Abstract:
According to the present invention, there is provided a semiconductor integrated circuit comprising: a power controller which outputs a voltage select signal for selecting one of at least two types of voltages; a power supply voltage controller which generates and outputs a power supply voltage at an arbitrary voltage change rate on the basis of the voltage select signal; and a circuit portion which receives the power supply voltage and performs processing, wherein said circuit portion keeps operating while said power supply voltage controller is outputting the power supply voltage generated at the arbitrary voltage change rate.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims benefit of priority under 35 USC § 119 from the Japanese Patent Application No. 2005-28568, filed on Feb. 4, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor integrated circuit and, more particularly, to a semiconductor integrated circuit including a power supply voltage controller which limits the change rate when the power supply voltage is to be changed.  
         [0003]     Recently, to reduce the power consumption of a semiconductor integrated circuit, the power supply voltage is dropped to a minimum necessary voltage in accordance with the operation state.  
         [0004]     For example, a 100-MHz, 1.2-V semiconductor integrated circuit is sometimes operated by downing the power to 50 MHz and 0.9 V in accordance with the margin of the data processability. In this case, before the power supply voltages are switched, the clock is lowered from 100 MHz to 50 MHz to temporarily stop data processing. Then, the power supply voltage is lowered from 1.2 V to 0.9 V, and data processing is resumed after the voltage has stabilized.  
         [0005]     To return to 100 MHz and 1.2 V, data processing is temporarily stopped before the power supply voltages are switched. After the power supply voltage is raised from 0.9 V to 1.2 V and the voltage has stabilized, the clock is returned from 50 MHz to 100 MHz to resume data processing.  
         [0006]     This is so because, during the abrupt transition of the power supply voltage, a problem such as clock out of sync or circuit delay fluctuation arises, and it is necessary to avoid a period like this.  
         [0007]     It is also necessary to avoid the problem that a period during which the power supply voltage falls outside the operating voltage is produced by overshoot caused by an abrupt transition of the power supply voltage. That is, it is necessary to avoid the event in which, assuming that the designed voltages are 1.2 V±0.05 V and 0.9 V±0.05 V, no stable operation can be ensured any longer because these voltage ranges are exceeded.  
         [0008]     Conventionally, therefore, before and after the power supply voltages are switched, data processing must be temporarily stopped until the power supply voltage stabilizes, and this lowers the processing speed.  
         [0009]     A reference disclosing the conventional semiconductor integrated circuit is as follows.  
         [0010]     Japanese Patent Laid-Open No. 2003-330549  
       SUMMARY OF THE INVENTION  
       [0011]     According to one aspect of the invention, there is provided a semiconductor integrated circuit comprising:  
         [0012]     a power controller which outputs a voltage select signal for selecting one of at least two types of voltages;  
         [0013]     a power supply voltage controller which generates and outputs a power supply voltage at an arbitrary voltage change rate on the basis of the voltage select signal; and  
         [0014]     a circuit portion which receives the power supply voltage and performs processing,  
         [0015]     wherein said circuit portion keeps operating while said power supply voltage controller is outputting the power supply voltage generated at the arbitrary voltage change rate.  
         [0016]     According to one aspect of the invention, there is provided a semiconductor integrated circuit comprising:  
         [0017]     a power controller which outputs a voltage select signal for selecting one of at least two types of voltages, and a clock select signal for selecting one of at least two types of clock frequencies as a clock;  
         [0018]     a clock generator which receives the clock select signal, and generates and outputs the clock;  
         [0019]     a power supply voltage controller which receives the voltage select signal, and generates and outputs a power supply voltage at an arbitrary voltage change rate; and  
         [0020]     a circuit portion which receives the clock and power supply voltage and performs processing,  
         [0021]     wherein when the power supply voltage is to be switched to a second voltage lower than a first voltage while said circuit portion is performing processing by using a clock having a first frequency and a power supply voltage having the first voltage,  
         [0022]     said clock generator outputs a clock having a second frequency lower than the first frequency, and, after that,  
         [0023]     said power supply voltage controller outputs a power supply voltage which is changed from the first voltage to the second voltage at the arbitrary voltage change rate,  
         [0024]     on the basis of signals from said power controller. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a circuit diagram showing the arrangement of a semiconductor integrated circuit according to the first embodiment of the present invention;  
         [0026]      FIG. 2  is a circuit diagram showing the arrangement of a power supply voltage controller included in a semiconductor integrated circuit according to the second embodiment of the present invention;  
         [0027]      FIG. 3  is a timing chart showing the transition of the voltage and clock and data processing when the power supply voltages are switched in the second embodiment;  
         [0028]      FIG. 4  is a timing chart showing the transition of the voltage and clock and data processing when a maximum voltage change rate is adjusted during the switching between the power supply voltages in the second embodiment;  
         [0029]      FIG. 5  is a circuit diagram showing the arrangement of a power supply voltage controller included in a semiconductor integrated circuit according to a comparative example;  
         [0030]      FIG. 6  is a timing chart showing the transition of the voltage and clock and data processing when the power supply voltages are switched in the comparative example;  
         [0031]      FIG. 7  is a circuit diagram showing the arrangement of a power supply voltage controller included in a semiconductor integrated circuit according to the third embodiment of the present invention;  
         [0032]      FIG. 8  is a circuit diagram showing the arrangement of a power supply voltage controller included in a semiconductor integrated circuit according to the fourth embodiment of the present invention; and  
         [0033]      FIG. 9  is a circuit diagram showing the arrangement of a power supply voltage controller included in a semiconductor integrated circuit according to the fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     Embodiments of the present invention will be described below with reference to the accompanying drawings.  
       (1) First Embodiment  
       [0035]      FIG. 1  shows the circuit configuration of a semiconductor integrated circuit according to the first embodiment of the present invention.  
         [0036]     This semiconductor integrated circuit includes a power controller PC, power supply voltage controller SVC, clock generator CG, and circuit portion CP.  
         [0037]     The power controller PC outputs, to the power supply voltage controller SVC, a voltage select signal SEL for selecting a power supply voltage, and current control signals CCS 1  and CCS 2  for limiting the current amount of a variable current source, and also outputs a clock select signal CLKS for selecting the frequency of a clock to the clock generator CG.  
         [0038]     Upon receiving the voltage select signal SEL, the power supply voltage controller SVC selects a normally used voltage (e.g., 1.2 V) or at least one type of a relatively low voltage (e.g., 0.9 V) which is used to down the power, and outputs a power supply voltage VDDV. A maximum voltage change rate when the voltage is dropped or raised can be adjusted by the current control signals CCS 1  and CCS 2 .  
         [0039]     Also, when changing the power supply voltage VDDV, the power supply voltage controller SVC outputs, to the power controller PC or clock generator CG, a voltage state signal VSTAT indicating that the voltage state has stabilized after the change.  
         [0040]     The clock generator CG receives a basic clock CLK 0 , and, on the basis of the clock select signal CLKS, outputs one of at least two types of frequencies (e.g., 100 and 50 MHz) as a clock CLK.  
         [0041]     The circuit portion CP performs predetermined data processing by receiving the output clock CLK and power supply voltage VDDV.  
         [0042]     The first embodiment having the above arrangement operates as follows in order to change the power supply voltage.  
         [0043]     First, to down the power from the normally used high voltage (e.g., 1.2 V) to the low voltage (e.g., 0.9 V), the power controller PC outputs the clock select signal CLKS to the clock generator CG. On the basis of the clock select signal CLKS, the clock generator CG lowers the frequency of the clock CLK from a normally used high frequency (e.g., 100 MHz) to a low frequency (e.g., 50 MHz) which is used to down the power.  
         [0044]     After that, the power controller PC outputs the voltage select signal SEL and current control signals CCS 1  and CCS 2  to the power supply voltage controller SVC, without stopping the data processing in the circuit portion CP. On the basis of the voltage select signal SEL, the power supply voltage controller SVC lowers the power supply voltage VDDV from the normally used high voltage to the low voltage. The power supply voltage controller SVC has a function of limiting the maximum voltage change rate, and lowers the voltage at a predetermined voltage change rate controllable by the current control signals CCS 1  and CCS 2 .  
         [0045]     When the low voltage is to be returned to the high normal voltage, the data processing in the circuit portion CP does not stop. In this case, the power controller PC outputs the voltage select signal SEL and current control signals CCS 1  and CCS 2  to the power supply voltage controller SVC. On the basis of the voltage select signal SEL, the power supply voltage controller SVC raises the power supply voltage VDDV from the low voltage to the normally used high voltage. Since the power supply voltage controller SVC has the function of limiting the maximum voltage change rate, it raises the voltage at a predetermined voltage change rate controllable by the current control signals CCS 1  and CCS 2 .  
         [0046]     Then, the power controller PC outputs the clock select signal CLKS to the clock generator CG. On the basis of the clock select signal CLKS, the clock generator CG returns the frequency of the clock CLK from the low frequency to the normally used high frequency.  
         [0047]     As described above, by limiting the maximum voltage change rate when the power supply voltage VDDV is changed, the transition of the power supply voltage VDDV can be performed by a moderate and constant voltage change. This allows a clock synchronization adjusting circuit (not shown) to adjust clock out of sync while the circuit operation continues. As a consequence, clock out of sync can be reduced to a level which does not interfere with the circuit operation.  
         [0048]     Also, since the transition of the power supply voltage VDDV is moderate, almost no overshoot occurs, so the power supply voltage VDDV does not fall outside the operating voltage standard.  
         [0049]     In the conventional circuits as described earlier, the transition of the power supply voltage is abruptly performed within a short time period. This causes clock out of sync, or produces a period during which the power supply voltage falls outside the operating voltage owing to overshoot. This makes it unavoidable to stop data processing. By contrast, in the first embodiment described above, the transition of the power supply voltage is gently performed, so the production of a period like this can be prevented. Consequently, the voltages can be switched without stopping data processing in the circuit portion CP, and the processing speed can be increased.  
       (2) Second Embodiment  
       [0050]      FIG. 2  shows the circuit configuration of a power supply voltage controller included in a semiconductor integrated circuit according to the second embodiment of the present invention. The second embodiment relates to a practical circuit configuration of the power supply voltage controller SVC according to the first embodiment.  
         [0051]     The power supply voltage controller SVC includes a maximum voltage change rate limiting unit  1 , and a voltage output unit having voltage comparators CM 3  and CM 4  each of which is, e.g., an arithmetic amplifier, a P-channel MOS transistor T 1 , an N-channel MOS transistor T 2 , and an output terminal OUT.  
         [0052]     The maximum voltage change rate limiting unit  1  receives two types of voltages of 1.2 V and 0.9 V, selects one voltage on the basis of a voltage select signal SEL, and outputs a reference voltage VREF_V.  
         [0053]     Between a voltage VDD terminal and reference voltage VREF_V output terminal, a variable current source CS 1  whose current amount is controllable by a current control signal CCS 1  and a transmission gate switch SW 101  for injecting an electric current are connected in series. A transmission gate switch SW 1  is connected between an input terminal which receives 1.2 V and the reference voltage VREF_V output terminal. A transmission gate switch SW 2  is connected between an input terminal which receives 0.9 V and the reference voltage VREF_V output terminal. Between the reference voltage VREF_V output terminal and a ground voltage VSS terminal, a transmission gate switch SW 102  for discharging an electric current and a variable current source CS 2  whose current amount is controllable by a current control signal CCS 2  are connected in series.  
         [0054]     Also, the voltage comparator CM 1  has a non-inversion input terminal which receives 1.2 V, and an inversion input terminal which is connected to the reference voltage VREF_V output terminal. The voltage comparator CM 2  has a non-inversion input terminal which is connected to the reference voltage VREF_V output terminal, and an inversion input terminal which receives 0.9 V.  
         [0055]     The voltage comparator CM 1  outputs a signal indicating whether the voltage of the reference voltage VREF_V output terminal is 1.2 V or more. The voltage comparator CM 2  outputs a signal indicating whether the voltage of the reference voltage VREF_V output terminal is 0.9 V or less.  
         [0056]     The output signals from the voltage comparators CM 1  and CM 2  are input to a switching controller SWC 1 . The switching controller SWC 1  receives the voltage select signal SEL and the signals indicative of the comparison results from the voltage comparators CM 1  and CM 2 , and controls ON/OFF of each of the transmission gate switches SW 101 , SW 1 , SW 2 , and SW 102  as will be described later.  
         [0057]     A capacitor C is connected between the reference voltage VREF_V output terminal and ground voltage VSS terminal. When the transmission gate switch SW 101  is ON and the transmission gate switch SW 102  is OFF, the variable current source CS 1  injects an electric current into the capacitor C to raise the electric potential of the reference voltage VREF_V output terminal. When the transmission gate switch SW 101  is OFF and the transmission gate switch SW 102  is ON, the variable current source CS 1  discharges an electric current from the capacitor C to drop the electric potential of the reference voltage VREF_V output terminal.  
         [0058]     The maximum voltage change rate limiting unit  1  having the above arrangement has a function of moderating the potential change of the reference voltage VREF_V output terminal by injecting or discharging an electric current into or from the capacitor C, in order to limit a maximum change rate during the voltage transition.  
         [0059]     The output voltage VREF_V from the maximum voltage change rate limiting unit  1  is input to the inversion input terminals of the voltage comparators CM 3  and CM 4 . The non-inversion input terminals of the voltage comparators CM 3  and CM 4  are connected to the output terminal OUT.  
         [0060]     Both the voltage comparators CM 3  and CM 4  check whether the electric potential of the output terminal OUT is lower than the reference voltage VREF_V. If the electric potential of the output terminal OUT is lower than the reference voltage VREF_V, the voltage comparators CM 3  and CM 4  output a low-level voltage. If the electric potential of the output terminal OUT is higher than the reference voltage VREF_V, the voltage comparators CM 3  and CM 4  output a high-level voltage.  
         [0061]     Between the power supply voltage VDD terminal and output terminal OUT, the source and drain of the P-channel MOS transistor T 1  are connected. Between the output terminal OUT and ground voltage VSS terminal, the drain and source of the N-channel MOS transistor T 2  are connected.  
         [0062]     If the voltage comparators CM 3  and CM 4  output the low level which indicates that the electric potential of the output terminal OUT is equal to or lower than the reference voltage VREF_V, the transistor T 1  is turned on, the transistor T 2  is turned off, and the electric potential of the output terminal OUT rises. If the voltage comparators CM 3  and CM 4  output the high level which indicates that the electric potential of the output terminal OUT is higher than the reference voltage VREF_V, the transistor T 1  is turned off, the transistor T 2  is turned on, and the electric potential of the output terminal OUT drops. In this manner, the two voltages are matched.  
         [0063]     The voltage controller SVC according to the second embodiment having the above arrangement operates as follows.  
         [0064]     In the maximum voltage change rate limiting unit  1 , the switching controller SWC 1  switches ON/OFF of the four transmission gate switches SW 101 , SW 1 , SW 2 , and SW 102 . In this way, one of 1.2 V and 0.9 V is output as the reference voltage VREF_V.  
         [0065]     Of the two voltages, the transmission gate switch SW 1  outputs 1.2 V to the reference voltage VREF_V output terminal, and the transmission gate switch SW 1  outputs 0.9 V to the reference voltage VREF_V output terminal.  
         [0066]     The transmission gate switch SW 101  injects an electric current into the capacitor C connected to the reference voltage VREF_V output terminal from the voltage VDD terminal via the variable current source CS 1 . The transmission gate switch SW 102  discharges an electric current from the capacitor C to the ground voltage VSS terminal via the variable current source CS 2 .  
         [0067]     To drop the voltage from 1.2 V to 0.9 V, the transmission gate switches SW 1  and SW 2  are turned off, the transmission gate switch SW 101  is turned off, and the transmission gate switch SW 102  is turned on, thereby discharging an electric current from the capacitor C and dropping the reference voltage VREF_V of the reference voltage VREF_V output terminal.  
         [0068]     To raise the voltage from 0.9 V to 1.2 V, the transmission gate switches SW 1  and SW 2  are turned off, the transmission gate switch SW 102  is turned off, and the transmission gate switch SW 101  is turned on, thereby injecting an electric current into the capacitor C and raising the voltage.  
         [0069]     The two voltage comparators CM 1  and CM 2  check whether the reference voltage VREF_V of the reference voltage VREF_V output terminal has completed its transition between 1.2 V and 0.9 V and reached one of these voltages.  
         [0070]     That is, the voltage comparator CM 1  checks whether the reference voltage VREF_V has reached 1.2 V or more. If the reference voltage VREF_V has risen to 1.2 V, the voltage comparator CM 1  notifies the switching controller SWC 1  of this information. The voltage comparator CM 2  checks whether the reference voltage VREF_V has reached 0.9 V or less. If the reference voltage VREF_V has dropped to 0.9 V, the voltage comparator CM 2  notifies the switching controller SWC 1  of this information.  
         [0071]     On the basis of the voltage select signal SEL, the switching controller SWC 1  switches ON/OFF of the transmission gate switch SW 101  for current injection or the transmission gate switch SW 102  for current discharge, thereby dropping or raising the voltage of the reference voltage VREF_V output terminal. On the basis of the notification from the voltage comparator CM 1  or CM 2 , when the reference voltage VREF_V has reached the target voltage, the switching controller SWC 1  turns on one of the transmission gate switches SW 1  and SW 2 , which connects the input terminal of one of the two voltage levels and the reference voltage VREF_V output terminal which generates the reference voltage VREF_V and turns off the other.  
         [0072]     The output reference voltage VREF_V is input to the inversion input terminals of the comparators CM 3  and CM 4 . The electric potential of the output terminal VDDV is input to the non-inversion input terminals of the comparators CM 3  and CM 4 .  
         [0073]     The operations of the comparators CM 3  and CM 4  and transistors T 1  and T 2  are the same as in the first embodiment, so an explanation thereof will be omitted.  
         [0074]     In the second embodiment, when the power supply voltage VDDV is dropped from 1.2 V to 0.9 V or returned from 0.9 V to 1.2 V, the transmission gate switch SW 101  or SW 102  controls the current injection amount or current discharge amount of the capacitor C of the reference voltage VREF_V output terminal in accordance with the current control signals CCS 1  and CCS 2 . Accordingly, the maximum voltage change rate of the power supply voltage VDDV can be limited.  
         [0075]     By thus limiting the maximum voltage change rate, the transition of the power supply voltage VDDV can be performed by a moderate and constant voltage change. This allows a clock synchronization adjusting circuit to adjust clock out of sync while the circuit operation continues. Consequently, clock out of sync can be reduced to a level which does not interfere with the circuit operation.  
         [0076]     Also, since the transition of the power supply voltage VDDV is moderate, almost no overshoot occurs, so the power supply voltage VDDV does not fall outside the operating voltage standard. As a consequence, the voltages can be switched without stopping data processing, so the processing speed increases.  
         [0077]     For example, consider a case in which a 100-MHz, 1.2-V semiconductor integrated circuit is used by downing the power to 50 MHz and 0.9 V in accordance with the margin of the data processability at that point.  
         [0078]     As shown in  FIG. 3 , before the power supply voltage VDDV is switched from 1.2 V to 0.9 V, the clock is dropped from 100 MHz to 50 MHz.  
         [0079]     Then, at time t 1 , the power supply voltage VDDV is slowly lowered from 1.2 V without stopping data processing. At time t 2 , the power supply voltage VDDV stabilizes at 0.9 V.  
         [0080]     On the other hand, to return to 100 MHz and 1.2 V, the power supply voltage VDDV is gently raised at time t 3  from 0.9 without stopping data processing, before the clock is returned from 50 MHz to 100 MHz. After the power supply voltage VDDV has reached 1.2 V and stabilized at time t 4 , the clock is returned from 50 MHz to 100 MHz.  
         [0081]     In the second embodiment as described above, the power supply voltage VDDV can be changed from 1.2 V to 0.9 V or vice versa, while data processing is not stopped but continued. This increases the data processing speed.  
         [0082]     When the power supply voltage VDDV is dropped or raised, an optimum transition time changes in accordance with, e.g., a clock distribution method or clock skew control method, or with, e.g., the size of the parasitic capacitance between the power supply voltage VDD terminal and ground voltage VSS terminal.  
         [0083]     When, therefore, the transition time can be shortened, for example, as shown in  FIG. 4 , the power supply voltage VDDV is dropped from 1.2 V at time t 1 , and stabilized at 0.9 V at time t 11  earlier than time t 2 . To return to 1.2 V, the power supply voltage VDDV is raised from time t 3 , and stabilized at time t 12  earlier than time t 4 . On the other hand, when it is necessary to prolong the transition time, the power supply voltage VDDV is dropped from 1.2 V at time t 1 , and stabilized at 0.9 V later than time t 2 . To return to 1.2 V, the power supply voltage VDDV is raised from time t 3 , and stabilized at 1.2 V later than time t 4 . The transition time can be set in this way.  
         [0084]     In the second embodiment as described above, even after the fabrication of the semiconductor integrated circuit, the transition time can be adjusted by controlling the current injection rate or current discharge rate of the capacitor C by the current control signals CCS 1  and CCS 2 .  
         [0085]      FIG. 5  shows the arrangement of a power supply voltage controller of a semiconductor integrated circuit as a comparative example of the above second embodiment.  
         [0086]     In this comparative example, a transmission gate switch SW 201  is connected between a reference voltage VREF_V output terminal which outputs a reference voltage VREF_V and an input terminal which receives 1.2 V, and a transmission gate switch SW 202  is connected between the reference voltage VREF_V output terminal and an input terminal which receives 0.9 V. The transmission gate switches SW 201  and SW 202  are controlled by a voltage select signal SEL. The rest of the arrangement is the same as the first embodiment described earlier, so an explanation thereof will be omitted.  
         [0087]     In the comparative example having the above arrangement, a power supply voltage VDDV changes as shown in  FIG. 6  when dropped from 1.2 V to 0.9 V and when returned to 1.2 V.  
         [0088]     Before the power supply voltage VDDV is switched from 1.2 V to 0.9 V, a clock is dropped from 100 MHz to 50 MHz.  
         [0089]     Then, data processing is stopped, and the power supply voltage VDDV is dropped at time t 101  from 1.2 V to 0.9 V faster than in the above embodiment. Although the power supply voltage VDDV reaches 0.9 V at time t 102 , a long time is required before time t 103  at which the voltage stabilizes. Data processing is resumed after time t 103 .  
         [0090]     To return to 100 MHz and 1.2 V, data processing is stopped first, and then at time t 104  the power supply voltage VDDV is rapidly raised from 0.9 V to 1.2 V. Although the power supply voltage VDDV reaches 1.2 V at time t 105 , a long time is required before time t 106  at which the voltage stabilizes. After time t 106 , the clock is returned from 50 MHz to 100 MHz to resume data processing.  
         [0091]     In this comparative example as described above, the voltage switching is controlled only by ON/OFF of the transmission gate switches SW 201  and SW 202 . This increases the rates of voltage drop and rise, and prolongs the time required before the voltage stabilizes. Since data processing must be stopped until the voltage stabilizes, the processing efficiency lowers.  
         [0092]     By contrast, in the first and second embodiments, the voltage can be gently changed by controlling the maximum voltage change rate, so the voltage transition can be performed while data processing continues. As a consequence, the processing efficiency can be increased.  
       (3) Third Embodiment  
       [0093]      FIG. 7  shows the arrangement of a power supply voltage controller of a semiconductor integrated circuit according to the third embodiment of the present invention. The third embodiment differs from the second embodiment in the arrangement of a maximum voltage change rate limiting unit  2 .  
         [0094]     In the maximum voltage change rate limiting unit  2 , a variable current source CS 11  and transmission gate switch SW 11  are connected in series between a power supply voltage VDD terminal and reference voltage VREF_V output terminal, and a transmission gate switch SW 12  and variable current source CS 12  are connected in series between the reference voltage VREF_V output terminal and a ground voltage VSS terminal.  
         [0095]     When turned on, the transmission gate switch SW 11  injects, via the variable current source CS 11 , an electric current into a capacitor C connected to the reference voltage VREF_V output terminal. When turned on, the transmission gate switch SW 12  discharges an electric current from the capacitor C of the reference voltage VREF_V output terminal via the variable current source CS 12 .  
         [0096]     A comparator CM 11  has a non-inversion input terminal which receives an input voltage VIN, and an inversion input terminal connected to the reference voltage VREF_V output terminal. The input voltage VIN is generated when 1.2 V and 0.9 V are input to an input voltage generator IVG, and one of these voltages is selected by a voltage select signal SEL.  
         [0097]     When the comparator CM 11  determines that a reference voltage VREF_V of the reference voltage output terminal matches the input voltage VIN, a switching controller SWC 2  is notified of this information.  
         [0098]     The switching controller SWC 2  is a device which controls ON/OFF of the transmission gate switches SW 11  and SW 12  such that the reference voltage VREF_V of the reference voltage VREF_V output terminal matches the input voltage VIN. The rest of the arrangement is the same as the second embodiment described above so an explanation thereof will be omitted.  
         [0099]     In the third embodiment, even when the input voltage VIN discretely changes, the maximum voltage change rate of the voltage transition is limited. Accordingly, the same effects as in the first and second embodiments are obtained.  
       (4) Fourth Embodiment  
       [0100]      FIG. 8  shows the arrangement of a power supply voltage controller of a semiconductor integrated circuit according to the fourth embodiment of the present invention. This power supply voltage controller of the fourth embodiment is characterized in that a maximum voltage change rate limiting unit  3  includes n (n is an integer of 2 or more) transmission gate switches and an up/down counter UDC.  
         [0101]     A transmission gate switch SW 21  is connected in series between an input terminal which receives a voltage of 1.20 V and a reference voltage VREF_V output terminal, a transmission gate switch SW 22  is connected in series between an input terminal which receives a voltage of 1.18 V and the reference voltage VREF_V output terminal, a transmission gate switch SW 23  is connected in series between an input terminal which receives a voltage of 1.16 V and the reference voltage VREF_V output terminal, a transmission gate switch SW 24  is connected in series between an input terminal which receives a voltage of 1.14 V and the reference voltage VREF_V output terminal, . . . , a transmission gate switch SW 36  is connected in series between an input terminal which receives a voltage of 0.90 V and the reference voltage VREF_V output terminal.  
         [0102]     ON/OFF operations of the transmission gate switches SW 21  to SW 36  are sequentially controlled by the up/down counter UDC.  
         [0103]     For example, when a voltage of 1.20 V is supplied, the transmission gate switch SW 21  is initially ON, the transmission gate switches SW 22  to SW 36  are initially OFF, and 1.20 V is initially output as a reference voltage VREF_V to the reference voltage VREF_V output terminal via the transmission gate switch SW 21 .  
         [0104]     To drop the voltage to 0.90 V from this state, 1.18 V is output by turning on only the transmission gate switch SW 22  and turning off all the other switches, 1.16 V is output by turning on only the transmission gate switch SW 23  and turning off all the other switches, . . . , 0.90 V is output as the reference voltage VREF_V by turning on only the transmission gate switch SW 36  and turning off all the other switches. In this manner, the voltage can be dropped step by step at an interval of 0.02 V.  
         [0105]     On the other hand, to raise the voltage from 0.90 V to 1.20 V, the transmission gate switch SW 36  is initially ON, the transmission gate switches SW 21  to SW 35  are initially OFF, and 0.90 V is initially output as the reference voltage VREF_V to the reference voltage VREF_V output terminal. From this state, 0.92 V is output by turning on only the transmission gate switch SW 34  and turning off all the other switches, . . . , 1.20 V is output by turning on only the transmission gate switch SW 21  and turning off all the other switches.  
         [0106]     In the fourth embodiment as described above, the levels of the reference voltage VREF_V are prepared from 1.2 V to 0.9 V at an interval of 0.02 V, and the operations of the transmission gate switches SW 21  to SW 36  are continuously switched by the up/down counter UDC. Consequently, the maximum change rate of a power supply voltage VDDV can be limited as in the first to third embodiments described above.  
         [0107]     A maximum voltage change rate per unit time is determined by the interval of the voltage steps and the switching rate. For example, to set the maximum voltage change rate at a lowest value, all the transmission gate switches SW 21  to SW 36  are sequentially turned on or off at an interval of 0.02 V as described above.  
         [0108]     To increase the maximum voltage change rate, ON/OFF of every other transmission gate switch, i.e., the transmission gate switches SW 21 , SW 23 , SW 25 , . . . , SW 33 , SW 35 , and SW 36  is controlled so that the voltage changes at an interval of, e.g., 0.04 V. To further increase the maximum voltage change rate, ON/OFF of every third transmission gate switch, i.e., the transmission gate switches SW 21 , SW 24 , SW 27 , SW 30 , SW 33 , and SW 36  is controlled so that the voltage changes at an interval of, e.g., 0.06 V.  
         [0109]     Alternatively, it is also possible to set the maximum voltage change rate by the switching rate, while the interval of the voltage steps is kept at 0.02 V.  
       (5) Fifth Embodiment  
       [0110]      FIG. 9  shows the arrangement of a semiconductor integrated circuit according to the fifth embodiment of the present invention. In the maximum voltage change rate limiting units  1  to  3  of the voltage controllers SVC according to the second to fourth embodiments described above, only two types of voltages, i.e., a normally used high voltage of 1.2 V and a low voltage of 0.9 V which is used to down the power are prepared. By contrast, in a maximum voltage change rate limiting unit  4  of a voltage controller SVC according to the fifth embodiment, voltages V 1  to Vn (n is an integer of 2 or more, and V 1 &gt;V 2 &gt; . . . &gt;Vn) which are not limited to two types but can be three types or more are prepared.  
         [0111]     In the maximum voltage change rate limiting unit  4 , a variable current source CS 1  and a transmission gate switch SW 101  for injecting an electric current are connected in series between a voltage VDD terminal and reference voltage VREF_V output terminal, a transmission gate switch SW 1  is connected between an input terminal which receives the highest voltage V 1  and the reference voltage VREF_V output terminal, a transmission gate switch SW 2  is connected between an input terminal which receives the second highest voltage V 2  and the reference voltage VREF_V output terminal, . . . , a transmission gate switch SW(n−1) (not shown) is connected between an input terminal which receives the second lowest voltage V(n−1) and the reference voltage VREF_V output terminal, a transmission gate switch SWn is connected between an input terminal which receives the lowest voltage Vn and the reference voltage VREF_V output terminal, and a transmission gate switch SW 102  for discharging an electric current and a variable current source CS 2  are connected in series between the reference voltage VREF_V output terminal and a ground voltage VSS terminal.  
         [0112]     A voltage comparator CM 1  has a non-inversion input terminal which receives the voltage V 1 , and an inversion input terminal which is connected to the reference voltage VREF_V output terminal, a voltage comparator CM 2  has a non-inversion input terminal which is connected to the reference voltage VREF_V output terminal, and an inversion input terminal which receives the voltage V 2 , a voltage comparator CM 3  (not shown) has a non-inversion input terminal which is connected to the reference voltage VREF_V output terminal, and an inversion input terminal which receives the voltage V 3 , . . . , a voltage comparator CMn has a non-inversion input terminal which is connected to the reference voltage VREF_V output terminal, and an inversion input terminal which receives the voltage Vn.  
         [0113]     The voltage comparator CM 1  outputs a signal indicating whether the voltage of the reference voltage VREF_V output terminal is the voltage V 1  or more. The voltage comparator CM 2  outputs a signal indicating whether the voltage of the reference voltage VREF_V output terminal is the voltage V 2  or less.  
         [0114]     The voltage comparator CM 3  outputs a signal indicating whether the voltage of the reference voltage VREF_V output terminal is the voltage V 3  or less, . . . , the voltage comparator CMn outputs a signal indicating whether the voltage of the reference voltage VREF_V output terminal is lower than the voltage Vn.  
         [0115]     The output signals from the voltage comparators CM 1  to CMn are input to a switching controller SWC 4 . The switching controller SWC 4  receives a voltage select signal SEL and the signals indicative of the comparison results from the voltage comparators CM 1  to CMn, and controls ON/OFF of each of the transmission gate switches SW 1  to SWn. The rest of the arrangement is the same as the power supply voltage controller according to the second embodiment, so an explanation thereof will be omitted.  
         [0116]     In the fifth embodiment, to drop the voltage from the normally used highest voltage V 1  in order to down the power, a desired voltage is selected from the voltages V 2  to Vn by the voltage select signal SEL. If, for example, the voltage V 3  is selected, the switching controller SWC 4  turns off all the transmission gate switches SW 101  and SW 1  to SWn, and turns on only the transmission gate switch SW 102 , thereby discharging an electric current from a capacitor C of the reference voltage VREF_V output terminal. Accordingly, the electric potential of the reference voltage VREF_V output terminal gradually drops.  
         [0117]     When the voltage comparator CM 3  detects that the electric potential of the reference voltage VREF_V output terminal is the same as the voltage V 3 , the switching controller SWC 4  turns on only the transmission gate switch SW 3 , and turns off all the other transmission gate switches SW 101 , SW 1 , SW 2 , SW 4  to SWn, and SW 102 . As a consequence, a reference voltage VREF_V having the stable voltage V 3  is output from the reference voltage VREF_V output terminal.  
         [0118]     To return to the voltage V 1  from the voltage V 3 , the voltage select signal SEL selects the voltage V 1 , the switching controller SWC 4  turns off all the transmission gate switches SW 1  to SWn, and turns on only the transmission gate switch SW 101 , thereby injecting an electric current into the capacitor C of the reference voltage VREF_V output terminal. Accordingly, the electric potential of the reference voltage VREF_V output terminal gradually rises.  
         [0119]     When the voltage comparator CM 1  detects that the electric potential of the reference voltage VREF_V output terminal is the same as the voltage V 1 , the switching controller SWC 4  turns on only the transmission gate switch SW 1 , and turns off all the other transmission gate switches SW 101 , SW 2  to SWn, and SW 102 . As a consequence, the reference voltage VREF_V equal to the voltage V 1  is output from the reference voltage VREF_V output terminal.  
         [0120]     As in the second embodiment described above, the electric potential of the power supply voltage VDDV can be slowly changed in the fifth embodiment as well by limiting a maximum change rate by the maximum voltage change rate limiting unit  4  during the transition of a selected voltage.  
         [0121]     In each of the first to fifth embodiments as explained above, when at least two types of voltages are switched in a semiconductor integrated circuit including a power supply voltage controller, a maximum voltage change rate can be limited during the transition of the power supply voltage.  
         [0122]     In this way, the transition of the power supply voltage is performed by a moderate and constant voltage change, clock out of sync and circuit delay fluctuation are reduced to a level which does not interfere with the circuit operation, and almost no overshoot occurs. Therefore, voltage switching can be performed without stopping data processing. This increases the efficiency of data processing.  
         [0123]     As described above, an optimum transition time of the power supply voltage changes in accordance with, e.g., a clock distribution method or clock skew control of a circuit which supplies the power supply voltage, or the capacitance between the power supply voltage terminal and ground terminal. Therefore, even after a trial product is fabricated, it is possible, by limiting a maximum voltage change rate by a control signal, to adjust the return time from the power down state, i.e., the transition time during which the voltage rises, or to adjust the transition time to the power down state. Consequently, the performance can be further improved.  
         [0124]     In the semiconductor integrated circuits of the above embodiments, the operation can continue even while the power supply voltages are switched. This increases the data processing speed.  
         [0125]     The above embodiments are merely examples, and hence do not limit the present invention. Therefore, these embodiments can be variously modified within the technical scope of the present invention.  
         [0126]     For example, the current amount of the variable current source need not be controlled by the current control signal as in the second embodiment. That is, it is also possible to store a predetermined value by using a memory such as a fuse memory, and control an electric current to flow on the basis of this value.  
         [0127]     Also, if the voltage transition is linear and moderate, the semiconductor integrated circuit can determine an optimum power supply voltage by using its own self-diagnostic function in combination with the above function of the present invention. In this case, it is possible to stop the power supply voltage at an intermediate level during the course of transition, and change the voltage set before the transition.