Abstract:
An electric circuit device operable under a power supply includes: a circuit; a first switch connected between the power supply and the circuit; a capacitor tending to produce a first leakage current; a second switch connected between the power supply and the capacitor, the second switch producing a second leakage current when it is cut off, the second leakage current being less than the first leakage current; and a switch controller for turning on the second switch while both the first switch and the second switch are turned off, and after a first time passes for turning on the first switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2007-330245 filed on Dec. 21, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     This art is related to an electronic circuit device for controlling a switch which connects a circuit to a power supply. 
     2. Description of the Related Art 
     As one of functions for reducing the power consumption of a semiconductor integrated circuit used in an electronic device, there is a power shut-off function. The power shut-off function stops the power supply to a circuit of a predetermined block constituting an electronic circuit device such as a semiconductor integrated circuit. The function can reduce unnecessary power consumption in the standby state of the circuit and increase the time of continuous operation of the electronic device. To stabilize the operation of the circuit, the circuit is normally connected in parallel to a capacitive element for stabilizing a power supply voltage. If the connection between the circuit and the power supply is interrupted, the connection between the capacitive element and the power supply is also simultaneously interrupted. Meanwhile, the circuit and the capacitive element are constantly connected to each other. Thus, if the connection of the circuit to the power supply is interrupted, the capacitive element is discharged by the circuit. When the circuit is reconnected to the power supply, therefore, the capacitive element needs to be charged. This phenomenon causes a delay in the activation of the circuit. 
     As conventional techniques, there are techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 2001-358294 and 2004-327820. The technique of the former publication turns off a switch connected in series to a capacitive element to prevent the charged capacitive element from being discharged. The technique of the latter publication turns off a switch connected in series to a MOS (Metal Oxide Semiconductor) capacitor to prevent gate leakage caused by the MOS capacitor. 
     SUMMARY 
     According to an aspect of an embodiment, an electric circuit device operable under a power supply includes: a circuit; a first switch connected between the power supply and the circuit; a capacitor tending to produce a first leakage current; a second switch connected between the power supply and the capacitor, the second switch producing a second leakage current when it is cut off, the second leakage current being less than the first leakage current; and a switch controller for turning on the second switch while both the first switch and the second switch are turned off, and after a first time passes for turning on the first switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an overall configuration diagram; 
         FIG. 2  shows a configuration diagram of a switch controller; 
         FIGS. 3A to 3C  show truth value tables of operation modes; 
         FIG. 4  shows a state transition diagram; 
         FIG. 5  shows an operational flowchart; 
         FIG. 6  shows a state transition diagram; and 
         FIG. 7  shows an operational flowchart. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description will be made below of embodiments of the present invention. The present invention is not limited to the embodiments described below. 
       FIG. 1  shows a configuration diagram of a semiconductor device including an electronic circuit device according to one of the present embodiments. The semiconductor device includes a power supply line  101 , a reference potential line  102 , a third circuit  130 , a block  103 , a block  104 , and a switch controller  131 . The blocks  103  and  104  and the third circuit  130  are supplied with a power supply voltage from a power supply  100  via the power supply line  101 . The switch controller  131  is formed by a PMU (Power Management Unit), for example. Detailed description of the switch controller  131  will be made with reference to  FIG. 2 . The electronic circuit device includes a plurality of blocks. In the present embodiment, the electronic circuit device includes two blocks. 
     The third circuit  130  is a logic circuit constantly connected to the power supply line  101  and the reference potential line  102 . Meanwhile, the blocks  103  and  104  are connected to the power supply line  101  on the basis of switch control signals  132   a ,  132   b ,  132   c , and  132   d  transmitted from the switch controller  131 . The block  103  includes a switch  110 , a first circuit  111 , a switch  112 , and a capacitive element  113 . The switch  110  turns on and off the connection between the first circuit  111  and the power supply line  101 . The switch  112  turns on and off the connection between the capacitive element  113  and the power supply line  101 . The switches  110  and  112  are controlled by switch control signals  132   a  and  132   b , respectively, which are output from the switch controller  131 . The capacitive element  113  is for stabilizing the level of the power supply voltage supplied to the first circuit  111 . 
     The block  104  includes a switch  120 , a second circuit  121 , a switch  122 , and a capacitive element  123 . The switch  120  turns on and off the connection between the second circuit  121  and the power supply line  101 . The switch  122  turns on and off the connection between the capacitive element  123  and the power supply line  101 . The switches  120  and  122  are controlled by switch control signals  132   c  and  132   d , respectively, which are output from the switch controller  131 . The capacitive element  123  is for stabilizing the level of the power supply voltage supplied to the second circuit  121 . 
     The switch controller  131  outputs the switch control signals  132   a ,  132   b ,  132   c , and  132   d  on the basis of a state transition signal  133  which determines the next operational state of each of the blocks. The state transition signal  133  is a signal transmitted from a not-illustrated microcomputer. On the basis of the state transition signal  133 , the switch controller  131  determines which block is to be transited to which operation mode. The switch controller  131  transmits to each of the switches the switch control signals  132   a ,  132   b ,  132   c , and  132   d  according to the determined operation mode. 
     The switches  112  and  122  are provided to prevent current consumption by the leakage current of the capacitive elements  113  and  123 . Therefore, the off-state leakage current of each of the switches  112  and  122  needs to be less than the leakage current of the corresponding one of the capacitive elements  113  and  123 . When each of the capacitive elements  113  and  123  is formed by a MOS capacitor, a large capacitance value can be ensured by reduction in thickness of an oxide film formed between the gate and the drain of the MOS capacitor. Meanwhile, the reduction in thickness of the oxide film results in an increase in the leakage current. Therefore, when each of the switches  112  and  122  is formed by a MOS transistor, for example, if the oxide film of each of the switches  112  and  122  is made thickener than the oxide film of the corresponding one of the capacitive elements  113  and  123 , the off-state leakage current of the switch can be made less than the leakage current of the capacitive element. According to the present embodiment, therefore, the capacitive element  113  or  123  has been charged when the first circuit  111  or the second circuit  121  is connected to the power supply  100 . As a result, a voltage drop occurring when the first circuit  111  or the second circuit  121  is connected to the power supply  100  can be prevented. Accordingly, the first circuit  111  or the second circuit  121  can be activated at high speed. 
       FIG. 2  is a functional block diagram for explaining the switch controller  131 . The switch controller  131  is constituted by a signal output unit  210 , a mode determination unit  211 , a state storage unit  212 , a decoder  213 , a time measurement unit  214 , a comparator  216 , a clock source  222 , and a time storage unit  218 . 
     The switch controller  131  receives the state transition signal  133  from an external device of a microcomputer  200 , and starts a process of determining the next operation mode. The switch controller  131  determines the next operation mode on the basis of the state transition signal  133  input therein, and outputs the result of the determination as the switch control signals  132   a ,  132   b ,  132   c , and  132   d.    
     The state storage unit  212  stores the current operation mode. The mode determination unit  211  stores a correspondence table of the next operation mode determined by the combination of the current operation mode and the state transition signal  133 . With the use of the state transition signal  133  input from the microcomputer  200  and the current operation mode stored in the state storage unit  212 , and on the basis of the correspondence table described later, the mode determination unit  211  performs a process of determining the next operation mode. The next operation mode determined by the mode determination unit  211  is stored in the state storage unit  212 . Each operation mode is defined as a binary number including a plurality of bits. Further, if the operation mode is stored in the state storage unit  212  as a nonvolatile memory, and if the operation mode is used at the time of activation, the state of the switch controller  131  at the time of activation can be determined. 
     The state storage unit  212  stores the current operation mode and the next operation mode. On the basis of the two operation modes, the time storage unit  218  described later determines the number of counts output to the comparator  216 . 
     The state storage unit  212  outputs an operation mode signal of the next operation mode to the decoder  213  and the time storage unit  218 . The decoder  213  decodes the input operation mode signal into an operation mode signal for each of the blocks  103  and  104 , and outputs the decoded operation mode signal to the signal output unit  210 . The signal output unit  210  decodes the input operation mode signal into the switch control signals  132   a ,  132   b ,  132   c , and  132   d , and outputs the decoded switch control signals  132   a ,  132   b ,  132   c , and  132   d . Further, upon receipt of the operation mode signal, the signal output unit  210  outputs a count start signal  230 . The time storage unit  218  stores the time required to charge each of the capacitive elements  113  and  123  and so forth, as time information in the number of counts of a clock signal, for example. The number of counts is determined on the basis of the clock period of the clock source  222 . The time storage unit  218  further stores a correspondence table of the change of the operation mode and the capacitive element to be charged according to the change. The time storage unit  218  outputs to the comparator  216  the number of counts corresponding to the current operation mode and the next operation mode input therein. 
     The relationship between the number of counts stored in the time storage unit  218  and the capacitance value of the capacitive element is defined as follows, for example. If the capacitance value of the capacitive element is represented as C, and if the on-resistance value at the turn-on of the switch which connects the capacitive element to the power supply is represented as R, a charging time t for charging the capacitive element is determined as t=C×R. The time t is time constant. Therefore, if the clock period of the clock source  222  supplied to a counter is represented as T, the number of counts N corresponding to the charging time t of the capacitive element can be obtained as N=t÷T. 
     The time measurement unit  214  measures the time elapsed since the receipt of the count start signal  230 , and records the result of the measurement as time information. The time measurement unit  214  can be formed by a counter, for example. The time measurement unit  214  receives the count start signal  230  output from the signal output unit  210 , starts counting on the basis of the clock signal output from the clock source  222  and having the clock period T, and outputs to the comparator  216  the number of counts accumulated since the start of the counting. 
     The comparator  216  compares the number of counts output from the time measurement unit  214  with the number of counts output from the time storage unit  218 . Then, if the two values become equal, the comparator  216  outputs an activation signal  220 . The activation signal  220  is a signal for notifying the other devices and so forth that the power supply to the first circuit  111 , the second circuit  121 , and so forth has been started. The signal can notify a clock supply unit that each of the blocks has been activated, for example, to thereby start the supply of the clock signal to the target block. The output timing of the activation signal  220  is not limited to the timing at which the number of counts output from the time measurement unit  214  becomes equal to the number of counts output from the time storage unit  218 . Thus, the output timing may be set to the timing at which the difference between the two numbers of counts becomes a predetermined value. 
     Each of  FIGS. 3A to 3C  represents the relationship between the operation mode and the switch control signal  132   a ,  132   b ,  132   c , and  132   d .  FIG. 3A  represents the relationship between the operation mode of the semiconductor device and the operation mode of each of the blocks.  FIG. 3B  represents the relationship between the operation mode of the block  103  and the operational state of each of the switches included in the block, and  FIG. 3C  represents the relationship between the operation mode of the block  104  and the operational state of each of the switches included in the block. The table of  FIG. 3A  is stored in the decoder  213 . The decoder  213  decodes the input signal in accordance with  FIG. 3A . A column C represents the operation mode signals input to the decoder  213 . Columns A and B represent the operation mode signals of the blocks  103  and  104 , respectively. In  FIG. 3A , the operation mode ON represents the state in which the switch connected in series to the circuit and the switch connected in series to the capacitive element are both in the ON state. The operation mode SLEEP represents the state in which only the switch connected in series to the capacitive element is in the ON state. The operation mode OFF represents the state in which the two switches are both in the OFF state. 
     The tables of  FIGS. 3B and 3C  are stored in the signal output unit  210 . On the basis of the decoded signal output from the decoder  213 , the signal output unit  210  outputs the switch control signals  132   a ,  132   b ,  132   c , and  132   d . Columns A and B represent the operation mode signals input to the signal output unit  210  for the blocks  103  and  104 , respectively. Columns  110 ,  112 ,  120 , and  122  represent the switch control signals  132   a ,  132   b ,  132   c , and  132   d  to be transmitted to the switches  110 ,  112 ,  120 , and  122 , respectively. In the present embodiment, “1” represents the ON state, and “0” represents the OFF state. 
     For example, if the operation mode stored in the state storage unit  212  is c 1 , the decoder  213  decodes the operation mode signal c 1  to set both of the blocks  103  and  104  in the operation mode ON. The decoded operation mode signal is output to the signal output unit  210 . On the basis of the input operation mode signal and  FIGS. 3B and 3C , the signal output unit  210  outputs the switch control signals  132   a ,  132   b ,  132   c , and  132   d  corresponding to the operation mode ON of each of the blocks. Specifically, the signal output unit  210  outputs to each of the switches  110 ,  112 ,  120 , and  122  the switch control signal  132   a ,  132   b ,  132   c , and  132   d  having a logical value “1” for turning on all of the switches. 
       FIG. 4  is a state transition diagram illustrating a state transition in accordance with the current operation mode and the input state transition signal  133 . The condition of the state transition of  FIG. 4  is stored in the mode determination unit  211  of  FIG. 2 . In the drawing, c 1  to c 5  represent the operation modes, which are equal to the operation modes of the column C in  FIG. 3A . If the number of transition paths is increased, the processing by the mode determination unit  211  becomes complicated. However, if the degree of freedom of mode transition is increased, a less redundant switch control can be performed. 
     A two-digit number accompanying each of arrows located between the respective operation modes represents the state transition signal  133  input from the microcomputer  200 . For example, if the current operation mode is c 3 , and if the input state transition signal  133  is “11,” the current operation mode c 3  transits to the operation mode c 2 . If the input state transition signal  133  is “00,” the current operation mode c 3  transits to the operation mode c 5 . If the input state transition signal  133  is “10,” the current operation mode c 3  transits to the operation mode c 4 . In the above-described manner, the next operation mode can be determined on the basis of the current operation mode and the input state transition signal  133 . 
     The operation modes c 1  to c 5  represent the operation mode of the entire semiconductor device. Meanwhile, the operation mode of each of the blocks can be defined on the basis of  FIG. 3A . For example, if the operation mode transits from c 1  to c 2 , the operation mode of the block  103  transits from ON to SLEEP. Similarly, the operation mode of the block  104  also transits from ON to SLEEP. Meanwhile, if the operation mode transits from c 2  to c 3 , the operation mode of the block  103  is unchanged, and the operation mode of the block  104  transits from SLEEP to OFF. As described above, a plurality of operation modes are provided for the entire semiconductor device, and each of the operation modes is assigned with the operation modes of the respective blocks. Accordingly, it is possible to separately control the operation modes of the respective blocks while reducing the capacity required to store the operation modes. 
       FIG. 5  is a flowchart illustrating a transition process of the operation mode performed in each of the blocks on the basis of the state transition signals  133  of  FIG. 4 . The operation modes ON, SLEEP, and OFF of  FIG. 5  are the same as the operation modes ON, SLEEP, and OFF of  FIGS. 3A to 3C . The blocks  103  and  104  perform the same operation on the basis of the flowchart of  FIG. 5 . Herein, description will be made of the state transition process of the block  103 , as an example. 
     At Step S 50 , the switch controller  131  performs a process of determining the next operation mode on the basis of the input state transition signal  133 . It is now assumed that the state storage unit  212  stores the information that the current operation mode is c 5 . Further, it is assumed that “10” has been input as the state transition signal  133 . In this case, on the basis of the state transition signal  133  and the current operation mode c 5 , the mode determination unit  211  determines from the state transition diagram of  FIG. 4  that the next operation mode is c 1 . The operation mode c 1  is written in the state storage unit  212 , and the switch controller  131  performs the process of Step S 52 . 
     At Step S 52 , on the basis of the operation mode c 1  stored in the state storage unit  212  and the truth value table of  FIG. 3A , the decoder  213  outputs a mode signal to the signal output unit  210  to set the block  103  in the operation mode ON. On the basis of  FIG. 3B , the signal output unit  210  outputs a switch control signal for turning on the switch  110  connected to the first circuit  111  and the switch  112  connected to the capacitive element  113 . 
     At Step S 54 , the signal output unit  210  outputs a switch control signal to the time measurement unit  214  to start counting the time until the charging of the capacitive element  113  is completed. At Step S 56 , the comparator  216  compares the number of counts output from the time measurement unit  214  with the value stored in the time storage unit  218 . If the two values become equal, the comparator  216  completes the counting process and outputs the activation signal  220  at Step S 58 . Thereby, the block  103  is set in the operation mode ON. 
     Meanwhile, if “11” is input as the state transition signal  133 , the mode determination unit  211  determines from the state transition diagram of  FIG. 4  that the next operation mode is c 3 . The operation mode c 3  is written in the state storage unit  212 , and the switch controller  131  performs the process of Step S 80 . 
     At Step S 80 , on the basis of the operation mode c 3  stored in the state storage unit  212 , the decoder  213  outputs a mode signal to the signal output unit  210  to set the block  103  in the operation mode SLEEP. The signal output unit  210  performs a process of outputting a signal for turning on the switch  112  connected to the capacitive element  113 . At Step S 82 , the signal output unit  210  outputs a signal to the time measurement unit  214  to start counting the time until the charging of the capacitive element  113  is completed. At Step S 84 , the comparator  216  compares the number of counts output from the time measurement unit  214  with the value stored in the time storage unit  218 . Then, if the two values become equal, the comparator  216  completes the counting process. The comparator  216  then outputs, as the activation signal  220 , the information that the charging of the capacitive element  113  has been completed. Thereby, the block  103  is set in the operation mode SLEEP. 
     At Step S 60 , the mode determination unit  211  remains in the standby state until the input of the state transition signal  133 . At Step S 62 , upon input of the state transition signal  133  to the mode determination unit  211 , the comparator  216  outputs, as the activation signal  220 , the information that the block  103  is no longer in the operation mode ON. 
     At Step S 64 , the mode determination unit  211  determines the next operation mode on the basis of the input state transition signal  133  and the current operation mode stored in the state storage unit  212 . If the state transition signal  133  is “00,” the mode determination unit  211  determines that the next operation mode is c 5 . Then, the mode determination unit  211  outputs the operation mode c 5  to the state storage unit  212 . 
     At Step S 66 , the switch controller  131  outputs a switch control signal for turning off the switch  110  connected to the first circuit  111  and the switch  112  connected to the capacitive element  113 . Thereby, the block  103  is set in the operation mode OFF. 
     Meanwhile, at Step S 64 , if “01” is input as the state transition signal  133 , the mode determination unit  211  determines the next operation mode as c 2 . At Step S 68 , the mode determination unit  211  outputs the operation mode c 2  signal to the state storage unit  212 . Further, at Step S 68 , the switch controller  131  outputs a signal for turning off the switch  110  connected to the first circuit  111 . Thereby, the operation mode of the block  103  is set in SLEEP. 
     At Step S 70 , the mode determination unit  211  remains in the standby state until the input of the state transition signal  133 . The state storage unit  212  stores the operation mode c 2 . If the state transition signal  133  is “11,” the mode determination unit  211  determines that the next operation mode is c 1 . At Step S 70 , the mode determination unit  211  determines the next operation mode as c 1 , and outputs the operation mode c 1  to the state storage unit  212 . Then, the switch controller  131  proceeds to the process of Step S 74 . 
     At Step S 74 , the signal output unit  210  outputs a switch control signal for turning on the switch  110  connected to the first circuit  111 . At Step S 76 , the signal output unit  210  instructs the time measurement unit  214  to start counting to determine whether the parasitic capacitance of the first circuit  111  has been charged. At Step S 78 , on the basis of the number of counts output from the time measurement unit  214  and the number of counts output from the time storage unit  218 , the comparator  216  determines whether the parasitic capacitance of the first circuit  111  has been charged. At Step S 58 , the comparator  216  outputs the activation signal  220 . Thereby, the operation mode of the block  103  is set in ON. 
     Meanwhile, if “01” is input as the state transition signal  133  at Step S 70 , the mode determination unit  211  determines the next operation mode as c 3 . Thus, the operation mode of the block  103  remains in SLEEP. If “00” is further input as the state transition signal  133 , the mode determination unit  211  determines the next operation mode as c 5 . Then, the switch controller  131  proceeds to the process of Step S 72 . 
     At Step S 72 , the signal output unit  210  outputs a switch control signal for turning off the switch  112 . Thereby, the operation mode of the block  103  is set in OFF. 
     If a switch connected to a capacitive element is turned off, the capacitive element maintains the charged state for a short time. After the lapse of a long time, however, the capacitive element is discharged due to a parasitic resistance of the capacitive element and so forth. Thus, the operation mode SLEEP is provided in the transition of the operation mode from OFF to ON. Thereby, the capacitive element connected in parallel to a circuit is recharged before the start of the power supply to the circuit. Accordingly, it is possible to suppress a drop in the power supply voltage occurring when the circuit is connected to the power supply, and to activate the circuit at high speed. 
       FIG. 6  is a state transition diagram illustrating a state transition in accordance with the current operation mode and the input state transition signal  133 . In the drawing, c 1  to c 5  represent the operation modes, which are equal to the operation modes of the semiconductor device in  FIG. 3A . The present example includes a smaller number of transition paths than in the state transition diagram of  FIG. 4 . Thus, the circuit operation becomes redundant. However, the circuit size of the switch controller  131  can be reduced. 
     A two-digit number accompanying each of arrows located between the respective operation modes represents the input state transition signal  133 . For example, if the current operation mode is c 3 , and if the input state transition signal  133  is “11,” the current operation mode c 3  transits to the operation mode c 2 . If the input state transition signal  133  is “00,” the current operation mode c 3  transits to the operation mode c 5 . If the input state transition signal  133  is “10,” the current operation mode c 3  transits to the operation mode c 4 . In the above-described manner, the next operation mode can be determined on the basis of the current operation mode and the input state transition signal  133 . 
     The operation modes c 1  to c 5  represent the operation mode of the entire semiconductor device. Meanwhile, the operation mode of each of the blocks can be defined on the basis of  FIG. 3A . For example, if the operation mode transits from c 1  to c 2 , the operation mode of the block  103  transits from ON to SLEEP. Similarly, the operation mode of the block  104  also transits from ON to SLEEP. Meanwhile, if the operation mode transits from c 2  to c 3 , the operation mode of the block  103  is unchanged, and the operation mode of the block  104  transits from SLEEP to OFF. As described above, a plurality of operation modes are provided for the entire semiconductor device, and each of the operation modes is assigned with the operation modes of the respective blocks. Accordingly, the operation modes of the respective blocks can be separately controlled. 
       FIG. 7  is a flowchart illustrating a transition process of the operation mode performed in each of the blocks on the basis of the state transition signals  133  of  FIG. 6 . The operation modes ON, SLEEP, and OFF of  FIG. 7  are the same as the operation modes ON, SLEEP, and OFF of  FIGS. 3A to 3C . The blocks  103  and  104  perform the same operation on the basis of the flowchart of  FIG. 7 . Herein, description will be made of the state transition process of the block  103 , as an example. 
     At Step S 10 , the mode determination unit  211  remains in the standby state until the input of the state transition signal  133 . It is now assumed that the state storage unit  212  stores the operation mode c 5 . At Step S 10 , on the basis of “11” input as the state transition signal  133  and the operation mode c 5  stored in the state storage unit  212 , the mode determination unit  211  determines that the next operation mode is c 3 . 
     At Step S 12 , the signal output unit  210  outputs a switch control signal  132   b  for turning on the switch  112 . At Step S 14 , the signal output unit  210  outputs a signal for instructing the time measurement unit  214  to start counting. At Step S 16 , the comparator  216  compares the number of counts output from the time measurement unit  214  with the number of counts output from the time storage unit  218 . Then, if the two values become equal, the comparator  216  outputs the activation signal  220 . 
     At Step S 18 , the mode determination unit  211  is in the standby state. The operation mode stored in the state storage unit  212  is c 3 . If “00” is input as the state transition signal  133 , the mode determination unit  211  determines the next operation mode as c 5 . Then, the mode determination unit  211  stores the operation mode c 5  in the state storage unit  212 , and the procedure proceeds to Step S 20 . Meanwhile, if “10” is input, the mode determination unit  211  determines the next operation mode as c 4 . Then, the mode determination unit  211  stores the operation mode c 4  in the state storage unit  212 , and the procedure proceeds to Step S 22 . At Step S 20 , the signal output unit  210  outputs a signal for turning off the switch  112 . 
     At Step S 22 , the signal output unit  210  outputs a switch control signal  132   a  for turning on the switch  110 . At Step S 24 , the signal output unit  210  outputs the count start signal  230  to the time measurement unit  214 . At Step S 26 , the comparator  216  compares the number of counts output from the time measurement unit  214  with the time required to charge the parasitic capacitance of the first circuit  111 , which is stored in the time storage unit  218 . Then, if the two values become equal, the comparator  216  outputs the activation signal  220  at Step S 28 . 
     At Step S 30 , the mode determination unit  211  is in the standby state. The state storage unit  212  stores c 4  as the operation mode. If “00” is input as the state transition signal  133 , the mode determination unit  211  determines the next operation mode as c 3 . Then, the mode determination unit  211  stores the operation mode c 3  in the state storage unit  212 . At Step S 32 , the comparator  216  outputs, as the activation signal  220 , the information that the first circuit  111  is to be brought into the stopped state. At Step S 34 , the signal output unit  210  outputs a signal for turning off the switch  110 . 
     If a switch connected to a capacitive element is turned off, the capacitive element maintains the charged state for a short time. After the lapse of a long time, however, the capacitive element is discharged due to a parasitic resistance of the capacitive element and so forth. Thus, the operation mode SLEEP is provided in the transition of the operation mode from OFF to ON. Thereby, the capacitive element connected in parallel to a circuit is recharged before the start of the power supply to the circuit. Accordingly, it is possible to suppress a drop in the power supply voltage occurring when the circuit is connected to the power supply, and to activate the circuit at high speed.