Abstract:
A latch circuit includes a first circuit including an N-MOS transistor having a first electrode receiving a signal, a second electrode outputting the signal, a gate electrode, and a P-well, and a first inverter including input and output terminals. The second electrode of the N-MOS transistor is electrically connected to the input terminal of the first inverter, and the gate electrode of the N-MOS transistor is electrically connected to the P-well of the N-MOS transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a D-type latch circuit and a D-type flip-flop circuit that are composed of MOS field effect transistors. 
     2. Description of the Related Art 
     FIG. 6 illustrates a configuration of a conventional dynamic D-type flip-flop circuit  30 . 
     The dynamic D-type flip-flop circuit  30  includes a master latch  31  and a slave latch  32 . The master latch  31  includes a transfer gate  23  and an inverter  24 . The slave latch  32  includes a transfer gate  25  and an inverter  26 . 
     FIG. 7 illustrates a configuration of the transfer gates  23  and  25  shown in FIG.  6 . The transfer gate shown in FIG. 7 includes a p-channel MOS field effect transistor  27  (hereinafter referred to as P-MOS transistor) and an n-channel MOS field effect transistor  28  (hereinafter referred to as N-MOS transistor). The source and drain of the P-MOS transistor  27  are connected to the source and drain of the N-MOS transistor  28 . 
     Referring to FIG. 7, when an input signal S is at a high level (hereinafter referred to as H level), an inverse input signal −S of the input signal S is at a low level (hereinafter referred to as L level). In this case, the P-MOS transistor  27  and the N-MOS transistor  28  are in the open state, and thus a signal A input to the transfer gate is output as a signal Y from the transfer gate. 
     When the input signal S is at the L level, the inverse input signal −S is at the high level. In this case, the P-MOS transistor  27  and the N-MOS transistor  28  are in the closed state, and thus the signal A input to the transfer gate is not output from the transfer gate. 
     FIG. 8 illustrates a configuration of an inverter. The inverter shown in FIG. 8 includes a P-MOS transistor  29  and an N-MOS transistor  30 . The gate of the P-MOS transistor  29  is connected to the gate of the N-MOS transistor  30 . The source of the P-MOS transistor  29  is connected to a power source V DD . The source of the N-MOS transistor  30  is connected to a ground GND. The drain of the P-MOS transistor  29  is connected to the drain of the N-MOS transistor  30 . 
     The dynamic D-type flip-flop circuit  30  shown in FIG. 6 receives clock signals BCK and −BCK. FIG. 9 illustrates a clock generating circuit for generating the clock signals BCK and −BCK. The clock generating circuit shown in FIG. 9 includes inverters  51  and  52 . The clock generating circuit shown in FIG. 9 generates the clock signals BCK and −BCK from a clock signal CK. 
     FIG. 10 illustrates a configuration of a conventional static D-type flip-flop circuit  60 . The static D-type flip-flop circuit  60  includes a master latch  61  and a slave latch  62 . The master latch  61  includes transfer gates  35  and  38 , and inverters  36  and  37 . The slave latch  62  includes transfer gates  39  and  42 , and Inverters  40  and  41 . The transfer gates  35 ,  38 ,  39 , and  42  have the same configuration as that shown in FIG.  7 . 
     The conventional static D-type flip-flop circuit  60  is the same operation as that of the dynamic D-type flip-flop circuit  30  shown in FIG.  6 . However, for example, when the transfer gate  35  of the static D-type flip-flop circuit  60  is in the closed state, the transfer gate  38  is in the open state while holding a signal, which has been input to the transfer gate  35  in the immediately previous open state, in a circuit of the transfer gate  38  and the inverters  36  and  37 . Therefore, even when the transfer gate  35  is in the closed state, the signal which has been input to the transfer gate  35  in the immediately previous open state is output from the master latch  61 . The same applies to the slave latch  62 . 
     The conventional flip-flop circuits  30  and  60  require the clock signals BCK and −BCK having reversed polarities. In order to obtain the clock signals BCK and −BCK using the clock signal CK, the clock generating circuit shown in FIG. 9, i.e., the inverter, is necessary. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention, a latch circuit includes a first circuit including an N-MOS transistor having a first electrode receiving a signal, a second electrode outputting the signal, a gate electrode, and a P-well, and a first inverter including input and output terminals. The second electrode of the N-MOS transistor is electrically connected to the input terminal of the first inverter, and the gate electrode of the N-MOS transistor is electrically connected to the P-well of the N-MOS transistor. 
     According to another aspect of this invention, a latch circuit includes a P-MOS transistor having a first electrode receiving a signal, a second electrode outputting the signal, a gate electrode, and an N-well, and a first inverter including input and output terminals. The second electrode of the P-MOS transistor is electrically connected to the input terminal of the first inverter, and the gate electrode of the P-MOS transistor is electrically connected to the N-well of the P-MOS transistor. 
     In one embodiment of the present invention, a latch circuit further includes a second circuit including a P-MOS transistor having a first electrode receiving a signal, a second electrode outputting the signal, a gate electrode, and an N-well, and a second inverter including input and output terminals. The second electrode of the P-MOS transistor is electrically connected to the input terminal of the second inverter, and the gate electrode of the P-MOS transistor is electrically connected to the N-well of the P-MOS transistor. 
     In one embodiment of the present invention, the first circuit serves as a master latch and the second circuit serves as a slave latch, a clock signal having a first or second level is input to the first and second circuits, the first circuit is in the open state when receiving the clock signal having the first level, the first circuit is in the closed state when receiving the clock signal having the second level, the second circuit is in the closed state when receiving the clock signal having the first level, and the second circuit is in the open state when receiving the clock signal having the second level. 
     In one embodiment of the present invention, the first circuit serves as a slave latch and the second circuit serves as a master latch, a clock signal having a first or second level is input to the first and second circuits, the first circuit is in the open state when receiving the clock signal having the first level, the first circuit is in the closed state when receiving the clock signal having the second level, the second circuit is in the closed state when receiving the clock signal having the first level, and the second circuit is in the open state when receiving the clock signal having the second level. 
     In one embodiment of the present invention, the first inverter includes an N-MOS transistor having a P-well and a gate electrode, and a P-MOS transistor having an N-well and a gate electrode. 
     In one embodiment of the present invention, the second inverter includes an N-MOS transistor having a P-well and a gate electrode, and a P-MOS transistor having an N-well and a gate electrode. 
     In one embodiment of the present invention, the absolute value of a threshold voltage of the N-MOS transistor included in the first circuit is smaller than the absolute value of a threshold voltage of the P-MOS transistor included in the first inverter. 
     In one embodiment of the present invention, the absolute value of a threshold voltage of the P-MOS transistor included in the second circuit is smaller than the absolute value of a threshold voltage of the N-MOS transistor included in the second inverter. 
     In one embodiment of the present invention, the first inverter includes an N-MOS transistor having a P-well and a gate electrode, and a P-MOS transistor having an N-well and a gate electrode, the second inverter includes an N-MOS transistor having a P-well and a gate electrode, and a P-MOS transistor having an N-well and a gate electrode, and the P-wells of the N-MOS transistors included in the first and second inverters are electrically connected to the gate electrodes of the N-MOS transistor included in the first and second inverters, respectively, and the N-wells of the P-MOS transistors included in the first and second inverters are electrically connected to the gate electrodes of the P-MOS transistor included in the first and second inverters, respectively. 
     In one embodiment of the present invention, the clock signal has one phase and one polarity. 
     Hereinafter, functions of the present invention will be described. 
     In the latch circuit according to the present invention, the gate electrode and P-well of the N-MOS transistor are electrically connected to each other. When such an N-MOS transistor is in the closed state, it has the same threshold voltage as that of a general N-MOS transistor. When the N-MOS transistor is in the open state, it has a smaller threshold voltage than that of the general N-MOS transistor. Therefore, the absolute value of the threshold voltage of the N-MOS transistor serving as a transfer gate in the open state is smaller than the absolute value of the threshold voltage of the P-MOS transistor included in the inverter positioned in a stage following the transfer gate. Therefore, the voltage of an output signal having the H level from the N-MOS transistor operating as the transfer gate is sufficiently prevented from decreasing. 
     As a result, a current passing across the inverter connected to a stage following the N-MOS transistor is substantially suppressed. In the latch circuit according to the present invention, power consumption can be largely reduced. 
     In another latch circuit according to the present invention, the gate electrode and N-well of the P-MOS transistor are electrically connected to each other. When such a P-MOS transistor is in the closed state, it has the same threshold voltage as that of a general P-MOS transistor. When the P-MOS transistor wherein the gate electrode and N-well thereof are connected to each other is in the open state, it has a smaller threshold voltage than that of the general P-MOS transistor. Therefore, the absolute value of the threshold voltage of the P-MOS transistor serving as a transfer gate in the open state is smaller than the absolute value of the threshold voltage of the N-MOS transistor included in the inverter positioned in a stage following the transfer gate. Therefore, the voltage of an output signal having the L level from the P-MOS transistor operating as a transfer gate is sufficiently prevented from increasing. As a result, power consumption caused by a current passing across the inverter is largely reduced. Moreover, when the P-MOS transistor is in the closed state, there is substantially no leak current therein. 
     Thus, the invention described herein makes possible the advantages of providing a latch circuit and a flip-flop circuit which include a small number of elements and require low power consumption. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating a dynamic flip-flop circuit according to Example 1 of the present invention. 
     FIG. 2 is a diagram illustrating a symbol representing a P-DTMOS transistor shown in FIG.  1 . 
     FIG. 2B is a diagram illustrating a configuration of the P-TMOS transistor shown in FIG.  1 . 
     FIG. 3A is a diagram illustrating a symbol representing an N-DTMOS transistor shown in FIG.  1 . 
     FIG. 3B is a diagram illustrating a configuration of the N-DTMOS transistor shown in FIG.  1 . 
     FIG. 4A is a diagram illustrating a symbol representing inverters shown in FIG.  1 . 
     FIG. 4B is a diagram illustrating a configuration of the inverters shown in FIG.  1 . 
     FIG. 5 is a diagram illustrating a static flip-flop circuit according to Example 2 of the present invention. 
     FIG. 6 is a diagram illustrating a conventional dynamic flip-flop circuit. 
     FIG. 7 is a diagram illustrating a configuration of transfer gates shown in FIG.  6 . 
     FIG. 8 is a diagram illustrating a configuration of inverters shown FIG.  6 . 
     FIG. 9 is a diagram illustrating a circuit for generating clock signals BCK and −BCK. 
     FIG. 10 is a diagram illustrating a configuration of a conventional static D-type flip-flop circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     A dynamic D-type flip-flop circuit according to Example 1 of the present invention will be described below with reference to FIGS. 1,  2 A,  2 B,  3 A,  3 B,  4 A, and  4 B. 
     FIG. 1 illustrates the dynamic flip-flop circuit of Example 1. 
     The dynamic flip-flop circuit  10 includes a master latch  1  and a slave latch  2 . The master latch  1  includes a transfer gate which is a P-type dynamic threshold (DT) MOS transistor  3  (hereinafter referred to as P-DTMOS transistor), and an inverter  4 . The slave latch  2  includes a transfer gate which is an N-type DTMOS transistor  5  (hereinafter referred to as N-DTMOS transistor), and an inverter  6 . 
     The P-type DTMOS transistor  3  and the N-type DTMOS transistor  5  will be described with reference to FIGS. 2A through 3B. 
     FIG. 2A illustrates a symbol representing the P-DTMOS transistor  3 . FIG. 2B illustrates a configuration of the P-DTMOS transistor  3 . The P-DTMOS transistor includes a source, a drain, a gate, and a gate insulator. The source is of P +  type. The drain is of P +  type. An active region is of N-well type. In the P-DTMOS transistor, the N-well is electrically connected to the gate. 
     FIG. 3A illustrates a symbol representing the N-DTMOS transistor  5 . FIG. 3B illustrates a configuration of the N-DTMOS transistor  5 . The N-DTMOS transistor includes a source, a drain, a gate, and a gate insulator. The source is of N +  type. The drain is of N +  type. An active region is of P-well type. In the N-DTMOS transistor, the P-well is electrically connected to the gate. As described above, the DTMOS transistor is such that the well including the active region is electrically connected to the gate. 
     Referring to FIG. 1, the P-DTMOS transistor  3  receives a clock signal CK at the gate thereof. The P-DTMOS transistor  3  also receives an input data signal D, and outputs the data signal to the inverter  4  in accordance with the clock signal CK. The inverter  4  inverts the received signal and outputs an inverse signal M. 
     The N-DTMOS transistor  5  receives a clock signal CK at the gate thereof. The N-DTMOS transistor  5  also receives the signal M, and outputs the signal M to the inverter  6  in accordance with the clock signal CK. The inverter  6  inverts the received signal and outputs an inverse signal Q. 
     In Example 1, the inverters  4  and  6  maybe composed of DTMOS transistors. FIG. 4A illustrates a symbol representing the inverters  4  and  6 . FIG. 4B illustrates a specific configuration of the inverters  4  and  6 . The inverters  4  and  6  each include a P-DTMOS transistor  7  and an N-DTMOS transistor  8 . 
     A threshold voltage V thpoff  is defined as a threshold voltage of the P-MOS transistor when the N-well of the P-MOS transistor is connected to a power source. A threshold voltage V thpon  is defined as a threshold voltage of the P-MOS transistor when the N-well of the P-MOS transistor is connected to a ground. A threshold voltage V thnoff  is defined as a threshold voltage of the N-MOS transistor when the P-well of the N-MOS transistor is connected to a ground. A threshold voltage V thnon  is defined as a threshold voltage of the N-MOS transistor when the P-well of the N-MOS transistor is connected to a power source. 
     As described above, the master latch  1  includes the P-DTMOS transistor  3 , and the inverter  4  includes the P-DTMOS transistor and the N-DTMOS transistor. A relationship between the threshold voltage V thpon  of the P-DTMOS transistor  3  in the open state and the threshold voltage V thnoff  of the N-DTMOS transistor in the closed state, of the inverter  4  is as follows. 
     |V thpon  (the threshold voltage of the P-DTMOS transistor  3 ) |&lt;|V thnoff  (the threshold voltage of the N-DTMOS transistor of the inverter  4 )| 
     Accordingly, when the inverter  4  receives a data signal D having the L level which is the absolute value |V thpon | of the threshold voltage of the P-DTMOS transistor  3  higher than a ground voltage, there is not a current passing across the N-DTMOS transistor of the inverter  4  in the master latch  1 . 
     The slave latch  2  includes the N-DTMOS transistor  5 , and the inverter  6  includes the P-DTMOS transistor and the N-DTMOS transistor. A relationship between the threshold voltage V thnon  of the N-DTMOS transistor  5  in the open state and the threshold voltage V thpoff  of the P-DTMOS transistor in the closed state, of the inverter  6  is as follows. 
     |V thnon  (the threshold voltage of the N-DTMOS transistor  5 ) |&lt;|V thpoff  (the threshold voltage of the P-DTMOS transistor of the inverter  6 )| 
     Accordingly, when the inverter  6  receives the input data signal M having the H level which is the absolute value |V thnon | of the threshold voltage of the N-DTMOS transistor  5  lower than a power source voltage, there is not a current passing across the P-DTMOS transistor of the inverter  6  included in the latch  2 . 
     Consequently, power consumption by the inverters  4  and  6  is largely reduced. 
     In the dynamic D-type flip-flop circuit  10  of Example 1, the inverters  4  and  6  as well as the transfer gates  3  and  5  include DTMOS transistors. Therefore, even when a low power source voltage is used for driving the dynamic D-type flip-flop circuit of Example 1, the driving ability is high and the operation speed is fast. 
     When the DTMOS transistor included in the transfer gate has the same threshold voltage as that of the DTMOS transistor included in the inverter, power consumption of the inverter is further increased than when the inverter is composed of general MOS transistors. Therefore, when power consumption is more important than speed, the general MOS transistor is more preferable to be used for constructing the inverter than the DTMOS transistor. 
     The number of transistors necessary for the conventional flip-flop circuit including the dynamic flip-flop circuit  30  (FIG. 6) and a clock generating circuit shown in FIG. 9 is twelve, but six for the dynamic D-type flip-flop circuit  10  of Example 1. Thus, the use of the dynamic D-type flip-flop circuit of Example 1 can largely reduce the number of transistors as compared with the conventional flip-flop circuit. 
     The number of transistors for generating input signals relating to a clock signal and transistors for receiving the input signals is eight for the conventional flip-flop circuit, but two for the dynamic D-type flip-flop circuit  10  of Example 1. Thus, the use of the dynamic D-type flip-flop circuit can largely reduce power consumption caused by transferring the clock signal as compared with the conventional flip-flop circuit. 
     The number of elements of the conventional dynamic flip-flop circuit  30  can be reduced by omitting the P-MOS transistor  27  from the transfer gate  25 . However, this may pose the following problem. 
     When the N-MOS transistor  28  which is used as the transfer gate  25  outputs a signal having the H level, the level of the signal decreases from the power source voltage V DD  by a threshold voltage V thn  of the N-MOS transistor  28 . 
     When |V thn  |≧|V thp | is satisfied where V thp  is a threshold voltage of a P-MOS transistor of the inverter  26  positioned in a stage following the transfer gate  25 , the P-MOS transistor is in the open state. This results in a direct current path from the power source to the ground, so that a current passes across the inverter  26 . 
     When the threshold voltage V thn  of the N-MOS transistor  28  as the transfer gate  25  is low, a leak current occurs therein, thereby making it impossible to hold a signal output from the N-MOS transistor  28 . 
     In a similar manner, the number of elements of the conventional dynamic flip-flop circuit  30  can be reduced by omitting the N-MOS transistor  28  from the transfer gate  23 . However, this may pose the following problem. 
     When the P-MOS transistor  27  which is used as the transfer gate  23  outputs a signal having the L level, the level of the signal increases from the ground voltage by the absolute value |V thp | of the threshold voltage of the P-MOS transistor  27 . When |V thp |≧|V thn | is satisfied where V thn  is a threshold voltage of the N-MOS transistor of the inverter  24 , the N-MOS transistor positioned in a stage following the transfer gate is not in the fully closed state. This results in a current passing across the inverter  24 . 
     When the threshold voltage Vthp of the P-MOS transistor  27  as the transfer gate  23  is low, a leak current occurs therein, thereby making it impossible to hold a signal output from the P-MOS transistor  27 . 
     EXAMPLE 2 
     A static D-type flip-flop circuit according to Example 2 of the present invention will be described below with reference to FIG.  5 . 
     FIG. 5 illustrates the static D-type flip-flop circuit  20  of Example 2. 
     The static D-type flip-flop circuit  20  includes a master latch  21  and a slave latch  22 . The master latch  21  includes a P-DTMOS transistor  11  and an N-DTMOS transistor  14 , and inverters  12  and  13 . The slave latch  22  includes an N-DTMOS transistor  15  and a P-DTMOS transistor  18 , and inverters  16  and  17 . 
     The P-DTMOS transistor  11  and the N-DTMOS transistor  14  each receive a clock signal CK at the gates thereof. The P-DTMOS transistor  11  receives an input data signal D, and outputs the received signal to the inverter  12  in accordance with the clock signal CK. The inverter  12  inverts the receive signal and outputs an inverse signal M. 
     The N-DTMOS transistor  15  and the P-DTMOS transistor  18  each receive the clock signal CK at the gates thereof. The N-DTMOS transistor  15  receives the signal M and outputs the received signal to the inverter  16  in accordance with the clock signal CK. The inverter  16  inverts the received signal and outputs an inverse signal as an output data signal Q. 
     In Example 2, the inverters  12 ,  13 ,  16 , and  17  may include an N-DTMOS transistor and a P-DTMOS transistor shown in FIG.  4 B. In this case, the static D-type flip-flop circuit  20  satisfies the following conditions. 
     The master latch  21  satisfies |V thpon  (a threshold voltage of the P-DTMOS transistor  11  in the open state) |&lt;|V thnoff  (a threshold voltage of the N-DTMOS transistor of the inverter  12  in the closed state)|. The slave latch  22  satisfies |V thnon  (a threshold voltage of the N-DTMOS transistor  15  in the open state) |&lt;|V thpoff  (a threshold voltage of the P-DTMOS transistor of the inverter  16  in the closed state)|. 
     This results in suppression of currents passing from the power source to ground across the inverter  12  and the inverter  16 . Therefore, power consumption of the inverters  12  and  16  is largely reduced. 
     All the transistors included in the static D-type flip-flop circuit  20  of Example 2 maybe DTMOS transistors, thereby obtaining a static D-type flip-flop circuit which has a high degree of driving force and operates fast by a low power source voltage. 
     When the DTMOS transistor included in the transfer gate has the same threshold voltage as that of the DTMOS transistor included in the inverter, power consumption of the inverter is further increased than when the inverter is composed of general MOS transistors. Therefore, when power consumption is more important than speed, the general MOS transistor is more preferable to be used for constructing the inverter than the DTMOS transistor. 
     Furthermore, in Example 2, the N-DTMOS transistor  14  and the P-DTMOS transistor  18  are used as the transfer gates included in feedback circuits for enhancing the reliability of a signal. Therefore, the static D-type flip-flop circuit  20  operates only using a clock signal having one phase and one polarity as does the dynamic D-typeflip-flop circuit  10  shown in FIG.  1 . 
     The number of transistors necessary for the conventional flip-flop circuit including the static flip-flop circuit  60  (FIG.  10 ), and a clock generating circuit shown in FIG. 9 is twenty, but twelve for the static D-type flip-flop circuit  20  of Example 2. Thus, the use of the dynamic D-type flip-flop circuit of Example 2 can largely reduce the number of transistors as compared with the conventional flip-flop circuit. 
     The number of transistors for generating input signals relating to a clock signal and transistors for receiving the input signals is twelve for the conventional flip-flop circuit, but four for the static D-type flip-flop circuit  20  of Example 2. Thus, the static D-type flip-flop circuit  20  can largely reduce power consumption caused by transferring the clock signal as compared with the conventional flip-flop circuit. 
     The present invention is not limited to a flip-flop circuit. For example, at least one of the master latch  1 , the slave latch  2 , the master latch  21 , and the slave latch  22  may be used as a single independent latch circuit. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.