Abstract:
The disclosed invention provides a semiconductor device capable of suitably controlling the level of an enable signal to resolve NBTI in a PMOS transistor. An input node receives an input signal alternating between high and low levels during normal operation and fixed to a high level during standby. A detection unit receives a signal through the input node and outputs an enable signal. The detection unit sets the enable signal to a low level upon detecting that the input node remains at a high level for a predetermined period. A signal transmission unit includes a P-channel MOS transistor and transmits a signal input to the input node according to control by the enable signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Divisional Application of U.S. Ser. No. 13/769,000 filed Feb. 15, 2013, which claims priority from Japanese Patent Application No. 2012-032671 filed on Feb. 17, 2012, the subject matter of each is incorporated herein by reference in entirety. 
    
    
     BACKGROUND 
     The present invention relates to a semiconductor device. 
     It is known that a waveform of pulses that are transmitted sequentially via logic circuits including P-channel MOS transistors is deteriorated by NBTI (Negative Bias Temperature Instability). 
     NBTI means that a threshold voltage of a PMOS transistor varies by continuous application of a negative gate bias to the PMOS transistor. 
     For example, in Patent Document 1 (Japanese Published Unexamined Patent Application No. 2006-33058), an embodiment is described in which a two-input NAND circuit is applied for clock gating. This embodiment sets forth that one input of the two-input NAND circuit is fixed to H and that a PMOS influenced by NBTI in the NAND circuit is divided into two parts to distribute the NBTI influence. 
     A semiconductor device described in Patent Document 2 (Japanese Published Unexamined Patent Application No. 2006-74746) includes a first semiconductor integrated circuit that has a predefined function and outputs a required output signal and a second semiconductor integrated circuit having a plurality of MOS elements that switch between conducting and non-conducting states independently of each other, in response to a plurality of gate signals with shifted timing, the MOS elements being coupled in parallel to the output or input of the first semiconductor integrated circuit. This semiconductor device further includes a pulse generating circuit that generates and outputs the gate signals with shifted timing to the MOS elements in the second semiconductor integrated circuit. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     [Patent Document 1] Japanese Published Unexamined Patent Application No. 2006-33058 
     [Patent Document 2] Japanese Published Unexamined Patent Application No. 2006-74746 
     SUMMARY 
     By the way, such a problem in a serial interface circuit is known that, when a serial clock stops, a negative gate bias is continuously applied to a PMOS transistor. To address this, it is possible to use a circuit scheme in which NAND circuits are cascade coupled and a clock signal is input to one input thereof and an enable signal is applied to the other input thereof, as described in Patent Document 1 and Patent Document 2. 
     However, there is no description about how to control the level of the enable signal to resolve NBTI in a PMOS transistor. 
     A semiconductor device according to an embodiment of the present invention includes: an input node that receives an input signal alternating between high and low levels during normal operation and fixed to a high level during standby; a detection unit that sets an enable signal to a low level upon detecting that the input node remains at a high level for a predetermined period; and a signal transmission unit that includes P-channel MOS transistors and transmits a signal input to the input node according to control by the enable signal. 
     The semiconductor device according to an embodiment of the present invention is capable of suitably setting the level of the enable signal to resolve NBTI in a PMOS transistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a structure of a semiconductor device according to an embodiment of the present invention. 
         FIG. 2  is a diagram showing a structure of a digital circuit included in a serial interface circuit SCIO according to a first embodiment. 
         FIG. 3  is a diagram showing a reference example of a digital circuit. 
         FIG. 4  is a diagram showing a structure of a NAND circuit  11 _ 1  shown in  FIG. 3 . 
         FIG. 5  is a timing chart for explaining the operation of the first embodiment. 
         FIG. 6  is a diagram showing a structure of a digital circuit included in a serial interface circuit SCIO according to a second embodiment. 
         FIG. 7  is a timing chart for explaining the operation of the second embodiment. 
         FIG. 8  is a diagram showing a structure of a digital circuit included in a serial interface circuit SCIO according to a third embodiment. 
         FIG. 9  is a timing chart for explaining the operation of the third embodiment. 
         FIG. 10  is a diagram showing a modification example of a reference voltage generating circuit. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a diagram showing a structure of a semiconductor device according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the semiconductor device  200  includes a central processing unit CPU, a memory  52 , a bus  51  that transfers data and addresses, a data transfer unit (direct memory access controller) DMAC, an analog-digital converter ADC, an interrupt controller INTC, a serial interface circuit SCIO, a system controller SYSC, a clock circuit  56 , a power supply circuit  54 , and a voltage detecting unit  55 . 
     The memory  52  includes a flash memory  53 , a ROM (Read Only Memory)  60 , and a RAM (Random Access Memory)  61 . The memory  52  stores data and programs. 
     The central processing unit CPU sequentially executes programs stored in the memory  52  and controls operation of the semiconductor device  200  as a whole. 
     The serial interface circuit SCIO stores externally input data into the memory  52 . The serial interface circuit SCIO receives a serial clock SCLK and serial data SDATA from outside. 
     The analog-digital converter ADC converts an externally input analog signal to a digital value and stores it into the memory  52 . 
     The data transfer unit DMAC controls data transfer via the bus  51 , when storing digital data from the serial interface circuit SCIO or the analog-digital converter ADC into the memory  52 . 
     The interrupt controller INTC receives an interrupt signal issued externally or by an internal functional unit and generates an interrupt to the central processing unit CPU. The central processing unit CPU performs a processing task requested by the interrupt. 
     The clock circuit  56  includes a plurality of clock sources. The clock circuit  56  generates a system clock SYSCLK and supplies it to each functional unit of the semiconductor device  200 . 
     The power supply circuit  54  steps down an external supply voltage VCC, generates an internal operating voltage VDD or the like, and supplies it to each component in the semiconductor device  200 . 
     The voltage detecting unit  55  has a power-on reset circuit POR that generates a reset signal triggering a power-on reset action in response to a voltage change of the external supply voltage VCC and a voltage drop detecting circuit LVD that generates an interrupt signal or reset signal in response to a voltage drop of the external supply voltage VCC. 
     The system controller SYSC controls operation of the semiconductor device  200  as a whole. The system controller SYSC controls supplying a clock and a power supply voltage to each of the functional blocks (that is, load circuits) including the CPU in the semiconductor device  200 . 
       FIG. 2  is a diagram showing a structure of a digital circuit included in the serial interface circuit SCIO according to the first embodiment.  FIG. 3  is a diagram showing a reference example of a digital circuit. First, the reference example of  FIG. 3  is described. 
     Referring to  FIG. 3 , a signal transmission unit  91  includes a plurality of stages of inverters IV 1  to IV 4  and sequentially delays a serial clock SCLK. A data path unit  189  includes a plurality of stages of flip-flops  13 _ 1  to  13 _ 2  and transmits serial data SDATA. 
     A signal that is output from an inverter IV 1  is input to a clock terminal of a flip-flop  13 _ 1 . A signal that is output from an inverter IV 2  is input to a clock terminal of a flip-flop  13 _ 2 . 
     During standby, when the serial clock SCLK remains at an “H” level for a long time, NBTI occurs in a P-channel MOS transistor P 11 . Specifically, the threshold voltage of the P-channel MOS transistor P 11  rises. The NBTI occurring in the P-channel MOS transistor P 11  deteriorates the duty ratio of the clock that is output from the inverter IV 2 . In consequence, the clock having a deteriorated duty ratio will be transmitted to the following stage. 
     If NBTI occurs in a P-channel MOS transistor P 14 , it further deteriorates the duty of the clock that is output from an inverter IV 4 . In consequence, the clock having a further deteriorated duty ratio will be transmitted to the following stage. 
     In this way, in the signal transmission unit  91  configured with a plurality of stages of inverters, the duty of the clock that is output from an inverter placed in a later stage is more deteriorated. Inconsequence, in the data path unit  189 , a flip-flop at a later stage undergoes a decrease in a setup margin. 
     Referring to  FIG. 2 , this digital circuit includes an input node IN, a detection unit  10 , a signal transmission unit  78 , and a data path unit  14 . 
     The data path unit  14  outputs serial data DATA to outside in accordance with a clock that is output from the signal transmission unit  78 . The data path unit  14  includes a plurality of stages of flip-flops  13 _ 1  to  13 _N and transmits serial data SDATA. 
     The input node IN receives a serial clock SCLK as an input signal. The serial clock SCLK is fixed to an “H” level during standby. 
     The signal transmission unit  78  includes a plurality of stages of NAND circuits  11 _ 1  to  11 _N and sequentially delays and transmits the serial clock SCLK in accordance with control by an enable signal EN that is output from the detection unit  10 . 
     One input terminal of a NAND circuit  11 _ 1  is coupled to the input node IN. The other input terminal of the NAND circuit  11 _ 1  receives the enable signal EN. When the enable signal EN is at an “L” level, the NAND circuit  11 _ 1  always outputs an “H” level irrespective of the level of the serial clock SCLK. 
     One input terminal of a NAND circuit  11 _ 2  is coupled to the output of the NAND circuit  11 _ 1 . The other input terminal of the NAND circuit  11 _ 2  receives the enable signal EN. When the enable signal EN is at an “L” level, the NAND circuit  11 _ 2  always outputs an “H” level irrespective of the level of the serial clock SCLK. 
     NAND circuits  11 _ 3  to  11 _N also operate in the same way as above.  FIG. 4  is a diagram showing a structure of a NAND circuit  11 _ 1  in  FIG. 3 . NAND circuits  11 _ 2  to  11 _N also have the same structure as shown here. 
     The NAND circuit includes P-channel MOS transistors P 1 , P 2 , and N-channel MOS transistors N 1 , N 2 . 
     An N-channel MOS transistor N 2  and a P-channel MOS transistor P 2  receive the serial clock SCLK. NBTI does not occur in the P-channel MOS transistor P 2 , because the level of the serial clock SCLK during standby is “H” level. 
     In the NAND circuits  11 _ 2  to  11 _N as well, because the signal that is output from a NAND circuit at the preceding stage is “H” level, NBTI does not occur in the P-channel MOS transistor P 2  in the NAND circuits  11 _ 2  to  11 _N. 
     An N-channel MOS transistor N 1  and a P-channel MOS transistor P 1  receive the enable signal EN. If the enable signal EN changes to an “L” level during standby, NBTI occurs in the P-channel MOS transistor P 1 . However, when the signal is transmitted, that is, when the level of the serial clock SCLK changes, the P-channel MOS transistor P 1  does not operate and, therefore, does not become a factor of deteriorating the waveform. 
     Referring to  FIG. 2  again, the detection unit  10  receives the serial clock SCLK from the input node IN. When the detection unit  10  detects that the serial clock SCLK is at an “H” level for a predetermined period, it sets the enable signal EN to an L level. 
     The detection unit  10  is configured with a shift register that performs a shift operation based on the system clock SYSCLK. By its overflow, the shift register sets the enable signal EN to an “L” level. The shift register resets the shift operation when the serial clock SYSCLK level has changed to “L”. 
     Specifically, the detection unit  10  includes an inverter  31  coupled to the input node IN and a plurality of stages of D type flip-flops  12 _ 1  to  12 _M. The above predetermined period is the frequency of the system clock SYSCLK×the number M of the D type flip-flops  12 _ 1  to  12 _M. 
     A set terminal of each of the D type flip-flops  12 _ 1  to  12 _M is coupled to the output of the inverter  31  and its level changes depending on the serial clock SCLK. 
     The system clock SYSCLK is input to a clock terminal of each of the D type flip-flops  12 _ 1  to  12 _M. 
     An input terminal of a D type flip-flop  12 _ 1  at a first stage receives a signal fixed to an “L” level. An input terminal of D type flip-flops  12 _ 2  to  12 _M at second and subsequent stages receives an output of a D type flip-flop at the preceding stage. From a D type flip-flop  12 _M at the last stage, the enable signal EN is output. 
       FIG. 5  is a timing chart for explaining the operation of the first embodiment. During normal operation, as the level of the serial clock SCLK cyclically changes between “H” and “L” levels, the level of the set terminal Set of each of the D type flip-flops  12 _ 1  to  12 _M iteratively changes between “H” and “L” levels. In consequence, the enable signal EN remains at an “H” level. On the other hand, as the level of the serial clock SCLK cyclically changes between “H” and “L” levels, the level of one input terminal of each of the stages of NAND circuits  11 _ 1  to  11 _N in the signal transmission unit  91  iteratively changes between “H” and “L” levels. In consequence, NBTI does not occur in the P-channel MOS transistor P 2  comprised in each of the NAND circuits  11 _ 1  to  11 _N. 
     During standby, when the serial clock SCLK is fixed to an “H” level, the level of the set terminal Set of each of the D type flip-flops  12 _ 1  to  12 _M remains at an “L” level. When the level of the set terminal Set remains at the “L” level for a predetermined time, the enable signal EN changes to an “L” level. In consequence, the level of the other input terminal of each of the stages of NAND circuits  11 _ 1  to  11 _N in the signal transmission unit  91  changes to an “L” level. Consequently, all the stages of NAND circuits  11 _ 1  to  11 _N output an “H” level, so that NBTI does not occur in the P-channel MOS transistor P 2  comprised in each of the NAND circuits  11 _ 1  to  11 _N. 
     As above, according to the present embodiment, the signal transmission unit is comprised of the stages of NAND circuits and the other input terminal of each of the NAND circuits is to change to the “L” level when a period in which the serial clock SCLK remains at the “H” level has exceeded the predetermined time. Thereby, it is possible to avoid NBTI in the PMOS transistors for signal transmission comprised in the NAND circuits. 
     Besides, because the system clock SYSCLK that is input to the detection unit is allowed to be adequately slower than the serial clock SCLK, power consumed by the detection unit can be reduced. 
     Second Embodiment 
       FIG. 6  is a diagram showing a structure of a digital circuit included in a serial interface circuit SCIO according to a second embodiment. 
     A point of difference of this digital circuit from the digital circuit of the first embodiment is a detection unit  110 . This detection unit  110 , in this embodiment, is configured with a time constant circuit including a constant current source and a capacitor. 
     Specifically, the detection unit  110  includes a charging unit  89  in which an amount of charge stored therein varies depending on a duty ratio of the serial clock SCLK and an output unit  88  that sets the level of the enable signal EN based on whether or not an amount of charge stored in the charging unit  89  exceeds a predetermined threshold value. 
     The charging unit  89  includes an inverter  22 , a constant current source  21 , an NMOS transistor N 3 , and a capacitive element  24 . 
     The inverter  22  receives the serial clock SCLK. The constant current source  21  supplies current to a node ND 1 . The NMOS transistor N 3  is installed between the node ND 1  and ground, receives the serial clock SCLK at its gate, and ON/OFF controlled in response to the serial clock SCLK. 
     The capacitive element  24  is installed between the node ND 1  and ground. The output unit  88  includes an inverter  23  that receives a voltage of the node ND 1 . The inverter  23  outputs an L-level enable signal EN, if the voltage of the node ND 1  exceeds a logical threshold voltage of the inverter. The inverter outputs an H-level enable signal EN, if the voltage of the node ND 1  is equal to or less than the logical threshold voltage of the inverter  23 . 
       FIG. 7  is a timing chart for explaining the operation of the second embodiment. During normal operation, as the level of the serial clock SCLK cyclically changes between “H” and “L” levels, the N-channel MOS transistor N 3  is cyclically switched between ON and OFF. Thereby, the capacitive element  24  is charged and discharged iteratively, so that an increase in the potential of the node ND 1  does not rise to an observable level. Thus, the potential of the node ND 1  does not exceed the logical threshold voltage Vth of the inverter  23  and the enable signal EN is at an “H” level. 
     During standby, when the serial clock SCLK is fixed to an “H” level, the N-channel MOS transistor is turned off by the inverter  22 . Inconsequence, current that is output from the constant current source  21  flows via the node ND 1  into the capacitive element  24  and the capacitive element  24  is charged. As the capacitive element  24  is charged continuously, the potential of the node ND  1  increases. When a period in which the serial clock SCLK is fixed to the “H” level exceeds a predetermined time and when the potential of the node ND 1  has exceeded the logical threshold voltage Vth of the inverter  23 , the enable signal changes to an “L” level. 
     As above, in the present embodiment, the other input terminal of each of the NAND circuits changes to the “L” level when a period in which the serial clock SCLK remains at the “H” level has exceeded the predetermined time, as is the case for the first embodiment, and it is possible to avoid NBTI in the PMOS transistors for signal transmission comprised in the NAND circuits. 
     Besides, because no system clock SYSCLK is required in the present embodiment, it becomes possible to further reduce power consumption. It may be expedient to provide both the detection unit  10  of the first embodiment and the detection unit  110  of the second embodiment. In this case, an OR circuit may be provided that outputs a logical sum of an enable signal EN which is output from the detection unit  10  of the first embodiment and an enable signal which is output from the detection unit  110  of the second embodiment and an output of the OR circuit may be supplied to the signal transmission unit. 
     Third Embodiment 
       FIG. 8  is a diagram showing a structure of a digital circuit included in a serial interface circuit SCIO according to a third embodiment. 
     This digital circuit differs from the digital circuit of the first embodiment in two points: a correction circuit  41  that it includes additionally and a signal transmission unit  92 . 
     The signal transmission unit  92  is comprised of a plurality of stages of inverters IV 1  to IVN to transmit a serial clock SCLK input to the input node IN, as described with regard to the reference example for the first embodiment. As described with regard to the reference example for the first embodiment, when the serial clock SCLK remains at an “H” level for a long time, an “L” level voltage is applied for a long time to P-channel MOS transistors P 11 , P 13 , . . . at even stages and NBTI occurs therein. 
     It is the correction circuit  41  to solve this problem. The correction circuit  41  controls the back gates of the P-channel MOS transistors P 11 , P 13 , . . . , in which NBTI occurs, depending on the duty ratio a signal that is output from an inverter IVN at the last stage. Through this control, the correction circuit  41  controls the threshold voltages of the P-channel MOS transistors P 11 , P 13 , . . . comprised in the inverters at even stages. 
     Specifically, the correction circuit  41  is configured with a reference voltage generating circuit  47  comprised of resistive dividers, a filter  83 , and a differential amplifier  46  whose inputs are coupled to the filter  83  and the reference voltage generating circuit  47  and whose output is coupled to the back gates of the P-channel MOS transistors P 11 , P 13 , . . . comprised in the inverters at even stages. 
     The reference voltage generating circuit  47  includes resistors R 12  and R 13  coupled in series between a VDD power supply terminal and ground. A reference voltage Vref (=VDD/2) of a node ND 2  between the resistors R 12  and R 13  is input to a negative input terminal of the differential amplifier  46 . 
     The filter  83  includes a resistor R 11  installed between a node ND 5  and the output of the inverter IVN at the last stage in the signal transmission unit  92  and a capacitive element C 11  installed between the node ND 5  and ground. By the filter  83 , the node ND  5  has a voltage produced by integrating the output voltage OUT of the inverter IVN at the last stage. The potential of the node ND 5  is input to a positive input terminal of the differential amplifier  46 . 
     The differential amplifier  46  amplifies a difference between the voltage of the node ND 5  and the reference voltage Vref of the node ND 2  and applies an amplified voltage to the back gates of the PMOS transistors P 11 , P 13 , etc. That is, the differential amplifier  46  controls the voltage that is applied to the back gates of the PMOS transistors P 11 , P 13 , . . . so that the voltage of the node ND 5  and the reference voltage Vref become equal. 
     An unbalanced duty ratio of the serial clock SCLK makes the voltage of the node ND 5  smaller than the reference voltage Vref. This results in a decrease in the voltage of the output gate ND 3  of the differential amplifier  46 , coupled to the back gates of the PMOS transistors P 11 , P 13 , . . . . This in turn decreases the threshold voltages Vth, which increased due to NBTI, of the PMOS transistors P 11 , P 13 , . . . comprised in the inverters at even stages. 
       FIG. 9  is a timing chart for explaining the operation of the third embodiment. During standby, when the serial clock SCLK remains at an “L” level for a long time, an “L” level voltage is applied for a long time to the P-channel MOS transistors P 11 , P 13 , . . . comprised in the inverters at even stages and NBTI occurs therein. As a result, this deteriorates the duty ratio of the voltage OUT of the inverter IVN. At this time, the voltage of the node ND 5  becomes smaller than the reference voltage Vref (VDD/2). This results in a decrease in the potential of the output node ND 3  of the differential amplifier  46 . Because the node ND 3  is coupled to the back gates of the PMOS transistors P 11 , P 13 , . . . , this in turn decreases the threshold voltages Vth of the PMOS transistors P 11 , P 13 , . . . . In consequence, the threshold voltages Vth of the PMOS transistors P 11 , P 13 , . . . , which increased due to NBTI, can recover. 
     As above, in the present embodiment, in a case in which the threshold voltages of PMOS transistors comprised in the inverters in the signal transmission unit increase and NBTI should occur therein during standby, it is possible to stop NBTI from occurring by decreasing the threshold voltages of these PMOS transistors. 
     In the present embodiment, the reference voltage generating circuit  47  of a resistive divider type is used, but this is not to be regarded as limiting; it may be of a type that generates a local threshold voltage, as shown in  FIG. 10 . 
     In the present embodiment, because the serial clock SCLK is at an “H” level during standby, the correction circuit was arranged to control the back gate voltages of PMOS transistors comprised in inverters at even stages among the stages of inverters, depending on a duty ratio of a signal that is output from an inverter at the last stage. If the serial clock SCLK is at an “L” level during standby, the correction circuit can be arranged to control the back gate voltages of PMOS transistors comprised in inverters at odd stages among the stages of inverters, depending on a duty ratio of a signal that is output from an inverter at the last stage. 
     Although the signal transmission unit in the present embodiment is comprised of a plurality of stages of inverters, the signal transmission unit may be comprised of a plurality of stages of NAND circuits, as is the case for the first and second embodiments. 
     The embodiments disclosed herein should be considered as illustrative in all respects, rather than restrictive. The scope of the present invention is indicated by the appended claims, rather than by the foregoing descriptions, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.