processing apparatus and processing state information on the image processing apparatus; and a changing unit configured to change a second CPU clock to the first CPU clock selected by the selecting unit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. P2008-041985 filed on Feb. 22, 2008, entitled “Image Processing Apparatus”, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image processing apparatus that performs image processing on inputted image data, and particularly, to an image processing apparatus having an energy saving function. 
         [0004]    2. Description of Related Art 
         [0005]    Typically, an energy saving function has been included in image processing apparatus, such as printers, copying machines, facsimile apparatus, scanner devices, and multifunction machines. The energy saving function causes the image processing apparatus to shift to a power saving state when a predetermined time passes after powering on or completion of a printing job and the next printing job is not executed, i.e., at the time of printing standby. 
         [0006]    For example, Japanese Patent Application Publication No. Hei 6-118836 discloses a technique for decreasing the image processing apparatus power consumption by stopping power to a fixing device at the time of printing standby. 
         [0007]    In recent years, further reduction of the power consumption is required in such an image processing apparatus. For example, an image processor, such as a controller board on which a CPU is mounted, has an edit-output function to perform editing of video data of an image to be printed, and to output the video data to an image formation unit. For that reason, the image processor is a component that consumes much power, as well as the above-mentioned fixing device. However, sufficient power saving measures for the image processor have not been taken in the conventional arts. There has been almost no difference between the power consumed by the image processor during printing standby and that during the edit-output processing execution, thereby wasting power. 
       SUMMARY OF THE INVENTION 
       [0008]    An aspect of the invention provides an image processing apparatus that processes image data on the basis of received data, the image processing apparatus comprising: a CPU operatable on multiple CPU clocks; a selecting unit configured to select a first CPU clock on the basis of an operation mode of the image processing apparatus and processing state information on the image processing apparatus; and a changing unit configured to change a second CPU clock to the first CPU clock selected by the selecting unit. 
         [0009]    According to the above-mentioned image processing apparatus, since the CPU clock is changed based on the operation mode and processing state of the image processing apparatus, power consumption by the CPU during standby can be reduced. 
         [0010]    Another aspect of the invention provides an image processing apparatus that processes image data on the basis of received data, the image processing apparatus comprising: an input unit configured to receive image data; an edit-output unit comprising a CPU operable on multiple frequencies, the edit-output unit configured to edit the image data received to generate video data, and output the video data; a formation unit configured to form an image on a medium based on the video data output; and a frequency changing unit configured to change an operation frequency of the CPU based on the processing status of the formation unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram showing a configuration of a printer according to Embodiment 1; 
           [0012]      FIG. 2  is an explanatory view showing an example of a determination table of Embodiment 1; 
           [0013]      FIG. 3  is an explanatory view showing an example of a setting information table of Embodiment 1; 
           [0014]      FIG. 4  is a circuit diagram showing a configuration of a voltage switching controller; 
           [0015]      FIG. 5  is a first flow chart showing print operation of Embodiment 1; 
           [0016]      FIG. 6  is a second flow chart showing print operation of Embodiment 1; 
           [0017]      FIG. 7  is a first time chart showing the print operation of Embodiment 1; 
           [0018]      FIG. 8  is a second time chart showing the print operation of Embodiment 1; 
           [0019]      FIG. 9  is a first flow chart showing switching operation of Embodiment 1; 
           [0020]      FIG. 10  is a second flow chart showing switching operation of Embodiment 1; 
           [0021]      FIG. 11  is a block diagram showing a configuration of a printer according to Embodiment 2; 
           [0022]      FIG. 12  is an explanatory view showing an example of a load information table; 
           [0023]      FIG. 13  is an explanatory view showing an example of a job information storage unit; 
           [0024]      FIG. 14  is an explanatory view showing an example of a determination table of Embodiment 2; 
           [0025]      FIG. 15  is an explanatory view showing an example of a setting information table of Embodiment 2; 
           [0026]      FIG. 16  is a first flow chart showing print operation of Embodiment 2; 
           [0027]      FIG. 17  is a second flow chart showing the print operation of Embodiment 2; 
           [0028]      FIG. 18  is a third flow chart showing the print operation of Embodiment 2; 
           [0029]      FIG. 19  is a first time chart showing print operation of Embodiment 2; and 
           [0030]      FIG. 20  is an explanatory view showing an example of calculation of a total load point of a print job. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0031]    Hereinafter, embodiments will be described, referring to the drawings. Same reference numerals will be given to elements common in the drawings. While as an example of an image forming apparatus an electrophotographic printer with an energy saving function will be described, the present invention is not limited to this, and can be implemented in many ways. 
       Embodiment 1 
       [0032]      FIG. 1  is a block diagram showing a configuration of a printer according to Embodiment 1. 
         [0033]    As shown in  FIG. 1 , printer  10  according to the embodiment is connected to personal computer  12  through communication line  11 . Printer  10  receives a print job including image data from personal computer  12 , and performs printing processing. 
         [0034]    As shown in  FIG. 1 , printer  10  serves as an image processing apparatus, and comprises communication controller  21 , image processor  22 , CPU  23 , image formation unit  24 , mode determination unit  25 , setting information storage unit  26 , mode setting unit  27 , CPU power supply unit  28 , and voltage switching controller  29 . 
         [0035]    Communication controller  21  includes host interfaces such as to a network and a USB port, and is connected to personal computer  12  through communication line  11 . As a receiver, communication controller  21  receives the print job including the image data from personal computer  12 . Communication controller  21  is also connected to image processor  22  through bus signal line  32  to output the received print job to image processor  22 . Moreover, communication controller  21  outputs an interrupt signal I-INT that indicates reception of the print job into mode determination unit  25 . Hereinafter, interrupt signal I-INT that is outputted by communication controller  21  into mode determination unit  25  is referred to as received signal  40 . 
         [0036]    As an edit-output unit, image processor  22  reads the image data from the print job input by communication controller  21  and performs image processing, such as gray level correction and dithering, on the image data. Then, image processor  22  generates rasterized video data, and subsequently, outputs the video data to image formation unit  24 . Hereinafter, the processing from input of the print job to generation of the video data in image processor  22  is referred to as input editing process, and the processing of outputting the video data to image formation unit  24  is referred to as output process. Image processor  22  is connected to CPU  23  through bus signal line  33 . 
         [0037]    When starting output of the video data to image formation unit  24 , image processor  22  outputs interrupt signal PS-INT that indicates start of output into mode determination unit  25 . When output of the video data to image formation unit  24  is terminated, image processor  22  outputs interrupt signal PE-INT that indicates termination of output into mode determination unit  25 . Interrupt signal PS-INT and interrupt signal PE-INT, which are output by image processor  22  into mode determination unit  25  are referred to as output start signal  41  and output termination signal  42 , respectively. 
         [0038]    CPU  23  controls image processor  22 , and is connected to image processor  22  through bus signal line  33 . CPU  23  can operate at different clock frequencies, and the mode in which CPU  23  operates is referred to as an operation mode. CPU  32  has a high speed operation mode in which CPU  32  operates at a higher frequency, and a low speed operation mode in which CPU  32  operates at a lower frequency. The high speed operation mode consumes more power than the low speed operation mode. In this embodiment, CPU  23  can operate at an operating voltage V CPU  of 1.26 V to 1.5 V and at operating frequency f CPU  of 500 MHz to 1000 MHz, and operates in either the low speed operation mode or high speed operation mode. However, is not limited to these two types of operation mode. For example, CPU  32  may have three types, four types, or more types of operation mode. The operation mode of CPU  23  is set by mode setting unit  27 . CPU  23  operates at a low voltage of V CPU =1.26 V and at a low frequency of f CPU =500 MHz in the low speed operation mode, and operates at a high voltage of V CPU =1.5 V and at a high frequency f CPU =1000 MHz in the high speed operation mode. 
         [0039]    Image formation unit  24  comprises a print head, various drums, a fixing device, a transfer device, a power supply, or the like, which are not shown, and is connected to image processor  22  through bus signal line  34 . Image formation unit  24  performs printing processing on a print medium on the basis of the video data inputted from image processor  22 . In this embodiment, image formation unit  24  is provided with a power saving mode wherein, when image formation unit  24  does not perform the printing processing for more than a specified period of time, power supply to the fixing device is stopped in response to an instruction from image processor  22 . 
         [0040]    Additionally, a temperature sensor is installed within the fixing device in image formation unit  24  to sense the surface temperature of the fixing roller. Image formation unit  24  outputs TTL level control signal FT into mode determination unit  25 . The TTL level control signal FT is a notification signal for notification of the temperature of the fixing device detected by the temperature sensor, and is referred to as fixing device temperature signal  43  below. Here, image formation unit  24  outputs an H level signal as fixing device temperature signal  43  when the fixing device temperature is within a range of ±20 degrees C. relative to predetermined fixing target temperature T c , and outputs an L level signal as fixing device temperature signal  43  when the fixing device temperature is out of the range of T c ±20 degree C. Fixing target temperature T c  is an optimal temperature for fixing a toner image onto the print medium, and is determined based on the temperature, humidity, or the like of the inside of printer  10 . 
         [0041]    Mode determination unit  25  contains determination table storage unit  30 . As a judging unit and a determination unit, mode determination unit  25  determines a setting mode on the basis of a determination table stored in determination table storage unit  30  when the interrupt signal is input from communication controller  21  or image processor  22 . Here, the setting mode indicates the operation mode to be set in CPU  23 . 
         [0042]      FIG. 2  is an explanatory view showing an example of the determination table of Embodiment 1. 
         [0043]    As shown in  FIG. 2 , setting mode information is set in the determination table in advance. Setting mode information indicates the setting mode and corresponds to signal states of the interrupt signal and control signal that are input from each unit. 
         [0044]    For example, in the determination table indicated in  FIG. 2 , the setting mode information “high speed mode” is stored in association with the signal state “rise” of received signal  40 , and the signal state “H level” of fixing device temperature signal  43 , while the setting mode information “switching unnecessary” is stored in association with the signal state “rise” of received signal  40  and the signal state “L level” of fixing device temperature signal  43 . This indicates that mode determination unit  25  determines the setting mode depending on a signal level of fixing device temperature signal  43  input from image formation unit  24  when received signal  40  is input from communication controller  21 . In other words, when fixing device temperature signal  43  is the H level signal, mode determination unit  25  determines that the setting mode is the high speed mode. On the other hand, when fixing device temperature signal  43  is the L level signal, mode determination unit  25  determines that the operation mode need not be switched. 
         [0045]    Moreover, in the determination table shown in  FIG. 2 , the setting mode information “high speed mode” is stored in association with the signal state “rise” of output start signal  41 , while the setting mode information “low speed mode” is stored in association with the signal state “rise” of output termination signal  42 . This indicates that mode determination unit  25  determines that the setting mode is the high speed mode when receiving output start signal  41  from image processor  22 , and determines that the setting mode is the low speed mode when receiving output termination signal  42 . 
         [0046]    Mode determination unit  25  also contains flag storage unit  31  as a storage unit. Flag information that indicates the current operation mode of CPU  23  is stored in flag storage unit  31 . The flag information is “H” when the operation mode of CPU  23  is the high speed mode, and is “L” when the operation mode is the low speed mode. 
         [0047]    After determining the setting mode on the basis of the input signal and the determination table ( FIG. 2 ), mode determination unit  25  reads the flag information from flag storage unit  31  to determine whether or not the setting mode and the operation mode coincide with each other. When determining that the setting mode and the operation mode do not coincide, mode determination unit  25  notifies mode setting unit  27  of the setting mode information in order to change the operation mode of CPU  23  to the determined setting mode. As an updating unit, mode determination unit  25  also updates the flag information stored in flag storage unit  31 . 
         [0048]    Setting information storage unit  26  is connected to mode setting unit  27  through bus signal line  36 , and stores a setting information table. 
         [0049]      FIG. 3  is an explanatory view showing an example of the setting information table of Embodiment 1. 
         [0050]    As shown in  FIG. 3 , in the setting information table, setting information comprises switching control signal information and set frequency information and is stored in association with each piece of the setting mode information. The switching control signal information indicates a signal level of switching control signal VC  44  output by mode setting unit  27 , the signal level being either an “H level” or “L level”. This switching control signal VC  44  is a signal output to voltage switching controller  29  to control operating voltage V CPU  of CPU  23 . The set frequency information indicates operating frequency f CPU  of CPU  23 , which is switched according to the setting mode. 
         [0051]    For example, in the setting information table shown in  FIG. 3 , the switching control signal information “H level” and the set frequency information “1000 MHz” are stored in association with the setting mode information “high speed mode”. Moreover, the switching control signal information “L level” and the set frequency information “500 MHz” are stored in association with the setting mode information “low speed mode”. 
         [0052]    Mode setting unit  27  is connected to mode determination unit  25  through bus signal line  35 , and is connected to setting information storage unit  26  through bus signal line  36 . Mode setting unit  27  is also connected to CPU  23  through bus signal line  37 , and, as a frequency switching unit, switches operating frequency f CPU  of CPU  23 . 
         [0053]    When notified by mode determination unit  25  of setting mode information having either the low speed mode or the high speed mode, mode setting unit  27  reads setting information corresponding to the setting mode information from the setting information table ( FIG. 3 ) of setting information storage unit  26 . Then, mode setting unit  27  writes set frequency information included in the thus-read setting information into an internal register of CPU  23  to change operating frequency f CPU . Based on switching control signal information included in the thus-read setting information, mode setting unit  27  switches the signal level of the switching control signal VC  44  to be output to voltage switching controller  29 . 
         [0054]    When notified by mode determination unit  25  of the setting mode information “high speed mode”, mode setting unit  27  switches switching control signal VC  44  to the H level signal on the basis of the switching control signal information “H level” read from the setting information table ( FIG. 3 ). Simultaneously, mode setting unit  27  changes operating frequency f CPU  of CPU  23  to 1000 MHz on the basis of the set frequency information “1000 MHz”. 
         [0055]    When notified by mode determination unit  25  of the setting mode information “low speed mode”, mode setting unit  27  switches switching control signal VC  44  to the L level signal on the basis of the switching control signal information “L level” read from the setting information table ( FIG. 3 ), and simultaneously, changes operating frequency f CPU  of CPU  23  to 500 MHz on the basis of the set frequency information “500 MHz”. 
         [0056]    CPU power supply unit  28  is a power circuit formed of a DC-DC controller or the like. In response to supply of power from a power supply unit of printer  10  not shown, CPU power supply unit  28  sets the power level according to a rating of CPU  23  and outputs operating voltage V CPU  to CPU  23 . CPU power supply unit  28  also has a feedback function to adjust output voltage V CPU  to hold constant output voltage V ref    46  from voltage switching controller  29 . 
         [0057]    As a voltage switching unit, voltage switching controller  29  outputs voltage V ref    46  to CPU power supply unit  28  to switch operating voltage V CPU  of CPU  23  on the basis of switching control signal VC  44  input from mode setting unit  27 . In the embodiment, output voltage V ref    46  from voltage switching controller  29  is 0.75 V. 
         [0058]      FIG. 4  is a circuit diagram showing a configuration of voltage switching controller  29 . 
         [0059]    As shown in  FIG. 4 , voltage switching controller  29  includes resistances R 1 , R 2 , and R 3 , and an FET. The voltage level of operating voltage V CPU    45  supplied to CPU  23  is controlled by controlling values of resistance values R 1 , R 2  and R 3  of resistances R 1 , R 2 , and R 3  and a current that flows into the FET. 
         [0060]    For example, when the H level signal is input as switching control signal VC  44  from mode setting unit  27  into voltage switching controller  29 , the FET operates, and a current flows into resistance R 2 . Therefore, operating voltage V CPU  is expressed by an equation V CPU =V ref ×(1+R 1 /R 23 ), where R 23  is a combined resistance value of resistances R 2  and R 3  and R 23 =R 2 ×R 3 /(R 2 +R 3 ). 
         [0061]    On the other hand, when the L level signal is input as switching control signal VC  44  from mode setting unit  27  into voltage switching controller  29 , the FET stops, and the current does not flow into resistance R 2 . Therefore, operating voltage V CPU  is expressed by an equation V CPU =V ref ×(1+R 1 /R 2 ). 
         [0062]    In the embodiment, resistance values R 1 , R 2  and R 3  of resistances R 1 , R 2 , and R 3  included in voltage switching controller  29  are respectively 1.5 kΩ, 4.7 kΩ, and 2.2 kΩ. Here, when operating voltage V CPU  is calculated based on the above-mentioned equations, V CPU =1.5 V when switching control signal VC  44  is the H level signal, and V CPU =1.26 V when switching control signal VC  44  is the L level signal. 
         [0063]    Generally, little current flows into a CMOS transistor circuit like the CPU except when a change occurs in a circuit signal in synchronization with clock timing. Therefore, the power consumption of the CPU is reduced in proportion to the decrease of the operating frequency and operating voltage of the CPU. In CPU  23  of the embodiment, when the operation mode is changed from the high speed mode to the low speed mode, operating voltage V CPU  drops by 16% from a high voltage of 1.5 V to a low voltage of 1.26 V, and operating frequency f CPU  drops by 50% from a high frequency of 1000 MHz to a low frequency of 500 MHz. Operating frequency f CPU  has an influence on the power consumption only at the time of switching. Therefore, assuming that this influence is estimated to be a half from dropping by 50% from a high frequency to a low frequency, i.e., 25%, the power consumption is reduced to 0.84×0.75×100=63% by switching the operation mode of CPU  23  from the high speed mode to the low speed mode. Therefore, when the power consumption of CPU  23  in the high speed mode is 10 W, the power consumption in the low speed mode is 6.3 W. 
         [0064]    Next, operation of printer  10  according to the embodiment will be described. 
         [0065]    Here, the flow of processing in printer  10  will be described when image processor  22  executes the input editing processing and the output processing on the print job received from personal computer  12 , and simultaneously, mode determination unit  25  executes determination and switching of the operation mode. 
         [0066]    When the power is switched on, printer  10  starts power supply to the fixing device, and heats the fixing roller within the fixing device. Simultaneously, printer  10  operates CPU  23  in the high speed mode to shift to a printable state in a short time. Subsequently, when a printing job is not executed for a specified period of time, printer  10  stops power supply to the fixing device, switches CPU  23  to the low speed mode, and shifts to a printing standby state. 
         [0067]    Referring to the flow charts shown in  FIGS. 5 and 6 , description will be given of a flow of processing performed when print job  1  and print job  2  are sequentially received from personal computer  12 , with printer  10  in the printing standby state. 
         [0068]      FIG. 5  is a first flow chart showing print operation of Embodiment 1 of the printer according to the invention, and  FIG. 6  is a second flow chart showing the print operation of Embodiment 1 of the printer according to the invention. 
         [0069]    First, along with the above-mentioned flow charts and a time chart shown in  FIG. 7 , description will be given, as an example, of a case where the fixing device temperature of image formation unit  24  is out of the range of T c ±20 degrees C. upon reception of print job  1  and print job  2 , or in other words, when the fixing device temperature is far from fixing target temperature T c  and it takes some time to bring image formation unit  24  into the printable state. 
         [0070]      FIG. 7  is a first time chart showing the print operation of Embodiment 1 of the printer according to the invention. 
         [0071]    In  FIG. 7 , symbols t 1  to t 7  shown below indicate time. 
         [0072]    When print job  1  is sent from personal computer  12  and received by printer  10 , communication controller  21  outputs received print job  1  to image processor  22 , and outputs received signal  40  that indicates reception of the print job. 
         [0073]    Image processor  22  starts the input editing processing on the print job  1  in response to input of print job  1  by communication controller  21 . A start time of this input editing processing is t 1  ( FIG. 7 ). Image processor  22  also resumes power supply to the fixing device to bring image formation unit  24  into the printable state. 
         [0074]    Synchronizing with start of the input editing processing on print job  1  by image processor  22  (t 1  (FIG.  7 )), received signal  40  is input into mode determination unit  25  from communication controller  21  (Step S 101 ). In response to input of received signal  40 , mode determination unit  25  determines, based on the determination table ( FIG. 2 ) stored in determination table storage unit  30 , whether or not fixing device temperature signal  43  input from image formation unit  24  is the L level signal, to determine whether or not to switch the operation mode (Step S 102 ). 
         [0075]    Image formation unit  24  has output, as fixing device temperature signal  43 , the L level signal indicating that the fixing device temperature is out of the range T c ±20 degrees C. ( FIG. 7 ). Therefore, mode determination unit  25  determines that fixing device temperature signal  43  is the L level signal (Step S 102 ). 
         [0076]    When determining that the fixing device temperature is the L level signal (Step S 102 ), mode determination unit  25  determines, based on the determination table ( FIG. 2 ), that the operation mode of CPU  23  need not be switched (Step S 103 ). 
         [0077]    Print job  2  is sent to printer  10  from personal computer  12  while image processor  22  is executing the input editing processing on print job  1 . Upon receipt of print job  2 , communication controller  21  outputs print job  2  to image processor  22 , and also outputs received signal  40 . 
         [0078]    Image processor  22  starts the input editing processing on print job  2  in response to input of print job  2  by communication controller  21  (t 2  ( FIG. 7 )). 
         [0079]    Synchronizing with start of the input editing processing on print job  2  by image processor  22  (t 2  (FIG.  7 )), received signal  40  is input into mode determination unit  25  from communication controller  21  (Step S 101 ). As is similar to the case of print job  1 , mode determination unit  25  determines the signal level of fixing device temperature signal  43  (Step S 102 ), and determines whether or not to switch the operation mode (Step S 103 ). Mode determination unit  25  determines that the operation mode of CPU  23  need not be switched (Step S 103 ). 
         [0080]    Upon termination of input editing processing on print job  1  and print job  2 , image processor  22  generates video data for each print job. Subsequently, in image formation unit  24 , when the fixing device temperature reaches a temperature within the range of fixing target temperature T c ±20 degrees C., fixing device temperature signal  43  output from image formation unit  24  becomes the H level signal. This time is shown as t 3  in  FIG. 7 . When fixing device temperature signal  43  output from image formation unit  24  becomes the H level signal, and further, a predetermined time passes, the fixing device temperature reaches fixing target temperature T c , so that image formation unit  24  is brought into the printable state. Upon shift to the printing possible state, image processor  22  outputs output start signal  41  (t 4  ( FIG. 7 )). 
         [0081]    Upon receipt of output start signal  41  from image processor  22  (Step S 104 ), mode determination unit  25  determines, based on the determination table ( FIG. 2 ), that the setting mode is the high speed mode. Subsequently, in order to determine whether or not to switch the operation mode, mode determination unit  25  reads the flag information from flag storage unit  31 , and determines whether or not the operation mode of CPU  23  is the high speed mode (Step S 105 ). 
         [0082]    Here, the operation mode of CPU  23  is the low speed mode, and the flag information “L” is stored in flag storage unit  31 . When reading the flag information “L”, mode determination unit  25  determines that the operation mode is the low speed mode, i.e., not the high speed mode (Step S 105 ). Then, mode determination unit  25  determines that the operation mode and the setting mode do not coincide, in other words, that the operation mode needs to be switched (Step S 106 ). 
         [0083]    Based on this determination, mode determination unit  25  updates the flag information of flag storage unit  31  to “H”, and simultaneously, notifies mode setting unit  27  of the setting mode information “high speed mode”. Then, mode setting unit  27  executes processing to switch the operation mode of CPU  23  to the high speed mode (Step S 107 ). The processing by mode setting unit  27  to switch the operation mode from the low speed mode to the high speed mode will be described in detail later. 
         [0084]    When the processing by mode setting unit  27  to switch the operation mode is terminated, image processor  22  sequentially outputs the video data generated based on print job  1  page by page (Step S 108 ). Then, image formation unit  24  executes printing processing on a print medium on the basis of the inputted video data. Since CPU  23  is in the high speed mode at the time of execution of the output processing by image processor  22 , the video data is continuously inputted into image formation unit  24  in accordance with a conveying speed of the print medium. Therefore, the output processing and printing processing of each page are sequentially executed. 
         [0085]    When output of the video data corresponding to print job  1  is terminated, image processor  22  outputs output start signal  41  to start output of the video data corresponding to print job  2  (t 5  ( FIG. 7 )). Upon receipt of output start signal  41  (Step S 104 ), mode determination unit  25  reads the flag information “H” from flag storage unit  31 , and determines that the operation mode of CPU  23  is the high speed mode (Step S 105 ). Then, mode determination unit  25  determines that the operation mode need not be switched (Step S 114 ). 
         [0086]    Then, image processor  22  outputs the video data corresponding to print job  2  (Step S 108 ). When output of all the video data generated and accumulated is terminated, image processor  22  outputs output termination signal  42  (t 6  ( FIG. 7 )). 
         [0087]    When output termination signal  42  is input into mode determination unit  25  from image processor  22  (Step S 109 ), in order to switch the operation mode of CPU  23  to the low speed mode on the basis of determination table ( FIG. 2 ), mode determination unit  25  updates the flag information of flag storage unit  31  to “L”, and notifies mode setting unit  27  of the setting mode information “low speed mode”. Then, mode setting unit  27  executes the processing to switch the operation mode of CPU  23  to the low speed mode (Step S 110 ). This switching processing from the high speed mode to the low speed mode will be also described in detail later. 
         [0088]    Then, upon completion of the printing processing based on the video data in image formation unit  24 , the printing job in printer  10  is terminated. When the specified period of time passes after termination of the printing job, power supply to the fixing device is stopped, and printer  10  shifts to the printing standby state. Fixing device temperature signal  43  output from image formation unit  24  is switched from the H level signal to the L level signal when the fixing device temperature falls to T c −20 degrees C. (t 7  ( FIG. 7 )). 
         [0089]    As mentioned above, when the print job is received with printer  10  in the printing standby state, image processor  22  performs the input editing processing while the operation mode of CPU  23  is maintained in the low speed mode. Then, the operation mode of CPU  23  is switched to the high speed mode along with start of output of the video data to image formation unit  24 . 
         [0090]    Next, with the flow charts of  FIGS. 5 and 6 , and a time chart shown in  FIG. 8 , description will given of, as an example, a case where the fixing device temperature of image formation unit  24  is out of the range of T c ±20 degrees C. upon reception of print job  1 , but reaches the temperature within the range of T c ±20 degrees C. upon reception of print job  2 , in other words, when the fixing device temperature is closer to fixing target temperature T c , and image formation unit  24  is in the printable state or in a state close to the printable state. 
         [0091]      FIG. 8  is a second time chart showing operation of Embodiment 1 of the printer according to the invention. In  FIG. 8 , symbols t 8  to t 14  shown below indicate time. 
         [0092]    In printer  10 , image processor  22  starts the input editing processing in response to input of print job  1  from communication controller  21  (t 8  ( FIG. 8 )). Image processor  22  also resumes power supply to the fixing device to bring image formation unit  24  into the printable state. 
         [0093]    When received signal  40  is inputted from communication controller  21  upon start of the input editing processing (Step S 101 ), mode determination unit  25  determines, based on the determination table ( FIG. 2 ), whether or not fixing device temperature signal  43  inputted from image formation unit  24  is the L level signal, to determine whether or not to switch the operation mode (Step S 102 ). 
         [0094]    Since fixing device temperature signal  43  is the L level signal in time  t   8  ( FIG. 8 ), mode determination unit  25  determines that fixing device temperature signal  43  is the L level signal (Step S 102 ), and determines that the operation mode of CPU  23  need not be switched (Step S 103 ). 
         [0095]    When the fixing device temperature reaches within the range of T c ±20 degrees C. during execution of the input editing processing on print job  1  in image processor  22 , image formation unit  24  switches fixing device temperature signal  43  to the H level signal. This time is shown as t 9  in  FIG. 8 . 
         [0096]    Next, print job  2  is outputted from communication controller  21 , and the input editing processing on print job  2  is started in image processor  22 . This time is shown as t 10  in  FIG. 8 . Here, t 9 &lt;t 10 . Simultaneously with start of the input editing processing, received signal  40  from communication controller  21  is inputted into mode determination unit  25  (Step S 101 ). 
         [0097]    Based on input of received signal  40 , mode determination unit  25  determines whether or not fixing device temperature signal  43  is the L level signal, to determine whether or not to switch the operation mode (Step S 102 ). Since fixing device temperature signal  43  is already switched to the H level signal ( FIG. 8 ), mode determination unit  25  determines that fixing device temperature signal  43  is the H level signal, i.e., not the L level signal (Step S 102 ). 
         [0098]    Next, mode determination unit  25  reads the flag information from flag storage unit  31  to determine whether or not the operation mode of CPU  23  is the high speed mode (Step S 111 ). 
         [0099]    When the read flag information is “H”, mode determination unit  25  determines that the operation mode is the high speed mode (Step S 111 ). Since the setting mode information “high speed mode” is stored in the determination table ( FIG. 2 ) in association with the signal state of received signal  40  “rise” and the signal state of fixing device temperature signal  43  “H level”, mode determination unit  25  determines that the setting mode and the operation mode coincide, namely, that the operation mode of CPU  23  need not be switched (Step S 103 ). 
         [0100]    When the read flag information is “L”, mode determination unit  25  determines that the operation mode is the low speed mode (Step S 111 ), and determines that the operation mode and the setting mode do not coincide, namely, that the operation mode needs to be switched (Step S 112 ). Mode determination unit  25  updates the flag information of flag storage unit  31  to “H”, and simultaneously, notifies mode setting unit  27  of the setting mode information “high speed mode”. Then, mode setting unit  27  executes the processing to switch the operation mode to the high speed mode (Step S 113 ). 
         [0101]    Since image formation unit  24  is brought into the printable state when the input editing processing on print job  1  and print job  2  is terminated in image processor  22 , image processor  22  outputs output start signal  41  (t 11  ( FIG. 8 )). 
         [0102]    Upon receipt of output start signal  41  from image processor  22  (Step S 104 ), mode determination unit  25  reads the flag information from flag storage unit  31  to determine whether or not to switch the operation mode, and determines whether or not the operation mode of CPU  23  is the high speed mode (Step S 105 ). Here, since CPU  23  is already switched to the high speed mode ( FIG. 8 ), the flag information “H” is stored in flag storage unit  31 . Therefore, mode determination unit  25  determines that the operation mode is the high speed mode (Step S 105 ), and determines that the operation mode need not be switched (Step S 114 ). 
         [0103]    Then, image processor  22  sequentially outputs the video data page by page on the basis of print job  1  (Step S 108 ), and image formation unit  24  executes the printing processing on a print medium on the basis of the inputted video data. After output of the video data corresponding to print job  1  is terminated, image processor  22  outputs output start signal  41  in order to start output of the video data corresponding to print job  2  (t 12  ( FIG. 8 )). 
         [0104]    Upon receipt of output start signal  41  (Step S 104 ), mode determination unit  25  reads the flag information “H” from flag storage unit  31 , determines that the operation mode of CPU  23  is the high speed mode (Step S 105 ), and determines that the operation mode need not be switched (Step S 114 ). Then, image processor  22  outputs the video data corresponding to print job  2  (Step S 108 ). Subsequently, when output of all the video data generated and accumulated is terminated, image processor  22  outputs output termination signal  42  (t 13  ( FIG. 8 )). 
         [0105]    Upon receipt of output termination signal  42  from image processor  22  (Step S 109 ), mode determination unit  25  updates the flag information of flag storage unit  31  to “L” to switch the operation mode of CPU  23  to the low speed mode, and simultaneously, notifies mode setting unit  27  of the setting mode information “low speed mode”. Then, mode setting unit  27  executes the processing to switch the operation mode of CPU  23  to the low speed mode (Step S 110 ), so that the printing job in printer  10  is terminated. When the specified period of time passes after termination of the printing job, power supply to the fixing device is stopped, and printer  10  shifts to the printing standby state. Fixing device temperature signal  43  outputted from image formation unit  24  is switched from the H level signal to the L level signal when the fixing device temperature falls to T c −20 degrees C. (t 14  ( FIG. 8 )). 
         [0106]    As mentioned above, when the print job is newly received after image formation unit  24  is brought into the printing possible state, printer  10  switches CPU  23  to the high speed mode, and executes the input editing processing by image processor  22 . 
         [0107]    Next, operation of switching the operation mode of CPU  23  in printer  10  will be described. 
         [0108]    First, with a flow chart shown in  FIG. 9 , description will be given of a flow of processing performed when the operation mode of CPU  23  is switched from the low speed mode to the high speed mode (Step S 107  ( FIG. 6 ) and Step S 113  ( FIG. 5 )). 
         [0109]      FIG. 9  is a first flowchart showing the switching operation of Embodiment 1 of the printer according to the invention. 
         [0110]    The flow of the processing of Step S 107  ( FIG. 6 ) corresponds to the flow of the processing from time t 4  in the time chart shown in  FIG. 7 , and the flow of the processing of Step S 113  ( FIG. 5 ) corresponds to the flow of the processing from time t 10  in the time chart shown in  FIG. 8 . 
         [0111]    When being notified by mode determination unit  25  of the setting mode information “high speed mode” in printer  10  (Step S 201 ), mode setting unit  27  switches, based on the notification, the operation mode of CPU  23  from the low speed mode to the high speed mode in accordance with a procedure shown below. 
         [0112]    First, mode setting unit  27  reads the setting information corresponding to the setting mode information “high speed mode”, i.e., switching control signal information “H level” as well as set frequency information “1000 MHz”, from the setting information table ( FIG. 3 ) stored in setting information storage unit  26  (Step S 202 ). 
         [0113]    Next, mode setting unit  27  switches switching control signal VC  44 , which is input into voltage switching controller  29 , on the basis of the read switching control signal information “H level”. Mode setting unit  27  switches switching control signal VC  44  to output from the L level signal to the H level signal (Step S 203 ). 
         [0114]    When switching control signal VC  44  input into voltage switching controller  29  is switched from the L level signal to the H level signal (Step S 203 ), the FET operates in voltage switching controller  29  ( FIG. 4 ), so that the current flows into resistance R 2 . Thereby, operating voltage V CPU  supplied from CPU power supply unit  28  to CPU  23  becomes 1.5 V. 
         [0115]    After switching the signal level of switching control signal VC  44 , mode setting unit  27  waits for a predetermined period of time to stabilize voltage fluctuation of operating voltage V CPU  (Step S 204 ). Here, wait time in the embodiment is less than 1 ms. 
         [0116]    Subsequently, mode setting unit  27  switches operating frequency f CPU  of CPU  23  on the basis of the read set frequency information “1000 MHz”. Mode setting unit  27  writes the set frequency information “1000 MHz” in the internal register of CPU  23  to switch operating frequency f CPU  from 500 MHz to 1000 MHz (Step S 205 ). Thereby, the operation mode switching processing of CPU  23  in printer  10  is terminated. 
         [0117]    As mentioned above, the signal level of switching control signal VC  44  is switched on the basis of setting information table ( FIG. 3 ), operating voltage V CPU  of CPU  23  is raised to a higher voltage, and subsequently, operating frequency f CPU  is switched to a higher frequency. 
         [0118]    Next, with a flow chart shown in  FIG. 10 , description will be given of a flow of processing performed when the operation mode of CPU  23  is switched from the high speed mode to the low speed mode (Step S 110  of  FIG. 6 ). 
         [0119]      FIG. 10  is a second flow chart showing switching operation of Embodiment 1 of the printer according to the invention. 
         [0120]    The flow of the processing of Step S 110  ( FIG. 6 ) corresponds to the flow of the processing from time t 6  in the time chart shown in  FIG. 7 , and corresponds to the flow of the processing from time t 13  in the time chart shown in  FIG. 8 . 
         [0121]    When being notified by mode determination unit  25  of the setting mode information “low speed mode” in printer  10  (Step S 301 ), mode setting unit  27  switches the operation mode of CPU  23  from the high speed mode to the low speed mode on the basis of the notification. 
         [0122]    First, mode setting unit  27  reads the setting information corresponding to the setting mode information “low speed mode”, i.e., the switching control signal information “L level” and the set frequency information “500 MHz”, from the setting information table ( FIG. 3 ) stored in setting information storage unit  26  (Step S 302 ). 
         [0123]    Next, mode setting unit  27  switches operating frequency f CPU  of CPU  23  on the basis of the read set frequency information “500 MHz”. Mode setting unit  27  writes the set frequency information “500 MHz” in the internal register of CPU  23  to switch operating frequency f CPU  from 1000 MHz to 500 MHz (Step S 303 ). 
         [0124]    After performing writing to the internal register, the mode setting unit  27  waits for a predetermined period of time in order to wait for stabilization of operating frequency f CPU  of CPU  23  (Step S 304 ). Here, the wait time in the embodiment is less than 1 ms. 
         [0125]    Subsequently, mode setting unit  27  switches the signal level of switching control signal VC  44  input into voltage switching controller  29  on the basis of the read switching control signal information “L level”. Mode setting unit  27  switches switching control signal VC  44  to output from the H level signal to the L level signal (Step S 305 ). When switching control signal VC  44  inputted into voltage switching controller  29  is switched from the H level signal to the L level signal (Step S 305 ), the FET stops in voltage switching controller  29  ( FIG. 4 ), so that operating voltage V CPU  supplied from CPU power supply unit  28  to CPU  23  becomes 1.26 V. Thereby, the operation mode switching processing of CPU  23  in printer  10  is terminated. 
         [0126]    As mentioned above, after operating frequency f CPU  is switched to a lower frequency on the basis of the setting information table ( FIG. 3 ), the signal level of switching control signal VC  44  is switched, and operating voltage V CPU  of CPU  23  is reduced to a lower voltage. 
         [0127]    As described above, during printing standby, printer  10  of the embodiment operates in the low speed mode by decreasing the operating frequency and operating voltage of CPU  23 . Simultaneously, during execution of the printing job, the operation mode of CPU  23  is maintained in the low speed mode when the fixing device temperature is far from the fixing target temperature, and image formation unit  24  is in the state where image formation unit  24  cannot print. Then, only when image formation unit  24  is brought into the state where image formation unit  24  can print, CPU  23  operates in the high speed mode by increasing the operating frequency and operating voltage of CPU  23 . Consequently, the power consumed by CPU  23  can be reduced without deteriorating the print speed. 
         [0128]    In the embodiment, mode determination unit  25  determines whether the operation mode of CPU  23  needs to be switched or not based on received signal  40  from communication controller  21 . Alternatively, whether or not switching is needed may be determined based not on received signal  40 , but on change in the signal level of fixing device temperature signal  43  input from image formation unit  24 . In this case, performance of printer  10  improves. 
       Embodiment 2 
       [0129]      FIG. 11  is a block diagram showing a configuration of a printer according to Embodiment 2 of the invention. 
         [0130]    Printer  50  of Embodiment 2 is different from the printer of Embodiment 1 in that communication controller  61  comprises load information storage unit  67  and calculation unit  68 , and that a medium speed mode is added to the operation mode of CPU  63 . 
         [0131]    In the embodiment, the same reference numerals will be given to the same components as those in Embodiment 1, and detailed description of these components will be omitted. 
         [0132]    As shown in  FIG. 11 , printer  50  of the embodiment is connected to personal computer  52  as a higher level device through communication line  51 , receives a print job from personal computer  52 , and performs printing processing. 
         [0133]    As shown in  FIG. 11 , printer  50  serves as an image processing apparatus, and comprises communication controller  61 , image processor  62 , CPU  63 , image formation unit  24 , mode determination unit  64 , setting information storage unit  65 , mode setting unit  66 , CPU power supply unit  28 , and voltage switching controller  29 . 
         [0134]    Communication controller  61  includes host interfaces such as a network and a USB. Communication controller  61  is connected to personal computer  52  through communication line  51 , and receives a print job including image data from the personal computer  52 . Communication controller  61  also includes load information storage unit  67  and calculation unit  68 , as shown in  FIG. 11 . 
         [0135]    Load information storage unit  67  comprises a nonvolatile memory area, and stores a load information table. This load information table is used by calculation unit  68  to calculate an amount of load on CPU  63  as a total load point, when the input editing processing and the output processing are executed by image processor  62 . 
         [0136]      FIG. 12  is an explanatory view showing an example of the load information table. 
         [0137]    As shown in  FIG. 12 , load point information pieces, each indicating a load point as a load unit, are set in the load information table in advance in association with respective parameters. In the embodiment, the parameters set to be used to calculate the total load point include: page description language (PDL) information that indicates a type of printer language; gradation and resolution information that indicates the gradation and resolution of the image data; and host interface information that indicates a type of host interface. In the embodiment, a larger numeric value of the load point information indicates that there is a larger amount of data to be processed in image processor  62 , and the larger load is placed on CPU  63 . 
         [0138]    For example, in the load information table in  FIG. 12 , the load point information “ 2 ” is set in association with the PDL information “PS”, and the load point information “ 0 ” is set in association with the PDL information “PCL”. 
         [0139]    Additionally, in the load information table, the load point information “ 0 ” is set in association with the gradation and resolution information “2 bits and 600 dpi×600 dpi”, and the load point information “ 5 ” is set in association with the resolution information “5 bits and 600 dpi×1200 dpi”. 
         [0140]    Moreover, in the load information table, the load point information “ 0 ” is set in association with the host interface information “Centro” and “Rs-232c”, the load point information “ 1 ” is set in association with the host interface information “USB-Full” and “LAN10/100”, and the load point information “ 2 ” is set in association with the host interface information “USB-High” and “G-LAN”. 
         [0141]    With respect to the print job received by communication controller  61 , calculation unit  68  analyzes the printer language used for description of the print job, and the gradation and resolution of the image data included in the print job, and acquires the PDL information and the gradation and resolution information. Calculation unit  68  also acquires the host interface information that indicates the type of the host interface used for reception of the print job. Calculation unit  68  calculates the total load point corresponding to the print job on the basis of the acquired PDL information, gradation and resolution information, and host interface information, as well as on the load information table ( FIG. 12 ) stored in load information storage unit  67 . 
         [0142]    For example, when the received print job is described in the printer language “PS”, the gradation of the image data is “2 bits”, the resolution is “600 dpi×600 dpi”, and the host interface used for reception of the print job is “USB-Full”, calculation unit  68  reads the load point information “ 2 ” corresponding to the PDL information “PS”, the load point information “ 0 ” corresponding to the gradation and resolution information “2 bits and 600 dpi×600 dpi” and the load point information “ 1 ” corresponding to the host interface information “USB-Full” from the load information table ( FIG. 12 ). Then, calculation unit  68  totals these points, and calculates the total load point to be “ 3 ”. 
         [0143]    Communication controller  61  is connected to image processor  62  through bus signal line  72 , and outputs the received print job to image processor  62 . At this time, communication controller  61  gives the print job the job identification information for identifying the print job, and outputs the job identification information together with the print job. Communication controller  61  also outputs received signal  40  that indicates reception of the print job, and outputs message signal  77  for notification of the job identification information and the calculated total load point. Received signal  40  and message signal  77  output by communication controller  61  are inputted into mode determination unit  64 . 
         [0144]    Image processor  62  is connected to CPU  63  through bus signal line  73 , and, as an edit-output unit, executes the input editing processing and output processing on each print job. Additionally, image processor  62  outputs output start signal  41  and output termination signal  42 , and further outputs interrupt signal G-INT that indicates termination of the input editing processing. Hereinafter, interrupt signal G-INT output by image processor  62  into mode determination unit  64  is referred to as editing termination signal  78 . Image processor  62  also outputs message signal  79  for notification of the job identification information corresponding to the print job. 
         [0145]    CPU  63  is connected to image processor  62  through bus signal line  73  to control image processor  62 . CPU  63  can operate at operating voltage V CPU  of 1.26 V to 1.5 V and at operating frequency f CPU  of 500 MHz to 1000 MHz, and in the embodiment, operates in either of the operation modes of the low speed mode, the medium speed mode, and the high speed mode. CPU  63  operates at a low voltage of V CPU =1.26 V and at a low frequency of f CPU =500 MHz in the low speed mode, and operates at a high voltage of V CPU =1.5 V and at a medium frequency of f CPU =700 MHz in the medium speed mode. CPU  63  also operates at a high voltage of V CPU =1.5V and at a high frequency of f CPU =1000 MHz in the high speed mode. 
         [0146]    As in the case of Embodiment 1, when the power consumption of CPU  63  is 10 W in the high speed mode, the power consumption in the low speed mode is 63% of that in the high speed mode, i.e., 6.3 W. In the medium speed mode, while maintaining operating voltage V CPU  at the high voltage of 1.5 V, operating frequency f CPU  is lowered by 30% from the high frequency of 1000 MHz to the medium frequency of 700 MHz. Therefore, when an influence on the power consumption by operating frequency f CPU  is estimated to be a half, i.e., 15%, the power consumption of CPU  63  in the medium speed mode is calculated to be 85% of the power consumption in the high speed mode, i.e., 8.5 W. 
         [0147]    Mode determination unit  64  has job information storage unit  69 , as shown in  FIG. 11 . Upon receipt of message signal  77  from communication controller  61 , mode determination unit  64  stores the job identification information and the total load point, which are included in the message signal, in job information storage unit  69 . Moreover, upon receipt of message signal  79  from image processor  62 , mode determination unit  64  deletes the job identification information and the corresponding total load point, which are included in the message signal, from job information storage unit  69 . 
         [0148]      FIG. 13  is an explanatory view showing an example of the job information storage unit. 
         [0149]    Job information storage unit  69  is a storage unit that stores the total load point of the print job on which the input editing processing is being executed in image processor  62 . As shown in  FIG. 13 , the total load point is stored in association with the job identification information for each print job. 
         [0150]    For example, in job information storage unit  69  in  FIG. 13 , the total load point “ 3 ” is stored in association with the job identification information “print job  1 ”, the total load point “ 4 ” is stored in association with the job identification information “print job  2 ”, and the total load point “ 6 ” is stored in association with the job identification information “print job  3 ”. At this time, the print jobs on which the input editing processing is being executed are print job  1 , print job  2 , and print job  3  in image processor  62 . Therefore, as a result of addition of these three total load points, “ 13 ” is obtained as the grand total load point that indicates the total of load on CPU  63 . 
         [0151]    Mode determination unit  64  also has determination table storage unit  70 . As a judging unit and a determination unit, when an interrupt signal is input from communication controller  61  or image processor  62 , mode determination unit  64  determines the setting mode on the basis of a determination table stored in determination table storage unit  70 . 
         [0152]      FIG. 14  is an explanatory view showing an example of the determination table in Embodiment 2. 
         [0153]    As shown in  FIG. 14 , the setting mode information pieces are set in the determination table of the embodiment in advance. Each setting mode information piece is associated with the grand total load point and the signal state of the interrupt signal and control signal inputted from each unit. 
         [0154]    For example in the determination table shown in  FIG. 14 , the setting mode information “high speed mode” is stored in association with the signal state “rise” of received signal  40  or editing termination signal  78 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 11  or more”. The setting mode information “medium speed mode” is stored in association with the signal state “rise” of received signal  40  or editing termination signal  81 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 5  to  10 ”. Furthermore, the setting mode information “low speed mode” is stored in association with the signal state “rise” of received signal  40  or editing termination signal  81 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 0  to  4 ”. This indicates that mode determination unit  64  determines the setting mode based on a value of the grand total load point when received signal  40  or editing termination signal  78  is inputted, and fixing device temperature signal  43  inputted from image formation unit  24  is the L level signal. Mode determination unit  64  calculates the grand total load point in accordance with job information storage unit  69 . When the calculated grand total load point is 11 or more, mode determination unit  64  determines that the setting mode is the high speed mode. When the grand total load points are 5 to 10, mode determination unit  64  determines that the setting mode is the medium speed mode. When the grand total load point is not more than 4, mode determination unit  64  determines that the setting mode is the low speed mode. 
         [0155]    Furthermore, mode determination unit  64  has flag storage unit  71  as a storage unit, and stores the flag information that indicates the current operation mode of CPU  63  in the flag storage unit  71 . The flag information is “H” when the operation mode of CPU  63  is the high speed mode, “M” when the operation mode is the medium speed mode, and “L” when the operation mode is the low speed mode. As an updating unit, mode determination unit  64  updates the flag information stored in flag storage unit  71 . 
         [0156]    Setting information storage unit  65  is connected to mode setting unit  66  through bus signal line  75 , and stores the setting information table. 
         [0157]      FIG. 15  is an explanatory view showing an example of the setting information table in Embodiment 2. 
         [0158]    As shown in  FIG. 15 , setting information including the switching control signal information and the set frequency information is stored in the setting information table in association with the setting mode information “high speed mode”, “medium speed mode”, and “low speed mode”. 
         [0159]    For example in the setting information table shown in  FIG. 15 , the switching control signal information “H level” and the set frequency information “1000 MHz” are stored in association with the setting mode information “high speed mode”. The switching control signal information “H level” and the set frequency information “700 MHz” are stored in association with the setting mode information “medium speed mode”. Moreover, the switching control signal information “L level” and the set frequency information “500 MHz” are stored in association with the setting mode information “low speed mode”. 
         [0160]    Mode setting unit  66  is connected to mode determination unit  64  through bus signal line  74 , and is connected to setting information storage unit  65  through bus signal line  75 . Mode setting unit  66  is also connected to CPU  63  through bus signal line  76 , and, as a frequency switching unit, switches operating frequency f CPU  of CPU  63  on the basis of the setting mode information notified from mode information unit  64 . Mode setting unit  66  outputs switching control signal VC  44  to voltage switching controller  29 , and switches switching control signal VC  44  on the basis of the above-mentioned setting mode information. 
         [0161]    For example, when mode setting unit  66  is notified of the setting mode information “medium speed mode” by mode determination unit  64 , mode setting unit  66  changes switching control signal VC  44  to the H level signal on the basis of the switching control signal information “H level”, and changes operating frequency f CPU  of CPU  63  into 700 MHz on the basis of the set frequency information “700 MHz”. 
         [0162]    Next, operation of printer  50  of the Embodiment 2 will be described. 
         [0163]    Here, with flow charts shown in  FIGS. 16 ,  17 , and  18 , and a time chart shown in  FIG. 19 , description will be given of a flow of processing performed when print job  1 , print job  2 , and print job  3  are sequentially received from personal computer  52  with printer  50  in the printing standby state. 
         [0164]      FIG. 16  is a first flow chart showing print operation of Embodiment 2 of the printer, and  FIG. 17  is a second flow chart showing the print operation of Embodiment 2 of the printer according.  FIG. 18  is a third flow chart showing the print operation of Embodiment 2 of the printer, and  FIG. 19  is a first time chart showing the print operation of Embodiment 2 of the printer. In  FIG. 19 , symbols t 21  to t 31  shown below indicate time. 
         [0165]    In the embodiment, description will be given, as an example, of a case where fixing device temperature of image formation unit  24  is out of the range of T c ±20 degrees C. upon receipt of print job  1 , print job  2 , and print job  3 , and upon termination of input editing processing on the print jobs, in other words, when the fixing device temperature is far from fixing target temperature T c , and it takes some time to bring image formation unit  24  into the printable state.  FIG. 20  shows received print job  1 , print job  2 , and print job  3  each in association with the PDL information, the gradation and resolution information, and the host interface information, as well as the total load point. 
         [0166]      FIG. 20  is an explanatory view showing an example of calculation of the total load point of the print job. 
         [0167]    First, communication controller  61  in printer  50  receives print job  1  from personal computer  52 . Communication controller  61  gives job identification information “print job  1 ” to received print job  1 . 
         [0168]    Calculation unit  68  then acquires PDL information, gradation and resolution information, and host interface information which correspond to print job  1 , and calculates the total load point on the basis of the acquired information. As shown in  FIG. 20 , print job  1  is received from personal computer  52  through the host interface “USB-Full”, is described in the printer language “PS”, and includes the image data of the gradation of “2 bits” and the resolution of “600 dpi×600 dpi”. Calculation unit  68  acquires the PDL information “PS”, the gradation and resolution information “2 bits and 600 dpi×600 dpi”, and the host interface information “USB-Full” from print job  1 . Then, from the load information table ( FIG. 12 ) of load information storage unit  67 , calculation unit  68  reads the load point information “2” corresponding to the PDL information “PS”, the load point information “0” corresponding to the gradation and resolution information “2 bits and 600 dpi×600 dpi”, and the load point information “1” corresponding to the host interface information “USB-Full”. Calculation unit  68  totals these points and calculates the total load point to be “3”. 
         [0169]    Communication controller  61  outputs received print job  1  and the job identification information “print job  1 ” given to the print job to image processor  62 . Simultaneously, communication controller  61  outputs received signal  40  that indicates reception of print job  1 , and message signal  77  for notification of the job identification information “print job  1 ” and the total load point “ 3 ”. 
         [0170]    Image processor  62  starts the input editing processing on the print job  1  in response to input of print job  1  by communication controller  61 . The start time of this input editing processing is t 21  ( FIG. 19 ). Image processor  62  also starts power supply to the fixing device to bring image formation unit  24  into the printable state. 
         [0171]    Synchronizing with start of the input editing processing on print job  1  by image processor  62  (t 21  (FIG.  19 )), received signal  40  and message signal  77  are input into mode determination unit  64  from communication controller  61  (Step S 401 ). In response to this input, mode determination unit  64  stores the job identification information “print job  1 ” and the total load point “ 3 ”, which are included in message signal  77 , in job information storage unit  69 , in association with each other (Step S 402 ). 
         [0172]    Next, in order to determine whether or not to switch the operation mode, mode determination unit  64  determines whether fixing device temperature signal  43  inputted from image formation unit  24  is the L level signal or not based on the determination table ( FIG. 14 ) stored in determination table storage unit  70  (Step S 102 ). Since image formation unit  24  has output the L level signal as fixing device temperature signal  43  ( FIG. 19 ), mode determination unit  64  determines that fixing device temperature signal  43  is the L level signal (Step S 102 ). 
         [0173]    When fixing device temperature signal  43  is determined to be the L level signal (Step S 102 ), mode determination unit  64  sums up total load points stored in job information storage unit  69  to obtain the grand total load point of the print jobs that are being processed in image processor  62 . Since only the total load point “ 3 ” corresponding to the job identification information “print job  1 ” is stored in job information storage unit  69 , mode determination unit  64  determines that the grand total load point is not more than 4 (Step S 403 ). 
         [0174]    In the determination table ( FIG. 14 ), the setting mode information “low speed mode” is stored in association with the signal state “rise” of received signal  40 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 0  to  4 ”. Therefore, mode determination unit  64  determines that the setting mode is the low speed mode. Then, in order to determine whether or not the operation mode of CPU  63  coincides with the setting mode, mode determination unit  64  reads the flag information from flag storage unit  71 , and on the basis of the flag information, determines whether the operation mode of CPU  63  is the low speed mode or not (Step S 404 ). Since the flag information “L” is stored in flag storage unit  71 , mode determination unit  64  determines that the operation mode of CPU  63  is the low speed mode (Step S 404 ). 
         [0175]    When the operation mode is determined to be the low speed mode (Step S 404 ), the setting mode and the operation mode coincide with each other. Therefore, mode determination unit  64  determines that the operation mode of CPU  63  need not be switched (Step S 405 ). 
         [0176]    Next, communication controller  61  receives print job  2  from personal computer  52 . At this time, image processor  62  is executing the input editing processing on print job  1  ( FIG. 19 ). Communication controller  61  gives job identification information “print job  2 ” to received print job  2 . Calculation unit  68  then acquires PDL information, gradation and resolution information, and host interface information which correspond to print job  2 , and calculates the total load point. As shown in  FIG. 20 , print job  2  is received from personal computer  52  through the host interface “G-LAN”, described in printer language “PS”, and includes the image data of the gradation “2 bits” and the resolution “600 dpi×600 dpi”. Therefore, calculation unit  68  acquires the PDL information “PS”, the gradation and resolution information “2 bits and 600 dpi×600 dpi”, and the host interface information “G-LAN” from print job  2  ( FIG. 20 ), and calculates total load point “ 4 ” on the basis of the load information table ( FIG. 12 ). 
         [0177]    Communication controller  61  outputs received print job  2  and given job identification information “print job  2 ” to image processor  62 , and simultaneously, outputs received signal  40  and message signal  77  for notification of the job identification information “print job  2 ” and the total load point “ 4 ”. 
         [0178]    In response to input of print job  2  by communication controller  61 , image processor  62  starts the input editing processing on the print job  2 . The start time of this input editing processing is t 22  ( FIG. 19 ). 
         [0179]    Synchronizing with start of the input editing processing on print job  2  by image processor  62  (t 22  (FIG.  19 )), received signal  40  and message signal  77  are inputted into mode determination unit  64  from communication controller  61  (Step S 401 ). In response to this input, mode determination unit  64  stores the job identification information “print job  2 ” and the total load point “ 4 ”, which are included in message signal  77 , in job information storage unit  69  in association with each other (Step S 402 ). 
         [0180]    Next, in order to determine whether or not to switch the operation mode, mode determination unit  64  determines whether fixing device temperature signal  43  input from image formation unit  24  is the L level signal or not based on the determination table ( FIG. 14 ) stored in determination table storage unit  70  (Step S 102 ). Since image formation unit  24  has output the L level signal as fixing device temperature signal  43  ( FIG. 19 ), mode determination unit  64  determines that fixing device temperature signal  43  is the L level signal (Step S 102 ). 
         [0181]    Next, mode determination unit  64  calculates the grand total load point of the print jobs that are being processed in image processor  62 . Since the total load point “ 3 ” corresponding to the job identification information “print job  1 ” and the total load point “ 4 ” corresponding to the job identification information “print job  2 ” are stored in job information storage unit  69 , mode determination unit  64  calculates the grand total load point to be “ 7 ” by summing up these total load points. Therefore, mode determination unit  64  determines that the grand total load point is 5 to 10 (Step S 403 , S 406 ). 
         [0182]    The setting mode information “medium speed mode” is stored in the determination table ( FIG. 14 ) in association with the signal state “rise” of received signal  40 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 5  to  10 ”. Therefore, mode determination unit  64  determines that the setting mode is the medium speed mode. Then, mode determination unit  64  reads the flag information from flag storage unit  71 , and determines whether or not the operation mode of CPU  63  is the medium speed mode (Step S 407 ). Since the flag information “L” is stored in flag storage unit  71 , mode determination unit  64  determines that the operation mode of CPU  63  is the low speed mode, i.e., not the medium speed mode (Step S 407 ). 
         [0183]    Determination that the operation mode is not the medium speed mode (Step S 407 ) indicates that the setting mode and the operation mode do not coincide with each other. Therefore, mode determination unit  64  determines that the operation mode of CPU  63  should be switched to the medium speed mode (Step S 408 ). 
         [0184]    Based on this determination, mode determination unit  64  updates the flag information of flag storage unit  71  to “M”, and simultaneously, notifies mode setting unit  66  of the setting mode information “medium speed mode”. Then, mode setting unit  66  executes the processing to switch the operation mode of CPU  63  to the medium speed mode (Step S 409 ). Mode setting unit  66  performs the processing to switch the operation mode from the low speed mode to the medium speed mode as follows. 
         [0185]    Mode setting unit  66  reads the setting information corresponding to the setting mode information “medium speed mode” i.e., the switching control signal information “H level” and the set frequency information “700 MHz”, from the setting information table ( FIG. 15 ) stored in setting information storage unit  65 . Then, mode setting unit  66  switches switching control signal VC  44  input into voltage switching controller  29  from the L level signal to the H level signal on the basis of the read switching control signal information “H level”. In response to this switching, operating voltage V CPU  supplied to CPU  63  from CPU power supply unit  28  is raised from the lower voltage of 1.26 V to the higher voltage of 1.5 V. 
         [0186]    After the switching of switching control signal VC  44 , mode setting unit  66  waits for stabilization of operating voltage V CPU , and switches operating frequency f CPU  of CPU  63 . Mode setting unit  66  writes the set frequency information “700 MHz” in the internal register of CPU  63 , and changes operating frequency f CPU  from the lower frequency of 500 MHz to the medium frequency of 700 MHz. Thereby, the switching processing is completed. 
         [0187]    Next, communication controller  61  receives print job  3  from personal computer  52 . At this time, image processor  62  is executing the input editing processing on print job  1  and print job  2  ( FIG. 18 ). Communication controller  61  gives job identification information “print job  3 ” to received print job  3 . Calculation unit  68  then acquires PDL information, gradation and resolution information, and host interface information which correspond to this print job  3 , and simultaneously, calculates the total load point. As shown in  FIG. 20 , print job  3  is received from personal computer  52  through the host interface “LAN10/100”, described in the printer language “PCL”, and includes the image data of the gradation “5 bits” and the resolution “600 dpi×1200 dpi”. Calculation unit  68  acquires the PDL information “PCL”, the gradation and resolution information “5 bits and 600 dpi×1200 dpi”, and the host interface information “LAN10/100” from this print job  3 , and based on the load information table ( FIG. 12 ), calculates total load point to be “ 6 ” ( FIG. 20 ). 
         [0188]    Communication controller  61  outputs the received print job and the given job identification information “print job  3 ” to image processor  62 . Simultaneously, communication controller  61  outputs received signal  40  and message signal  77  for notification of the job identification information “print job  3 ” and the total load point “ 6 ”. 
         [0189]    In response to input of print job  3  by communication controller  61 , image processor  62  starts the input editing processing on the print job  3 . The start time of this input editing processing is t 23  ( FIG. 19 ). 
         [0190]    Synchronizing with start of the input editing processing on print job  3  by image processor  62  (t 23  (FIG.  19 )), received signal  40  and message signal  77  are input into mode determination unit  64  from communication controller  61  (Step S 401 ). In response to this input, mode determination unit  64  stores the job identification information “print job  3 ” and the total load point “ 6 ”, which are included in message signal  77 , in job information storage unit  69  in association with each other (Step S 402 ). 
         [0191]    Next, in order to determine whether or not to switch the operation mode, mode determination unit  64  determines whether fixing device temperature signal  43  is the L level signal or not (Step S 102 ). Since image formation unit  24  has output the L level signal as fixing device temperature signal  43  ( FIG. 19 ), mode determination unit  64  determines that fixing device temperature signal  43  is the L level signal (Step S 102 ). 
         [0192]    Next, mode determination unit  64  calculates the grand total load point of the print jobs that are being processed in image processor  62 . The total load point “ 3 ” corresponding to the job identification information “print job  1 ”, the total load point “ 4 ” corresponding to the job identification information “print job  2 ”, and the total load point “ 6 ” corresponding to the job identification information “print job  3 ” are stored in job information storage unit  69 . Therefore, mode determination unit  64  calculates the grand total load point to be “ 13 ” by summing up these total load points. Accordingly, mode determination unit  64  determines that the total load point is not less than 11 (Step S 403 , S 406 ). 
         [0193]    The setting mode information “high speed mode” is stored in the determination table ( FIG. 14 ) in association with the signal state “rise” of received signal  40 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 11  or more”. Therefore, mode determination unit  64  determines that the setting mode is the high speed mode. Then, mode determination unit  64  reads the flag information from flag storage unit  71  to determine whether or not the operation mode of CPU  63  is the high speed mode (Step S 410 ). Since the flag information “M” is stored in flag storage unit  71 , mode determination unit  64  determines that the operation mode of CPU  63  is the medium speed mode, i.e., not the high speed mode (Step S 410 ). 
         [0194]    Determination that the operation mode is not the high speed mode (Step S 410 ) indicates that the setting mode and the operation mode do not coincide with each other. Accordingly, mode determination unit  64  determines to switch the operation mode of CPU  63  to the high speed mode (Step S 411 ). 
         [0195]    Based on this determination, mode determination unit  64  updates the flag information of flag storage unit  71  to “H”, and simultaneously, notifies mode setting unit  66  of the setting mode information “high speed mode”. Then, mode setting unit  66  executes the processing to switch the operation mode of CPU  63  to the high speed mode (Step S 412 ). Mode setting unit  66  performs the processing to switch the operation mode from the medium speed mode to the high speed mode as follows. 
         [0196]    Mode setting unit  66  reads the setting information corresponding to the setting mode information “high speed mode”, i.e., the switching control signal information “H level” and the set frequency information “1000 MHz”, from the setting information table ( FIG. 15 ) stored in setting information storage unit  65 . Since mode setting unit  66  is outputting the H level signal as switching control signal VC  44  ( FIG. 19 ), mode setting unit  66  determines that the signal level of switching control signal VC  44  need not be switched, and switches operating frequency f CPU  of CPU  63 . Mode setting unit  66  writes the set frequency information “1000 MHz” in the internal register of CPU  63 , and changes operating frequency f CPU  from the medium frequency of 700 MHz to the higher frequency of 1000 MHz. Thereby, the switching processing is completed. 
         [0197]    Next, when the input editing processing to print job  1  is terminated, image processor  62  outputs editing termination signal  78  and message signal  79  for notification of the job identification information “print job  1 ” (t 24  ( FIG. 19 )). 
         [0198]    When editing termination signal  78  and message signal  79  are input into mode determination unit  64  (Step S 413 ), mode determination unit  64  deletes the total load point “ 3 ” corresponding to job identification information “print job  1 ” from job information storage unit  69  ( FIG. 13 ) on the basis of the job identification information “print job  1 ” included in the message signal  79  (Step S 414 ). 
         [0199]    Next, mode determination unit  64  determines the signal level of fixing device temperature signal  43  to determine the setting mode (Step S 102 ). When mode determination unit  64  determines that the signal level of fixing device temperature signal  43  is the L level (Step S 102 ), mode determination unit  64  calculates the grand total load point. The total load point “ 4 ” corresponding to the job identification information “print job  2 ” and the total load point “ 6 ” corresponding to the job identification information “print job  3 ” are stored in job information storage unit  69 . Accordingly, mode determination unit  64  calculates the grand total load point to be “ 10 ”, and determines that the grand total load point is 5 to 10 (Step S 403 , S 406 ). 
         [0200]    Then, mode determination unit  64  reads the signal state “rise” of editing termination signal  78 , the signal state “L level” of fixing device temperature signal  43 , and the setting mode information “medium speed mode” corresponding to the grand total load point “ 5  to  10 ” from the determination table ( FIG. 14 ). Mode determination unit  64  reads the flag information “H” from flag storage unit  71 . When mode determination unit  64  determines that the operation mode is the high speed mode and not in the medium speed mode (Step S 407 ), mode determination unit  64  determines to switch the operation mode of CPU  63  from the high speed mode to the medium speed mode (Step S 408 ). 
         [0201]    Based on this determination, mode determination unit  64  updates flag information of flag storage unit  71  to “M”, and simultaneously, notifies mode setting unit  66  of the setting mode information “medium speed mode”. Then, mode setting unit  66  executes the processing to switch the operation mode of CPU  63  to the medium speed mode (Step S 409 ). Mode setting unit  66  performs the processing to switch the operation mode from the high speed mode to the medium speed mode as follows. 
         [0202]    Mode setting unit  66  reads the setting information corresponding to the setting mode information “medium speed mode”, i.e., the switching control signal information “H level” and the set frequency information “700 MHz”, from the setting information table ( FIG. 15 ) stored in setting information storage unit  65 . Since mode setting unit  66  is outputting the H level signal as switching control signal VC  44  in the high speed mode of CPU  63  ( FIG. 19 ), the signal level of switching control signal VC  44  need not be switched. Therefore, mode setting unit  66  switches operating frequency f CPU  of CPU  63 . Mode setting unit  66  writes the set frequency information “700 MHz” in the internal register of CPU  63 , and reduces operating frequency f CPU  from the higher frequency of 1000 MHz to the medium frequency of 700 MHz. Thereby, the switching processing is completed. 
         [0203]    Subsequently, when the input editing processing on print job  2  is terminated, image processor  62  outputs editing termination signal  78 , and message signal  79  for notification of the job identification information “print job  2 ” (t 25  ( FIG. 19 )). 
         [0204]    When editing termination signal  78  and message signal  79  are input into mode determination unit  64  (Step S 413 ), mode determination unit  64  deletes the total load point “ 4 ” corresponding to the job identification information “print job  2 ” from job information storage unit  69  on the basis of the job identification information “print job  2 ” included in the message signal  79  (Step S 414 ). 
         [0205]    Then, in order to determine the setting mode, mode determination unit  64  determines the signal level of fixing device temperature signal  43  (Step S 102 ). When mode determination unit  64  determines that the signal level of fixing device temperature signal  43  is the L level (Step S 102 ), mode determination unit  64  calculates the grand total load point. Since total load point “ 6 ” corresponding to the job identification information “print job  3 ” is stored in job information storage unit  69 , mode determination unit  64  calculates the grand total load point to be “ 6 ”, and determines that the grand total load point is 5 to 10 (Step S 403 , S 406 ). 
         [0206]    Then, mode determination unit  64  reads the signal state “rise” of editing termination signal  78 , the signal state “L level” of fixing device temperature signal  43 , and the setting mode information “medium speed mode” which correspond to the grand total load point “ 5  to  10 ” from the determination table ( FIG. 14 ). When determining that the operation mode is the medium speed mode by reading the flag information “M” from flag storage unit  71  (Step S 407 ), mode determination unit  64  determines that the operation mode of CPU  63  need not be switched (Step S 405 ). 
         [0207]    Subsequently, when the input editing processing on print job  3  is terminated, image processor  62  outputs editing termination signal  78  and message signal  79  for notification of the job identification information “print job  3 ” (t 26  ( FIG. 19 )). 
         [0208]    When editing termination signal  78  and message signal  79  are input into mode determination unit  64  (Step S 413 ), mode determination unit  64  deletes total load point “ 6 ” corresponding to the job identification information “print job  3 ” from job information storage unit  69  on the basis of the job identification information “print job  3 ” included in the message signal  79  (Step S 414 ). 
         [0209]    Subsequently, in order to determine the setting mode, mode determination unit  64  determines the signal level of fixing device temperature signal  43  (Step S 102 ). When determining that the signal level of fixing device temperature signal  43  is the L level (Step S 102 ), mode determination unit  64  calculates the grand total load point. Since the input editing processing on each print job in image processor  62  is terminated at this time, neither print job identification information nor grand total load point is stored in job information storage unit  69 . Therefore, mode determination unit  64  obtains the grand total load point “ 0 ”, and determines that the grand total load point is not more than 4 (Step S 403 ). 
         [0210]    Then, mode determination unit  64  reads the setting mode information “low speed mode” corresponding to the signal state “rise” of editing termination signal  78 , the signal state “L level” of fixing device temperature signal  43 , and the grand total load point “ 0  to  4 ” from the determination table ( FIG. 14 ). Additionally, mode determination unit  64  reads the flag information “M” from flag storage unit  71 , and therefore determines that the operation mode is the medium speed mode, i.e., not the low speed mode (Step S 404 ). Then, mode determination unit  64  determines to switch the operation mode of CPU  63  from the medium speed mode to the low speed mode (Step S 415 ). 
         [0211]    Based on this determination, mode determination unit  64  updates the flag information of flag storage unit  71  to “L”, and simultaneously, notifies mode setting unit  66  of the setting mode information “low speed mode”. Then, mode setting unit  66  executes the processing to switch the operation mode of CPU  63  to the low speed mode (Step S 416 ). Mode setting unit  66  performs the processing to switch the operation mode from the medium speed mode to the low speed mode as follows. 
         [0212]    Mode setting unit  66  reads the setting information corresponding to the setting mode information “low speed mode”, i.e., the switching control signal information “L level” and the set frequency information “500 MHz”, from the setting information table ( FIG. 15 ) stored in setting information storage unit  65 . Then, first, mode setting unit  66  writes the set frequency information “500 MHz” in the internal register of CPU  63 , and changes operating frequency f CPU  from the medium frequency of 700 MHz to the lower frequency of 500 MHz. 
         [0213]    Mode setting unit  66  waits for stabilization of operating frequency f CPU , and further switches the signal level of switching control signal VC  44 . Since mode setting unit  66  is outputting the H level signal as switching control signal VC  44  in the medium speed mode of CPU  63  ( FIG. 18 ), mode setting unit  66  switches the switching control signal VC  44  to the L level signal. In response to this switching, operating voltage V CPU  supplied from CPU power supply unit  28  to CPU  63  is reduced from the higher voltage of 1.5 V to the lower voltage of 1.26 V. Thereby, the switching processing is completed. 
         [0214]    In image processor  62 , all the input editing processing on print job  1 , print job  2 , and print job  3  is terminated, and the video data for each print job is generated. Subsequently, in image formation unit  24 , when the fixing device temperature reaches the temperature within the range of fixing target temperature T c ±20 degrees C., fixing device temperature signal  43  output from image formation unit  24  turns into the H level signal. This time is shown as t 27  in  FIG. 19 . After predetermined period of time passes after fixing device temperature signal  43  output from image formation unit  24  turns into the H level signal, fixing device temperature reaches fixing target temperature T c . Thereby, image formation unit  24  is brought into the printable state. Waiting for this printing possible state, image processor  62  outputs output start signal  41  (t 28  ( FIG. 19 )). 
         [0215]    Upon receipt of output start signal  41  from image processor  62  (Step S 104 ), mode determination unit  64  reads the flag information from flag storage unit  71  to determine whether or not the operation mode of CPU  63  is the high speed mode (Step S 417 ). Thereby, mode determination unit  64  determines whether or not to switch the operation mode on the basis of the determination table ( FIG. 14 ). 
         [0216]    Since the operation mode of CPU  63  is the low speed mode at this time, the flag information “L” is stored in flag storage unit  71 . When mode determination unit  64  reads the flag information “L”, mode determination unit  64  determined that the operation mode is the low speed mode, i.e., not the high speed mode (Step S 417 ). Then, mode determination unit  64  determines that the operation mode and the setting mode do not coincide with each other, in order words, that the high speed mode of the operation mode needs to be switched (Step S 418 ). 
         [0217]    Based on this determination, mode determination unit  64  updates the flag information of flag storage unit  71  to “H”, and simultaneously, notifies mode setting unit  66  of the setting mode information “high speed mode”. Then, mode setting unit  66  executes the processing to switch the operation mode of CPU  63  from the low speed mode to the high speed mode (Step S 419 ). Since a flow of the processing to switch the operation mode from the low speed mode to the high speed mode is the same as that of Embodiment 1 ( FIG. 9 ), description thereof will be omitted. 
         [0218]    When the switching processing of the operation mode by mode setting unit  66  is terminated, image processor  62  sequentially outputs the video data generated based on print job  1  page by page (Step S 108 ). Then, image formation unit  24  executes the printing processing on a print medium on the basis of the input video data. 
         [0219]    When output of the video data corresponding to print job  1  is terminated, image processor  62  outputs output start signal  41  to start output of the video data corresponding to print job  2  (t 29  ( FIG. 19 )). Upon receipt of output start signal  41  (Step S 104 ), mode determination unit  64  reads the flag information “H” from flag storage unit  71 , determines that the operation mode of CPU  23  is the high speed mode (Step S 417 ), and determines that the operation mode need not be switched (Step S 420 ). Then, image processor  62  outputs the video data corresponding to print job  2  (Step S 108 ). 
         [0220]    When output of the video data corresponding to print job  2  is terminated, image processor  62  outputs output start signal  41  to perform the same processing on print job  3  (t 30  ( FIG. 19 )). Upon receipt of output start signal  41  (Step S 104 ), mode determination unit  64  reads the flag information “H” from flag storage unit  71 , and determines that the operation mode of CPU  23  is the high speed mode (Step S 417 ). Thereby, mode determination unit  64  determines that the operation mode need not be switched (Step S 420 ). Then, the video data corresponding to print job  3  is outputted (Step S 108 ). 
         [0221]    When output of all the video data generated and accumulated is terminated, image processor  62  outputs output termination signal  42  (t 31  ( FIG. 19 )). 
         [0222]    When output termination signal  42  is input into mode determination unit  64  from image processor  62  (Step S 109 ), mode determination unit  64  updates the flag information of flag storage unit  71  to “L” to switch the operation mode of CPU  63  to the low speed mode on the basis of determination table ( FIG. 14 ), and simultaneously, notifies mode setting unit  66  of the setting mode information “low speed mode”. Then, mode setting unit  66  executes the processing to switch the operation mode of CPU  63  from the high speed mode to the low speed mode (Step S 421 ). Since the processing to switch the operation mode from the high speed mode to the low speed mode is the same as that of Embodiment 1 ( FIG. 10 ), description thereof will be omitted. 
         [0223]    Then, when the printing processing on the basis of each video data in image formation unit  24  is completed, the printing job in printer  50  is terminated. When the period of time passes after termination of the printing job, power supply to the fixing device is stopped so that printer  50  shifts to the printing standby state. Fixing device temperature signal  43  outputted from image formation unit  24  is switched from the H level signal to the L level signal when the fixing device temperature falls to T c −20 degrees C. (t 32  ( FIG. 19 )). 
         [0224]    As mentioned above, the amount of load placed on CPU  63  for each print job on which the input editing processing is being executed in image processor  62  is calculated, and switching of the operation mode of CPU  63  is executed on the basis of the amount of load. 
         [0225]    At Step S 410 , when determining that the operation mode of CPU  63  is the high speed mode, mode determination unit  64  determines that the switching of the operation mode of CPU  63  is unnecessary (Step S 422 ). Then, printer  50  executes the processing after Step S 413 . 
         [0226]    As mentioned above, printer  50  of the embodiment switches the CPU mode to either of the low speed mode, the medium speed mode, and the high speed mode depending on the state of image formation unit  24  as well as on the load of each print job and the processing state of the print job in image processor  24 . Consequently, further reduction of the power consumption is attained. 
         [0227]    While in each embodiment mentioned above, description has been given of a case where the image processing apparatus of the invention is applied to an electrophotographic printer, the image processing apparatus of the invention is not limited to this example. For example, the image processing apparatus of the invention can be applied to facsimiles, copying machines, multifunction machines, or the like. 
         [0228]    The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.