Abstract:
An oscillator circuit which generates an oscillator signal that ensures the generation of a stable internal supply voltage. The oscillator circuit has a periodic circuit which includes a switch circuit. A method of controlling the oscillator circuit includes the steps of operating the periodic circuit using the switch circuit in response to a first control signal when the first control signal is in a first state to generate a first oscillator signal having a first frequency, and operating the periodic circuit using the switch circuit in response to a second control signal when the first control signal is in a second state to generate a second oscillator signal having a period synchronized to a period of the second control signal having a second frequency.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an oscillator circuit, and more particularly, to an oscillator circuit that generates an oscillator signal supplied to a charge pump circuit of an internal power generator circuit.  
           [0002]    [0002]FIG. 1 is a schematic block diagram of a conventional internal power generator circuit built in a semiconductor device. The internal power generator circuit acts as a boosted power generator circuit  91  for boosting an external or internal supply voltage VDD to generate a boosted voltage VPP.  
           [0003]    The boosted power generator circuit  91  includes a first and a second detector circuit  92 ,  93  for detecting the boosted voltage VPP; a first and a second oscillator circuit  94 ,  95 ; a signal synthesizer circuit  96 ; and a charge pump circuit  97 . The boosted power generator circuit  91  has an active mode in which a current is supplied with a relatively large driving performance, and a standby mode in which a current is supplied with a relatively small driving performance.  
           [0004]    The active mode will be first described.  
           [0005]    In the active mode, the first detector circuit  92  is activated by an activation signal φ. The first detector  92  is a detector circuit for the active mode which has relatively large current consumption and operates at a relatively high reaction speed. The first detector circuit  92  detects the boosted voltage VPP in the active mode and generates a detection signal DET-A when the boosted voltage VPP decreases to a voltage equal to or smaller than a predetermined voltage. The first oscillator circuit  94  receives the detection signal DET-A from the first detector circuit  92 , and generates an oscillator signal OSC-A for the active mode which has a relatively short period (for example, several tens of nanoseconds (ns)) (i.e., a relatively high oscillating frequency).  
           [0006]    The signal synthesizer circuit  96  receives the oscillator signal OSC-A from the first oscillator circuit  94 , and generates an oscillator signal OSC in accordance with the oscillator signal OSC-A for the active mode. The charge pump circuit  97  performs a charge pump operation following the period of the oscillator signal OSC to boost the external or internal supply voltage VDD to generate the boosted voltage VPP.  
           [0007]    Next, the standby mode will be described.  
           [0008]    The second detector circuit  93  is activated without fail whenever the device is applied with the supply voltage VDD, not only in the standby mode. The second detector circuit  93  is a detector circuit for the standby mode which has relatively small current consumption and operates at a relatively low reaction speed. The second detector circuit  93  is activated at all times irrespective of whether the device is in the active mode or in the standby mode.  
           [0009]    The second detector circuit  93  detects the boosted voltage VPP, and generates a detection signal DET-S when the boosted voltage VPP decreases to a voltage equal to or smaller than a predetermined voltage. The second oscillator circuit  95  receives the detection signal DET-S from the second detector circuit  93 , and generates an oscillator signal OSC-S for the standby mode which has a relatively long period (for example, several hundreds of nanoseconds (ns)) (i.e., a relatively low oscillating frequency).  
           [0010]    The signal synthesizer circuit  96  receives the oscillator signal OSC-S for the standby mode from the second oscillator circuit  95 , and generates an oscillator signal OSC in accordance with the oscillator signal OSC-S for the standby mode. The charge pump circuit  97  performs a charge pump operation following the period of the oscillator signal OSC to boost the external or internal supply voltage VDD to generate the boosted voltage VPP.  
           [0011]    As described above, the boosted power generator circuit  91  operates at different frequencies in the active mode and standby mode. In the active mode, the boosted power generator circuit  91  supplies a larger current than in the standby mode to generate the boosted voltage VPP. In the standby mode, the boosted power generator circuit  91  consumes a smaller current to limit its power consumption.  
           [0012]    In the active mode, the first oscillator circuit  94  as well as the second oscillator circuit  95  are activated. However, since the first and second oscillator circuits  94 ,  95  are asynchronous to each other, a pulse having a shorter period than the period of the oscillator signals OSC-A, OSC-S of the oscillator circuits  94 ,  95  may be generated in some cases. Such a pulse may cause a malfunction of the charge pump circuit  97 . Specifically, a shorter pulse period would result in a failure in a sufficient charge pump operation, a reduced current supply capability, or increased power consumption.  
           [0013]    To solve this problem, it is contemplated to separately provide a charge pump circuit for the active mode and a charge pump circuit for the standby mode. However, since the charge pump circuit has a relatively large circuit area, separately provided charge pump circuits would result in an increased semiconductor die size and increased power consumption.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the present invention to provide an oscillator circuit which generates an oscillator signal that ensures the generation of a stable internal supply voltage.  
           [0015]    In a first aspect of the present invention, a method of controlling an oscillator circuit having a periodic circuit which includes a switch circuit is provided. The method includes the steps of operating the periodic circuit using the switch circuit in response to a first control signal when the first control signal is in a first state to generate a first oscillator signal having a first frequency, and operating the periodic circuit using the switch circuit in response to a second control signal when the first control signal is in a second state to generate a second oscillator signal having a period synchronized to a period of the second control signal having a second frequency.  
           [0016]    In a second aspect of the present invention, a method of controlling an oscillator circuit having a periodic circuit which includes a switch circuit is provided. The method includes the steps of operating the periodic circuit in response to a first control signal using the switch circuit to generate a first oscillator signal synchronized in phase to the first control signal, and operating the periodic circuit in response to a second control signal using the switch circuit to generate a second oscillator signal synchronized in phase to the second control signal.  
           [0017]    In a third aspect of the present invention, an oscillator circuit is provided that includes a synthesizer for synthesizing a first control signal with a pulse signal to generate a synthesized signal, a pulse generator circuit connected to the synthesizer for generating a pulse signal in response to a second control signal and supplying the pulse signal to the synthesizer, a control circuit connected to the synthesizer for generating a switch control signal in accordance with the synthesized signal, and a periodic circuit connected to the control circuit, and including a switch circuit responsive to the switch control signal. The periodic circuit generates one of a first oscillator signal having a first frequency and a second oscillator signal having a second frequency in accordance with an operation of the switch circuit.  
           [0018]    In a fourth aspect of the present invention, an oscillator circuit is provided that includes a first pulse generator circuit for generating a first pulse signal in response to a first control signal, a second pulse generator circuit for generating a second pulse signal in response to a second control signal, a synthesizer connected to the first and second pulse generator circuits for synthesizing the first pulse signal and the second pulse signal to generate a synthesized signal, a control circuit connected to the synthesizer for generating a switch control signal in accordance with the synthesized signal, and a periodic circuit connected to the control circuit, and including a switch circuit responsive to the switch control signal. The periodic circuit generates one of a first oscillator signal corresponding to the first control signal and having a first frequency and a second oscillator signal corresponding to the second control signal and having a second frequency in accordance with an operation of the switch circuit.  
           [0019]    In a fifth aspect of the present invention, a method of controlling an internal power generator circuit is provided. The internal power generator circuit includes an oscillator circuit having a periodic circuit including a switch circuit, and a charge pump circuit connected to the oscillator circuit. The method includes the steps of operating the periodic circuit in response to a first control signal using the switch circuit, when the first control signal is in a first state, to generate a first oscillator signal having a first frequency, generating a voltage in accordance with the first oscillator signal using the charge pump circuit, operating the periodic circuit in response to a second control signal using the switch circuit, when the first control signal is in a second state, to generate a second oscillator signal having a period synchronized to a period of the second control signal having a second frequency, and generating the voltage in accordance with the second oscillator signal using the charge pump circuit.  
           [0020]    In a sixth aspect of the present invention, a method of controlling an internal power generator circuit is provided. The internal power generator circuit includes an oscillator circuit including a periodic circuit having a switch circuit, and a switch control circuit for controlling the switch circuit, and a charge pump circuit connected to the oscillator circuit. The method includes the steps of controlling the switch circuit in response to a first control signal by the switch control circuit to operate the periodic circuit to generate a first oscillator signal having a period synchronized to a period of the first control signal, generating a voltage in accordance with the first oscillator signal using the charge pump circuit, controlling the switch circuit in response to a second control signal by the switch control circuit to operate the periodic circuit to generate a second oscillator signal having a period synchronized to a period of the second control signal, and generating the voltage in accordance with the second oscillator signal using the charge pump circuit.  
           [0021]    In a seventh aspect of the present invention, an internal power generator circuit is provided that includes a first oscillator circuit and a charge pump circuit. The first oscillator circuit includes a synthesizer for synthesizing a first control signal with a pulse signal to generate a synthesized signal, a pulse generator circuit connected to the synthesizer for generating a pulse signal in response to a second control signal and supplying the synthesizer with the pulse signal, a control circuit connected to the synthesizer for generating a switch control signal in accordance with the synthesized signal, and a periodic circuit connected to the control circuit and including a switch circuit responsive to the switch control signal. The periodic circuit generates one of a first oscillator signal having a first frequency and a second oscillator signal having a second frequency in accordance with an operation of the switch circuit. The charge pump circuit is connected to the periodic circuit and generates a voltage in accordance with one of the first oscillator signal and the second oscillator signal.  
           [0022]    In an eighth aspect of the present invention, an internal power generator circuit is provided that includes a first oscillator circuit and a charge pump circuit. The first oscillator circuit includes a first pulse generator circuit for generating a first pulse signal in response to a first control signal, a second pulse generator circuit for generating a second pulse signal in response to a second control signal, a synthesizer connected to the first and second pulse generator circuits for synthesizing the first pulse signal and the second pulse signal to generate a synthesized signal, a control circuit connected to the synthesizer for generating a switch control signal in accordance with the synthesized signal, and a periodic circuit connected to the control circuit, and including a switch circuit responsive to the switch control signal. The periodic circuit generates one of a first oscillator signal corresponding to the first control signal and having a first frequency and a second oscillator signal corresponding to the second control signal and having a second frequency in accordance with an operation of the switch circuit. The charge pump circuit is connected to the periodic circuit and generates a voltage in accordance with one of the first oscillator signal and the second oscillator signal.  
           [0023]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0025]    [0025]FIG. 1 is a schematic block diagram of a conventional boosted power generator circuit;  
         [0026]    [0026]FIG. 2 is a schematic block diagram of a boosted power generator circuit according to a first embodiment of the present invention;  
         [0027]    [0027]FIG. 3 is a schematic block diagram of the boosted power generator circuit of FIG. 2;  
         [0028]    [0028]FIG. 4 is a schematic circuit diagram of a first detector circuit in the boosted power generator circuit of FIG. 2;  
         [0029]    [0029]FIG. 5 is a schematic circuit diagram of a second detector circuit of the boosted power generator circuit of FIG. 2 FIG. 6 is a schematic circuit diagram of an oscillator circuit of the boosted power generator circuit of FIG. 2;  
         [0030]    [0030]FIG. 7 is a schematic circuit diagram of an incorrect pulse preventing circuit of the boosted power generator circuit of FIG. 2;  
         [0031]    [0031]FIG. 8 is a schematic circuit diagram of a charge pump circuit of the boosted power generator circuit of FIG. 2;  
         [0032]    FIGS.  9 ( a ) through  9 ( c ) are waveform charts for describing the operation of the boosted power generator circuit of FIG. 2;  
         [0033]    [0033]FIG. 10 is a graph showing the relationship between an input frequency and an output frequency of the boosted power generator circuit of FIG. 2;  
         [0034]    [0034]FIG. 11 is a schematic block diagram of a boosted power generator circuit according to a second embodiment of the present invention; and  
         [0035]    [0035]FIG. 12 is a schematic circuit diagram of an incorrect pulse preventing circuit of the boosted power generator circuit of FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    In the drawings, like numerals are used for like elements throughout.  
         [0037]    A boosted power generator circuit  11  according to a first embodiment of the present invention will be described with reference to FIGS. 2 through 10. As illustrated in FIG. 2, the boosted power generator circuit  11  includes first and second detector circuits  12 ,  13  for detecting a boosted voltage VPP; an incorrect pulse preventing circuit  14 ; an oscillator circuit  15 ; and a charge pump circuit  16 .  
         [0038]    The boosted power generator circuit  11  operates at different frequencies in an active mode (device operative mode) and a standby mode (device inoperative mode). The charge pump circuit  16  of the boosted power generator circuit  11  operates at a first frequency in the active mode to generate the boosted voltage VPP. The charge pump circuit  16  operates at a second frequency in the standby mode to output the boosted voltage VPP. The first frequency is greater than the second frequency, so that the boosted power generator circuit  11  supplies more currents in the active mode than in the standby mode.  
         [0039]    The first detector circuit  12  is activated by an activation signal φ in the active mode. The first detector circuit  12  is a detector circuit for the active mode which has relatively large current consumption and operates at a relatively high reaction speed. The first detector circuit  12  detects the boosted voltage VPP in the active mode, and generates a detection signal DET-A when the boosted voltage VPP decreases to a voltage equal to or smaller than a predetermined voltage.  
         [0040]    The second detector circuit  13  operates without fail whenever the device is applied with a supply voltage VDD, not only in the standby mode. The second detector circuit  13  is a detector circuit for the standby mode which has relatively small current consumption and operates at a relatively low reaction speed. The second detector circuit  13  detects the boosted voltage VPP, and generates a detection signal DET-S when the boosted voltage VPP decreases to a voltage equal to or smaller than a predetermined voltage.  
         [0041]    As illustrated in FIG. 3, the oscillator circuit  15  includes a periodic circuit  23 ; a switch control circuit  24 ; and an inverter gate  25 . The periodic circuit  23  includes a periodic delay circuit  21  and a switch circuit  22 . An output signal of the periodic delay circuit  21  is supplied to the switch circuit  22 , while an output signal (oscillator signal OSC-S) of the switch circuit  22  is fed back to the periodic delay circuit  21  as well as supplied to the switch control circuit  24 .  
         [0042]    The inverter gate  25  receives the detection signal DET-S from the second detector circuit  13 , inverts the detection signal DET-S, and supplies the inverted detection signal to the switch control circuit  24 . The switch control circuit  24  is activated by the inverted detection signal and the oscillator signal OSC-S to supply an activation signal to the switch circuit  22 . The switch circuit  22  is activated by the activation signal, so that the periodic circuit  23  operates as a ring oscillator to generate an oscillator signal OSC-S which has a second frequency.  
         [0043]    The incorrect pulse preventing circuit  14  includes a pulse generator circuit  31 ; a synthesizer  32 ; and an oscillator circuit  33 . The oscillator circuit  33  includes a periodic circuit  43  and a switch control circuit  44 . The periodic circuit  43  includes a periodic delay circuit  41  and a switch circuit  42 . An output signal of the periodic delay circuit  41  is supplied to the switch circuit  42 , while an output signal of the switch circuit  42  (oscillator signal OSC) is fed back to the periodic delay circuit  41  as well as supplied to the switch control circuit  44 .  
         [0044]    The pulse generator circuit  31  receives the oscillator signal OSC-S from the oscillator circuit  15  and generates a pulse signal P 1 . The synthesizer  32  receives the pulse signal P 1  and the detection signal DET-A from the first detector circuit  12 , and supplies a synthesized signal to the switch control circuit  44 .  
         [0045]    The switch control circuit  44  is activated by the synthesized signal from the synthesizer  32  and the oscillator signal OSC to supply an activation signal to the switch circuit  42 . The switch circuit  42  is activated by the activation signal, so that the periodic circuit  43  operates as a ring oscillator to generate the oscillator signal OSC.  
         [0046]    The incorrect pulse preventing circuit  14  receives the detection signal DET-A (first control signal IN 1 ) from the first detector circuit  12 , and the oscillator signal OSC-S (second control signal IN 2 ) from the oscillator circuit  15 , and generates an oscillator signal OSC having a first frequency or a second frequency. The oscillator circuit  33  is activated when the synthesizer  32  is supplied with the detection signal DET-A having H-level (when the control signal IN 1  is in a first state). In this event, the periodic circuit  43  of the oscillator circuit  33  oscillates at the first frequency (greater than the second frequency) to output the oscillator signal OSC having the first frequency from the periodic circuit  43 .  
         [0047]    When the synthesizer  32  is supplied with the detection signal DET-A having L-level (when the first control signal IN 1  is in a second state), the oscillator circuit  33  is activated by the pulse signal P 1  based on the oscillator signal OSC-S. In this event, the periodic circuit  43  of the oscillator circuit  33  oscillates at the second frequency to output the oscillator signal OSC at the second frequency from the periodic circuit  43 .  
         [0048]    The charge pump circuit  16  performs a charge pump operation in accordance with the oscillator signal OSC to generate a boosted voltage VPP which is a boosted one of the supply voltage VDD.  
         [0049]    As illustrated in FIG. 4, the first detector circuit  12  includes resistors R 1 , R 2 ; a current mirror  51 ; and an inverter  52 . The current mirror  51  includes two PMOS transistors Tr 1 , Tr 2 ; and three NMOS transistors Tr 3 , Tr 4 , Tr 5 .  
         [0050]    The boosted voltage VPP generated by the charge pump circuit  16  is divided by the resistors R 1 , R 2 , and a divided voltage is supplied to a first input terminal (a gate of the first NMOS transistor Tr 3 ) of the current mirror  51 . The current mirror  51  is also supplied with a reference voltage Vref at its second input terminal (a gate of the second NMOS transistor Tr 4 ).  
         [0051]    The third PMOS transistor Tr 6  is connected in parallel with the second PMOS transistor Tr 2 . The activation signal φ indicative of the active mode is supplied to a gate of the third PMOS transistor Tr 6  and a gate of the NMOS transistor Tr 5 .  
         [0052]    Output signals generated at drains of the second and third PMOS transistors Tr 2 , Tr 6 , and an output signal generated at a drain of the third NMOS transistor Tr 4  are supplied to the inverter  52 .  
         [0053]    The first detector circuit  12  is activated by the activation signal φ indicative of the active mode, and generates the detection signal DET-A at H-level for activating the oscillator circuit  33  when the divided voltage of the boosted voltage VPP is equal to or smaller than the reference voltage Vref.  
         [0054]    As illustrated in FIG. 5, the second detector circuit  13  includes resistors R 3 , R 4 ; a current mirror  61 ; and an inverter  62 . The current mirror  61  includes two PMOS transistors Tr 7 , Tr 8 ; and three transistors Tr 9 , Tr 10 , Tr 11 .  
         [0055]    The boosted voltage VPP generated by the charge pump circuit  16  is divided by the resistors R 3 , R 4 , so that a divided voltage is supplied to a first input terminal of the current mirror  61  (a gate of the first NMOS transistor Tr 9 ). A reference voltage Vref is supplied to a second input terminal of the current mirror  61  (a gate of the second NMOS transistor Tr 10 ). The supply voltage VDD is supplied to a gate of the third NMOS transistor Tr 11 . The second detector circuit  13  is activated at all times as long as the supply voltage VDD is supplied in the active mode and standby mode.  
         [0056]    The second detector circuit  13  operates irrespective of the active mode or standby mode, and generates the detection signal DET-S having H-level for activating the oscillator circuit  15  when the divided voltage of the boosted voltage VPP is equal to or smaller than the reference voltage Vref.  
         [0057]    As illustrated in FIG. 6, the oscillator circuit  15  includes a periodic circuit  23 ; a switch control circuit  24 ; and an inverter gate  25 . The periodic circuit  23  includes a periodic delay circuit  21  and a switch circuit  22 .  
         [0058]    The switch circuit  22  includes an inverter  22   a  comprised of a PMOS transistor Tr 12  and an NMOS transistor Tr 13 ; two PMOS transistors Tr 14 , Tr 15 ; and two NMOS transistors Tr 16 , Tr 17 . The periodic delay circuit  21  includes inverters  21   a  at an even number of stages (for example, four stages).  
         [0059]    An output terminal of the periodic delay circuit  21  (a node N 1 ) is connected to an input terminal of the inverter  22   a  (gates of the transistors Tr 12 , Tr 13 ) of the switch circuit  22 . An output terminal of the inverter  22   a  is connected to an input terminal of the periodic delay circuit  21  (the inverter at the first stage). An odd number of stages of inverters (the four inverters  21   a  and inverter  22   a ) are connected to the periodic circuit  23  in a loop configuration to form a ring oscillator.  
         [0060]    The PMOS transistor Trl 4  has its source connected to the supply voltage VDD, and its drain connected to a source of the PMOS transistor Tr 12  of the inverter  22   a.  The NMOS transistor Tr 16  has its source connected to a ground GND, and its drain connected to a source of the NMOS transistor Tr 13  of the inverter  22   a.  Therefore, when the transistors Tr 14 , Tr 16  are turned on, the periodic circuit  23  is activated to generate the oscillator signal OSC-S having the second frequency.  
         [0061]    The PMOS transistor Tr 15  has its source connected to the supply voltage VDD, and its gate connected to a gate of the NMOS transistor Tr 16 . The NMOS transistor Tr 17  has its source connected to the ground GND, and its gate connected to a gate of the transistor Tr 14 .  
         [0062]    The PMOS transistor Tr 15  and NMOS transistor Tr 17  have their drains connected to each other, and a connection node N 2  is connected to the output terminal of the inverter  22   a  (drains of the transistors Tr 12 , Tr 13 ). The oscillator signal OSC-S is output from the node N 2 .  
         [0063]    The switch control circuit  24  is comprised of a plurality of logic circuits. For example, the switch control circuit  24  includes an inverter  24   a;  a NAND circuit  24   b;  and a NOR circuit  24   c.    
         [0064]    The inverter  24   a  has its input terminal connected to an output terminal of the inverter gate  25  (node N 3 ), which is connected to a first input terminal of the NAND circuit  24   b . The NAND circuit  24   b  is supplied with the oscillator signal OSC-S at its second input terminal. The oscillator signal OSC-S is supplied to a first input terminal of the NOR circuit  24   c  which has a second input terminal supplied with an output signal of the inverter  24   a.    
         [0065]    An output signal of the NAND circuit  24   b  is supplied to a gate of the NMOS transistor Tr 16  and to a gate of the PMOS transistor Tr 15 . An output signal of the NOR circuit  24   c  is supplied to a gate of the NMOS transistor Tr 17  and to a gate of the PMOS transistor Tr 14 . In the foregoing configuration, the switch control circuit  24  controls activation/deactivation of the periodic circuit  23 .  
         [0066]    In the oscillator circuit  15 , when the inverter gate  25  is supplied with the detection signal DET-S having H-level from the second detector circuit  13 , the inverter gate  25  outputs an output signal having L-level. In other words, the voltage at the node N 3  is set to L-level. This causes the NAND circuit  24   b  to output a signal having H-level, and the NOR circuit  24   c  to output a signal having L-level, so that the transistors Tr 14 , Tr 16  are turned on, and the transistors Tr 15 , Tr 17  are turned off. Consequently, the inverter  22   a  is activated, and the periodic circuit  23  generates the oscillator signal OSC-S having the second frequency.  
         [0067]    When the inverter gate  25  is supplied with the detection signal having L-level from the second detector  13 , the inverter gate  25  outputs an output signal having H-level. In other words, the voltage at the node N 3  is set to H-level. In this state, when the periodic circuit  23  is outputting the oscillator signal OSC-S having L-level, the NAND circuit  24   b  and NOR circuit  24   c  both output signals having H-level. This causes the transistors Tr 14 , Tr 15  to turn off, and the transistors Tr 16 , Tr 17  to turn on. Consequently, the inverter  22   a  is deactivated to stop the operation of the periodic circuit  23 . In this event, the oscillator signal OSC-S is clamped to L-level (connected to the ground GND) by the transistor Tr 17  which has been turned on.  
         [0068]    Also, when the inverter gate  25  is outputting an output signal having H-level and the periodic circuit  23  is outputting the oscillator signal OSC-S having H-level, the NAND circuit  24   b  and NOR circuit  24   c  both output signals having L-level. This causes the transistors Tr 14 , Tr 15  to turn on, and the transistors Tr 16 , Tr 17  to turn off. Consequently, the inverter  22   a  is deactivated to stop the operation of the periodic circuit  23 . In this event, the oscillator signal OSC-S is clamped to H-level (connected to the supply voltage VDD) by the transistor Tr 15  which has been turned on.  
         [0069]    As illustrated in FIG. 7, the incorrect pulse preventing circuit  14  includes a pulse generator circuit  31 ; a synthesizer  32 ; and an oscillator circuit  33 .  
         [0070]    The pulse generator circuit  31 , which is comprised of a plurality of logic circuits, generates a pulse in response to a pulse edge of the oscillator signal OSC-S (second control signal IN 2 ) output from the oscillator circuit  15 . In other words, the pulse generator circuit  31  is an edge triggered circuit. The pulse generator circuit  31  includes two delay circuits  71 ,  72  each comprised of an odd number (for example, three stages) of inverters; an inverter  73 ; and three NAND circuits  74 ,  75 ,  76 .  
         [0071]    A first NAND circuit  74  is supplied at its first input terminal with the oscillator signal OSC-S, and supplied at its second input terminal with the oscillator signal OSC-S which is delayed by the first delay circuit  71  and inverted. The oscillator signal OSC-S is inverted by the inverter  73 , and the inverted oscillator signal OSC-S is supplied to a first input terminal of a second NAND circuit  75 . The second NAND circuit  75  is supplied at its second input terminal with an output signal of the inverter  73  which is delayed by the second delay circuit  72  and inverted. Output signals of the NAND circuits  74 ,  75  are supplied to a third NAND circuit  76 .  
         [0072]    In response to pulse edges (a rising edge and a falling edge) of the oscillator signal OSC-S having the second frequency, the third NAND circuit  76  outputs a pulse signal P 1 . Specifically, the pulse generator circuit  31  generates the pulse signal P 1  having an H-level pulse width corresponding to delay times of the delay circuits  71 ,  72  in response to a pulse edge of the oscillator signal OSC-S. In other words, the pulse signal P 1  has a frequency twice the second frequency.  
         [0073]    The periodic circuit  43  of the oscillator circuit  33  operates in a half-period (or a complete period) of the oscillator signal OSC-S having the second frequency in response to the pulse signal P 1 . Specifically, the pulse generator circuit  31  generates the pulse signal P 1  which has a pulse formed for a duration shorter than the time in which the periodic circuit  43  is operated in a half-period (or a complete period). The pulse width of the pulse signal P 1  is set according to the delay times of the first and second delay circuits  71 ,  72 .  
         [0074]    The synthesizer  32  is comprised of a NOR circuit. The NOR circuit receives the detection signal DET-A (first control signal IN 1 ) from the first detector circuit  12 , and the pulse signal P 1  from the pulse generator circuit  31 , and generates an output signal at L-level when the NOR circuit receives the detection signal DET-A at H-level for activating the oscillator circuit  33  or the pulse signal P 1  at H level.  
         [0075]    The oscillator circuit  33  includes a periodic circuit  43  and a switch control circuit  44 . The periodic circuit  43  includes a periodic delay circuit  41  and a switch circuit  42 . In other words, the oscillator circuit  33  has the same configuration as the oscillator circuit  15 .  
         [0076]    A transistor comprising each inverter  41   a  of the periodic delay circuit  41  has a device parameter (for example, a channel width) different from that of the transistor comprising each inverter  21   a  of the periodic delay circuit  21  of FIG. 6. Then, the oscillator circuit  33  generates an oscillator signal having an oscillating frequency (i.e., the first frequency) different from the oscillating frequency (second frequency) of the oscillator circuit  15  (first frequency&gt;second frequency). Alternatively, the number of inverters of the periodic delay circuit  41  may be chosen to be different from the number of inverters in the periodic delay circuit  21  to generate oscillator signals at different oscillating frequencies.  
         [0077]    The switch control circuit  44  controls activation/deactivation of the periodic circuit  43 , and the periodic circuit  43  outputs the oscillator signal OSC from a node N 5 . Specifically, when the synthesizer  32  outputs an output signal having L-level (when the node N 6  falls to L-level), the inverter  42   a  of the switch circuit  42  is causing the periodic circuit  43  to operate. When the synthesizer  32  outputs an output signal having H-level (when the node N 6  rises to H-level), the inverter  42   a  is deactivated, causing the periodic circuit  43  to stop operating. Then, when the periodic circuit  43  is outputting the oscillator signal OSC having L-level, the oscillator signal OSC is clamped to L-level by the transistor Tr 23  which has been turned on. Also, when the periodic circuit  43  is outputting the oscillator signal OSC having H-level, the oscillator signal OSC is clamped to H-level by the transistor Tr 21  which has been turned on.  
         [0078]    In the foregoing manner, the incorrect pulse preventing circuit  14  generates the oscillator signal OSC in response to the detection signal DET-A or oscillator signal OSC-S.  
         [0079]    Specifically, when the synthesizer  32  is supplied with the detection signal DET-A having H-level (active mode), the transistors Tr 20 , Tr 22  are turned on, while the transistors Tr 21 , Tr 23  are turned off. Consequently, the oscillator circuit  33  generates the oscillator signal OSC having the first frequency.  
         [0080]    When the synthesizer  32  is supplied with the detection signal DET-A having L-level (in the standby mode), the oscillator circuit  33  intermittently operates in response to the pulse signal P 1 . The pulse generator circuit  31  is triggered by an edge of the oscillator signal OSC-S to generate the pulse signal P 1  which has H-level for a predetermined duration. This causes the synthesizer  32  to output a pulse signal having L-level (see FIG. 9( b )), so that the inverter  42   a  of the switch circuit  42  of the oscillator circuit  33  is activated.  
         [0081]    In this event, as shown in FIG. 9( b ), when the voltage of the oscillator signal OSC is, for example, at L-level, the voltage at an output terminal (node N 4 ) of the periodic delay circuit  41  is also at L-level. Therefore, when the inverter  42   a  is activated, the oscillator circuit  33  immediately outputs the oscillator signal OSC having H-level. The pulse signal at L-level from the synthesizer  32  rises to H-level before the inverted oscillator signal OSC is transmitted to the node N 4  (or before the voltage at the node N 4  is changed by the periodic delay circuit  41 ), to deactivate the inverter  42   a.  A NAND circuit  44   b  of the switch control circuit  44  outputs a signal having L-level in response to the oscillator signal OSC having H-level. The L-level signal turns on the transistor Tr 21  which clamps the oscillator signal OSC to H-level. As a result, the periodic circuit  43  of the oscillator circuit  33  operates in a half-period of the oscillator signal OSC-S having the second frequency.  
         [0082]    Subsequently, when the synthesizer  32  outputs a pulse signal having L-level in response to the pulse signal P 1  having H-level, the voltage at the output terminal (node N 4 ) of the periodic delay circuit  41  is at H-level because the oscillator signal OSC is clamped to H-level. Therefore, when the inverter  42   a  is activated, the oscillator circuit  33  immediately outputs the oscillator signal OSC having L-level. The L-level pulse signal from the synthesizer  32  rises to H-level before the inverted oscillator signal OSC is transmitted to the node N 4  (or before the voltage at the node N 4  is changed by the periodic delay circuit  41 ), to deactivate the inverter  42   a.  Then, a NOR circuit  44   c  outputs a signal having H-level in response to the oscillator signal OSC having L-level. The H-level signal turns on the transistor Tr 23  which clamps the oscillator signal OSC to L-level. As a result, the periodic circuit  43  in the oscillator circuit  33  operates in a half-period.  
         [0083]    As described above, the oscillator circuit  33  generates the oscillator signal OSC having a period synchronized to the period of the second oscillator signal OSC-S when the synthesizer  32  is supplied with the detection signal DET-A at L-level (standby mode).  
         [0084]    The periodic circuit  43  of the oscillator circuit  33  may operate in a complete period in response to the pulse signal P 1  from the pulse generator circuit  31 , rather than operates in a half-period.  
         [0085]    As illustrated in FIG. 8, the charge pump circuit  16  includes transistors Tr 24 , Tr 25 , each functioning as a diode; and a capacitor C 1 . The capacitor C 1  has an output terminal connected to a node N 7  between the transistors Tr 24  and Tr 25 .  
         [0086]    The capacitor C 1  is supplied with the oscillator signal OSC at its input terminal. Each of the transistors Tr 24 , Tr 25  is, for example, comprised of an NMOS transistor having its gate connected to its drain to form a MOS diode. The transistor Tr 24  has a drain connected to a high potential power supply VDD, and the boosted voltage VPP is output from a source of the transistor Tr 25 .  
         [0087]    When the oscillator signal OSC at L-level is supplied, the capacitor C 1  is charged by the high potential power supply VDD, resulting in the node N 7  set at a voltage smaller by a threshold value of the transistor Tr 24  than the voltage of the high potential power supply VDD.  
         [0088]    Subsequently, when the oscillator signal OSC at H-level is supplied, the voltage at the node N 7  is boosted by capacitive coupling of the capacitor C 1 . Then, when the voltage at the node N 7  becomes greater by a threshold value of the transistor Tr 25  than the boosted voltage VPP, a charge from the capacitor C 1  is supplied to boost the boosted voltage VPP.  
         [0089]    In this manner, the oscillator signal OSC at H-level or L-level is repeatedly supplied to generate the boosted voltage VPP.  
         [0090]    Next, the operation of the boosted power generator circuit  11  will be described with reference to FIGS.  9 ( a )- 9 ( c ).  
         [0091]    As shown in FIG. 9( a ), when the first detector circuit  12  outputs the detection signal DET-A at H-level indicative of the active mode, the voltage at the output terminal (node N 6 ) of the synthesizer  32  falls to L-level. Then, the oscillator circuit  33  generates the oscillator signal OSC having the first frequency. Subsequently, when the first detector circuit  12  outputs the detection signal DET-A at L-level to release the active mode, the voltage at the node N 6  rises to H-level, thereby deactivating the oscillator circuit  33 . At this time, the oscillator signal OSC is clamped to H-level.  
         [0092]    Next, as shown in FIG. 9( b ), when the first detector circuit  12  outputs the detection signal DET-A at L-level, and the oscillator circuit  15  supplies the oscillator signal OSC-S, a pulse at L-level is generated at the node N 6 . At this time, the pulse at L-level is generated in response to a pulse edge of the oscillator signal OSC-S having the second frequency. Therefore, the oscillator circuit  33  oscillates substantially at the same period as the second oscillator signal OSC-S.  
         [0093]    Next, description will be given for the case where the oscillator signal OSC-S is supplied in a short time period after the first detector circuit  12  outputs the detection signal DET-A at L-level (after releasing the active mode).  
         [0094]    As shown in FIG. 9( c ), assume that the oscillator circuit  33  is now outputting the oscillator signal OSC having the first frequency in response to the detection signal DET-A at H-level.  
         [0095]    Next, as the oscillator circuit  33  is supplied with the detection signal DET-A at L-level, the voltage at the node N 6  rises to H-level, thereby deactivating the oscillator circuit  33 . In this event, the oscillator signal OSC is clamped to L-level.  
         [0096]    When a pulse (H-level pulse) of the oscillator signal OSC-S is supplied immediately after switching to the detection signal DET-A at L-level, an L-level pulse is generated at the node N 6 , thereby activating the inverter  42   a  to release the oscillator signal OSC clamped to L-level. However, since the oscillator signal OSC at L-level is delayed by the periodic delay circuit  41  at that time, the voltage at the node N 4  is still maintained at H-level. For this reason, the oscillator circuit  33  outputs the oscillator signal OSC at L-level for a predetermined time period. Further, as shown in FIG. 9( c ), since the L-level pulse generated at the node N 6  disappears within a short period, the inverter  42   a  of the switch circuit  42  is deactivated before the voltage at the node N 4  falls down. In other words, the periodic circuit  43  stops its operation with the oscillator signal OSC maintained at L-level.  
         [0097]    With the foregoing operation, the oscillator circuit  33  will not generate the oscillator signal OSC which has a period shorter than the period of the first frequency. This applies also to the case where the detection signal at H-level is supplied immediately after the oscillator circuit  15  supplies the oscillator signal OSC-S.  
         [0098]    [0098]FIG. 10 is a graph showing the relationship between an input frequency and an output frequency of the incorrect pulse preventing circuit  14 . The incorrect pulse preventing circuit  14  receives the oscillator signal OSC-S, and stably generates the oscillator signal OSC having a frequency smaller than the first frequency.  
         [0099]    The boosted power generator circuit  11  according to the first embodiment has the following advantages.  
         [0100]    (1) The oscillator circuit  33  of the incorrect pulse preventing circuit  14  generates the oscillator signal OSC having the first frequency when the synthesizer  32  is supplied with the detection signal DET-A in the active mode. The oscillator circuit  33  generates the oscillator signal OSC having the same period as the period of the oscillator signal OSC-S having the second frequency in response to the oscillator signal OSC-S supplied from the oscillator circuit  15  through the pulse generator circuit  31  in the standby mode. Therefore, no oscillator signal OSC having a period (pulse width) shorter than the period of the first frequency will be output from the incorrect pulse preventing circuit  14 . Consequently, the charge pump circuit  16  provides a stable charge pump operation, resulting in stabilization of the boosted voltage VPP and efficient supply thereof.  
         [0101]    (2) Even when the mode is switched, or when a change in the detection signal DET-A overlaps a change in the oscillator signal OSC-S, the incorrect pulse preventing circuit will not generate the oscillator signal OSC having a pulse width shorter than the period of the oscillator signal having the first frequency.  
         [0102]    (3) Since a single charge pump is connected to the oscillator circuits  15 ,  33 , an increase in the semiconductor die size is prevented.  
         [0103]    In the following, a boosted power generator circuit  81  according to a second embodiment of the present invention will be described with reference to FIGS. 11 and 12. The boosted power generator circuit  81  includes first and second detector circuits  12 ,  13 ; an incorrect pulse preventing circuit  82 ; oscillator circuits  15 ,  83 ; and a charge pump circuit  16 . In the second embodiment, the oscillator circuit  83  supplies the incorrect pulse preventing circuit  82  with an oscillator signal OSC-A (a first control signal IN 1 ) in response to a detection signal DET-A of the first detector circuit  12 .  
         [0104]    The oscillator circuit  83  has the same configuration as the oscillator circuit  15 , so that detailed description thereof is omitted. The oscillator circuit  83  generates an oscillator signal having a third frequency smaller than the first frequency of the oscillator circuit  33 . Also, the third frequency is greater than the second frequency of the oscillator circuit  15 . Expressed in another way, first frequency&gt;third frequency&gt;second frequency is established.  
         [0105]    As illustrated in FIG. 12, the incorrect pulse preventing circuit  82  includes a first and a second pulse generator circuit  31   a,    31   b;  a synthesizer  32 ; a periodic circuit  43 ; and a switch control circuit  44 . Each of the first and second pulse generator circuits  31   a,    31   b  has the same configuration as the pulse generator circuit  31  in FIG. 7. The synthesizer  32  is supplied with a pulse signal output from the first pulse generator  31   a  in response to the oscillator signal OSC-A (the first control signal IN 1 ), and with a pulse signal output from the second pulse generator circuit  31   b  in response to an oscillator signal OSC-S (a second control signal IN 2 ).  
         [0106]    Next, the operation of the boosted power generator circuit  81  will be described. The first detector circuit  12  supplies the third oscillator circuit  83  with the detection signal DET-A having H-level when a divided voltage of the boosted voltage VPP is equal to or smaller than a reference voltage Vref in the active mode. The third oscillator circuit  83  generates the oscillator signal OSC-A having the third frequency in response to the detection signal DET-A at H-level.  
         [0107]    The synthesizer  32  of the incorrect pulse preventing circuit  82  is supplied with the oscillator signal OSC-A through the first pulse generator circuit  31   a,  and the oscillator circuit  33  oscillates at the same frequency as the third frequency. Therefore, the incorrect pulse preventing circuit  82  outputs the oscillator signal having the same frequency as the third frequency.  
         [0108]    The charge pump circuit  16  performs a boost operation in accordance with the oscillator signal OSC to generate a boosted voltage VPP. When a divided voltage of the boosted voltage VPP rises to a voltage equal to or grater than the reference voltage Vref, the first detector circuit  12  outputs the detection signal DET-A having L-level, thereby deactivating the third oscillator circuit  83  and incorrect pulse preventing circuit  82 . Consequently, the charge pump circuit  16  stops its operation.  
         [0109]    The second detector circuit  13  supplies the oscillator circuit  15  with a detection signal DET-S having H-level, when the divided voltage of the boosted voltage VPP is equal to or smaller than the reference voltage Vref. The oscillator circuit  15  generates the oscillator signal OSC-S having the second frequency in response to the detection signal DET-S at H-level.  
         [0110]    The synthesizer  32  is supplied with the oscillator signal OSC-S through the second pulse generator circuit  31   b,  and the oscillator circuit  33  oscillates at the same frequency as the second frequency. Therefore, the incorrect pulse preventing circuit  82  outputs the oscillator signal having the same frequency as the second frequency.  
         [0111]    The charge pump circuit  16  performs a boost operation in accordance with the oscillator signal OSC to generate the boosted voltage VPP. The second detector circuit  13  outputs the detection signal DET-S having L-level when the divided voltage of the boosted voltage VPP rises to a voltage equal to or greater than the reference voltage Vref, thereby deactivating the oscillator circuit  15  and incorrect pulse preventing circuit  82 . Therefore, the charge pump circuit  16  stops its operation.  
         [0112]    In the second embodiment, the frequency of the oscillator signal OSC of the incorrect pulse preventing circuit  82  is switched each time the first and second control signals IN 1 , IN 2  (oscillator signals OSC-A, OSC-S) are switched. Therefore, the boosted power generator circuit  81  of the second embodiment has the same advantages as the boosted power generator circuit  11  of the first embodiment.  
         [0113]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
         [0114]    The periodic delay circuit  41  of the periodic circuit  43  may be comprised of an even number of inverters  41   a  such as two, six and the like.  
         [0115]    Oscillator signals having different oscillating frequencies may be generated by providing a number of inverters comprising the periodic delay circuit  41  of the oscillator circuit  33  different from the number of inverters comprising the periodic delay circuit  21  of the oscillator circuit  15 .  
         [0116]    The present invention may be applied to memory devices such as SDRAM and the like. In this case, the boosted power generator circuit may be supplied with a system clock as the first control signal IN 1  and with a self-refresh request signal as the second control signal IN 2 .  
         [0117]    The present invention may be applied to a negative power generator circuit as well as the boosted power generator circuits  11 ,  81 .  
         [0118]    Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.