Patent Document

CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority/priorities from Japanese Patent Application No. 2012-072515 filed on Mar. 27, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to semiconductor integrated circuits. 
     BACKGROUND 
     Reconfigurable integrated circuits (ICs), such as field programmable gate arrays (FPGAs), can reconfigure circuits to thereby implement arbitrary logic functions. A reconfigurable IC includes logic blocks and wiring portions. The logic blocks respectively implement arbitrary truth tables, and the wiring portions change the interconnection among the logic blocks. For example, look-up tables (LUTs) are used as elements for respectively configuring logic blocks, and switches are provided in the wiring portions. Data of the LUTs and data for switching connection/non-connection of the switches are stored in memories. The user can implement arbitrary logic-functions by appropriately writing information into the memories. 
     For example, in an LUT for implementing an N-input 1-output logic-circuit (“N” is a positive integer), 2 N  memories are provided. To realize the logical operation represented by a given truth table, data corresponding to outputs of the given truth table are written into the 2 N  memories. Then, the LUT outputs an appropriate signal by selecting one of the 2 N  memories according to inputs thereto. Here, as the number of inputs to the LUT increases, delay in output thereof increases. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  schematically illustrates an LUT according to a first embodiment. 
         FIG. 2  illustrates a circuit example of the LUT according to the first embodiment. 
         FIGS. 3A and 3B  illustrate examples of a switch. 
         FIG. 4  illustrates another circuit example of the LUT according to the first embodiment. 
         FIG. 5  illustrates an LUT according to a comparative example. 
         FIG. 6  illustrates a first modification of the first embodiment. 
         FIG. 7  illustrates a second modification of the first embodiment. 
         FIGS. 8 and 9  illustrate a third modification of the first embodiment. 
         FIG. 10  illustrates a fourth modification of the first embodiment. 
         FIG. 11  schematically illustrates an LUT according to a second embodiment. 
         FIG. 12  illustrates another example of the LUT according to the second embodiment. 
         FIGS. 13A to 13F  and  14  illustrate examples of a memory. 
         FIGS. 15 to 17  each illustrates still another example of the LUT according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments are described with reference to the drawings. 
     [First Embodiment] 
       FIG. 1  illustrates an LUT according to a first embodiment. The LUT  10  illustrated in  FIG. 1  is an (N+1)-input LUT. The LUT  10  includes two LUTs  11  and  12  and a multiplexer  13 . Each of the LUTs  11  and  12  is less in the number of inputs than the LUT  10 . The following description is made by assuming that each of the LUTs  11  and  12  is an N-input LUT. Thus, the N-input LUT  11  includes a memory group  21   a  of 2 N  memories and a multiplexer  22   a , and selectively outputs information stored in the memory group  21   a  using N input signals. The N-input LUT  12  includes a memory group  21   b  of 2 N  memories and a multiplexer  22   b , and selectively outputs information stored in the memory group  21   b  using N input signals. 
     The N-input LUTs  11  and  12  are connected to the N input signal wires, and output signals of the N-input LUTs  11  and  12  are input to the multiplexer  13 . One of the output signals input to the multiplexer  13  is selected to be output according to an (N+1)-th input signal. Consequently, an (N+1)-input 1-output LUT  10  can be implemented. Here, a direction of connecting the N-input LUT  11  to the N input signal wires is opposite to a direction of connecting the N-input LUT  12  to the N input signal wires. 
     By making the connection directions of the N-input LUTs  11  and  12  with respect to the N input signal wires to be opposite to one another, loads of the N input signal wires can be uniformized. Accordingly, a delay time from the input of each input signal to the output of the selected signal can be reduced. This effect is described hereinafter with reference to a circuit example of the LUT  10 . 
       FIG. 2  illustrates a circuit example of the LUT  10 . The LUT  10  illustrated in  FIG. 2  is a  4 -input LUT. The LUT  10  configures multiplexers  13 ,  22   a  and  22   b  using plural switches. In the case of the 4-input LUT  10 , 3 input wires A, B and C are connected to each of the multiplexers  22   a  and  22   b , and 3 input signals A, B and C are input to the 3 input wires A, B and C, respectively. 
     As illustrated in  FIG. 3A , e.g., a transfer gate configured by a parallel combination of an N-channel metal oxide semiconductor (NMOS) transistor and a P-channel metal oxide semiconductor (PMOS) transistor can be used as each of the transistors. Incidentally, in  FIG. 2  and later, the transfer gate is designated with a symbol illustrated in  FIG. 3B . Alternatively, as illustrated in  FIG. 4 , either an NMOS transistor switch or a PMOS transistor switch can be used as each of the switches of the LUT  10 . The memories of the memory groups  21   a  and  21   b  may be either volatile memories or nonvolatile memories. 
     The memories of the memory group  21  a are connected to a switch that is connected to the input wire A, whereas the memories of the memory group  21   b  are connected to a switch that is connected to the input wire C. As illustrated in  FIG. 2 , the stages of the multiplexers  22   a  and  22   b  having the largest number of switches are connected to the input wires A and C that are closest to the memory groups  21   a  and  21   b , respectively. Two switches are connected to the closest to output terminals of the multiplexers  22   a  and  22   b , respectively. Thus, by making the direction of connecting the multiplexer  22   a  to the input wires A to C and the direction of connecting the multiplexer  22   b  to the input wires A to C to be opposite to one another, the number of the switches of the multiplexer  22   a , which are connected to the input wire A, is larger than the number of the switches of the multiplexer  22   a , which are connected to the input wire C, while the number of the switches of the multiplexer  22 b, which are connected to the input wire A, is smaller than the number of the switches of the multiplexer  22   b , which are connected to the input wire C. Consequently, the loads on the input wires can be uniformized. 
     For example, if the direction of connecting the multiplexer  22   a  to the input wires and the direction of connecting the multiplexer  22   b  to the input wires are the same, as indicated in a comparative example illustrated in  FIG. 5 , the number of switches driven by signals input from the closest input wire (i.e., the input wire A) to each of the memory groups are  16 . 
     On the other hand, in the case of the LUT  10  illustrated in  FIG. 2 , the number of switches driven by the input signal A is  10 . Thus, time taken to charge and discharge the switch through the wire from which the signal A is input is shortened. Consequently, the delay time of the LUT can be shortened. 
     If the connection directions of the multiplexers  22   a  and  22   b  with respect to the input wires are the same, the number of switches connected to the closest input wire (i.e., the input wire A) to the memory group is twice or more the number of switches connected to the other input wires. Therefore, load is concentrated onto the closest input wire to the memory group. Delay time from the input of an input signal to the switch connected to this input wire to the output of the input signal is longer than the delay time of other switches. Thus, circuit delay has considerably changed depending on whether the critical path of the circuit uses the closest input wire to the memory group. 
     On the other hand, in the LUT  10  illustrated in  FIG. 2 , the number of switches driven by an input signal A is  10 . The number of switches driven by an input signal B is  8 . The number of switches driven by an input signal C is  10 . Thus, the loads on the input wires can be uniformized. Consequently, the necessity of considering the balance of loads on input terminals at the configuration of the circuit is reduced. 
     In the first embodiment, memory data stored in the memories of the memory group  21   a  is set so as to differ in the order of values from that stored in the memories of the memory group  21   b . As illustrated in  FIG. 2 , the memory group  22   b  is configured by reversing the memory group  22   a . Data arranged considering the above-mentioned relationship are stored in the memory groups  21   a  and  21   b . That is, the following data are respectively stored in the memories of the memory group  21   a , from the top, as viewed in  FIG. 2 . 
     [Expression 1] 
     Ā             B             C , Ā           B             C , Ā         B           C , A         B           C , Ā           B           C, A           B           C, Ā         B         C, and A         B         C.
     On the other hand, the following data are respectively stored in the memories of the memory group  21   b , from the top, as viewed in  FIG. 2 . 
     [Expression 2] 
     Ā             B             C , Ā           B           C, Ā         B           C , A         B         C, A           B             C , A           B           C, A         B           C , and A         B         C.
     Each overbar represents logical negation. For example, low voltage level is assigned to logical negation. The data respectively stored in the memories of the memory group  21  a, which are sequentially arranged from the top, differ in arrangement-sequence from the data respectively stored in the memories of the memory group  21   b , which are sequentially arranged from the top. However, all the possible values represented by the data stored in the memories of the memory group  21   a  correspond to those represented by the data stored in the memories of the memory group  21   b , respectively. The LUT  10  serves as a 4-input LUT by selecting which of the memory groups  21   a  and  21   b  corresponds to each of an input from the wire D and the logical negation of this input. On the other hand, in the case of a comparative example illustrated in  FIG. 5 , the arrangement-sequence determined according to input signals A, B and C is the same between a memory group illustrated at an upper portion and a lower memory group illustrated at a lower portion. 
     [First Modification] 
       FIG. 6  illustrates a first modification of the first embodiment. An (N+1)-input LUT  101  further includes wires for inputting, to an external circuit, signals output from two N-input LUTs  11  and  12 . Consequently, the LUT  101  can be used as either an (N+1)-input LUT or two N-input LUTs. 
     [Second Modification] 
       FIG. 7  illustrates a second modification of the first embodiment. An (N+ 2 )-input LUT  102  includes four N-input LUTs  11 ,  12 ,  14  and  15 . The connection directions of the LUTs  11  and  14  with respect to input wires are opposite to the connection directions of the LUTs  12  and  15  with respect to the input wires. Thus, an i-input LUT (“i” is a positive integer) may be configured by three or more j-input LUTs (“j” is a positive integer and less than the integer “i”). The number of the LUTs connected to the input wires in a first connection direction is not necessarily the same as the number of the LUTs connected to the input wires in a second connection direction that is opposite to the first connection direction. 
     The more largely the number of the j-input LUTs configuring the i-input LUT is increased, the more uniform the balance of loads on the input wires becomes. Consequently, whatever input wire the critical path of the circuit uses, the variation of the delay time of the LUT is reduced. 
     [Third Modification] 
       FIG. 8  illustrates a third modification of the first embodiment. An LUT  103  includes an N-input LUT  11  and an M-input LUT  12  (“M” is a positive integer) configured such that M is less than N. In the case of combining LUTs differing in size from one another, preferably, the small-size LUT  12  is connected to the input wires which are connected to switches close to the output terminal of the large-size LUT  11 , among input wires to which the large-size LUT  11  is connected. This is because of the facts that a load on the input wire connected to the switch close to the output terminal of the large-size LUT  11 , which is caused due to the switch of the large-size LUT  11 , is small, and that even if the LUT  12  is connected such an input wire, load on the input wire is suppressed. 
       FIG. 9  illustrates the case of combining a 3-input LUT and a 2-input LUT. Even in the LUT  103  illustrated in  FIG. 9 , the switches may be transfer gates, NMOS transistors, or PMOS transistors. Assuming that the input wires of the 3-input LUT  113  are an input wire A, an input wire B, and an input wire C, the input wire A is connected to a switch that is connected to the memory of the  3 -input LUT  113 . The input wire C is connected to a switch that is connected to an output of the  3 -input LUT  113 . A 2-input LUT  123  is connected to the input wire B and the input wire C. The 2-input LUT  123  can be connected to the input wires A and B, or to the input wires A and C. However, because the wire A is the input wire to which the largest number of switches of the 3-input LUT  113  are connected, the load on the wire A is large. Thus, the load on the wire A is large. Therefore, a lowest load circuit configuration for the LUT  103  is obtained by connecting the 2-input LUT  123  to the input wire B and the input wire C. In addition, the loads on the input wires are substantially equal to one another. Thus, configuration with small variation of the delay time can be implemented. Incidentally, this modification employs the combination of the 3-input LUT and the 2-input LUT by way of example. However, LUTs each having an optional number of inputs can be used. 
     [Fourth Modification] 
       FIG. 10  illustrates a fourth modification of the first embodiment. In an LUT  104 , signals output from the LUTs  11  and  12  are input to an external circuit, instead of selecting one of outputs of the LUT  11  and the LUT  12  with a multiplexer. For example, in the case of configuring an adder, an output therefrom is surely represented by plural bits. In this case, it is unnecessary that one of outputs from plural LUTs is selected by a multiplexer. Thus, since the LUT  104  can be configured without providing a multiplexer therein, the area of the circuit and the power consumption thereof can be reduced. 
     The above modifications may be combined with one another. For example, an i-input LUT may be configured by three or more j-input LUTs, and a wire for inputting, to an external circuit, three or more j-input LUTs may be added. At that time, a multiplexer for selecting one of output signals from the j-input LUTs is not necessarily provided. In addition, the plural LUTs may differ in the number of inputs from one another. 
     [Second Embodiment] 
       FIG. 11  illustrates an LUT  20  according to a second embodiment. The LUT  20  is configured such that PMOS power-supply-control switches  32   a  and  32   b  are connected to the memory groups  21   a  and  21   b , respectively. Power-supply-control memories  31   a  and  31   b  are connected to the gates of the power-supply-control switches  32   a  and  32   b , respectively. In the case of the LUT  20  illustrated in  FIG. 11 , the power-supply-control switches  32   a  and  32   b  are provided between the power supply wire and the memory group  21   a  and between the power supply wire and the memory group  21   b , respectively. However, a PMOS power-supply-control switch may be provided between the power supply wire and each of the memory groups  21   a  and  21   b . In addition, an NMOS power-supply-control switch may be provided between the ground wire and each of the memory groups  21   a  and  21   b . The power-supply-control switch may be provided only between the power supply wire and the memory group. Alternatively, the power-supply-control switch may be provided only between the memory group and the ground wire. The power supply wire and the ground wire may be referred correctively to as the power-supply/ground wire. The LUTs  11  and  12  and the multiplexer  13  may be configured similarly to those according to the first embodiment. 
     Thus, the power supply to the LUTs  11  and  12  may be interrupted by providing the power-supply-control switches. For example, in a case where the LUT  20  is not used, the power consumption of the entire LUT  20  can be reduced by shutting off both of the power-supply-control switches  32   a  and  32   b.    
     For example, in the case of using the (N+1)-input LUT  20  as an N-input LUT, an output of a predetermined one (e.g., the LUT  11 ) of the N-input LUTs  11  and  12  is selected by the multiplexer  13 . The selected signal is output from the LUT  20 . Thus, the (N+1)-input LUT  20  as an N-input LUT. At that time, it is unnecessary to use the LUT  12 . Then, the power supply to the power-supply-control switch  32   b  is interrupted. Consequently, power consumption can be reduced. 
     Incidentally,  FIG. 11  illustrates the LUT in which the power supply to the multiplexer  22   a  and the power supply to the multiplexer  22   b  are controlled according data stored in the power-supply-control memories  31   a  and  31   b , respectively, independent of each other. However, the power supply to both of the multiplexers  22   a  and  22   b  may be controlled in common according to data stored in the power-supply-control memories  31   a  and  31   b .  FIG. 12  illustrates such LUT  200 . In the LUT  200 , if “1” is stored in the power-supply-control memory  31   a , the power-supply-control switches  32   a  and  33   a  are put into an off-state. Thus, the power supply to the memories connected to the power-supply-control switches  32   a  and  33   a  is shut off. If “1” is stored in the power-supply-control memory  31   b , the power-supply-control switches  32   b  and  33   b  are brought into an off-state. Thus, the power supply to the memories connected to the power-supply-control switches  32   b  and  33   b  is shut off. In the LUT  200 , inverters provided on the input wires A, B, and C are shared by the multiplexers  22   a  and  22   b . Power-supply-control switches  32   c ,  33   c ,  32   d  and  33   d  are connected to inverters provided on the input wires A, B, C and D. The power-supply-control switches  32   c  and  33   c  are put into an off-state, if “1” is stored in the power-supply-control memory  31   b . The power-supply-control switches  32   d  and  33   d  are brought into an off-state, if “1” is stored in the power-supply-control memory  31   a . That is, the power supply to the inverters provided on the input wires A, B, C, and D is shut off if “1” is stored in both of the power-supply-control memories  31   a  and  31   b . Thus, the power supply to the multiplexers  22   a ,  22   b  and  13  provided in the LUT  200  can be shut off. 
     As illustrated in  FIGS. 11 and 12 , the power-supply-control memories  31   a  and  3  lb and the power-supply-control switches  32   a  and  32   b  for all LUT (i.e., the LUTs  11  and  12 ) configuring the LUT  20  are provided. However, the power-supply-control memories and the power-supply-control switches are not necessarily provided for all the LUTs. The LUT  20  may be configured such that if one of the LUTs  11  and  12  is always used, no power-supply-control memory and no power-supply-control switches are provided for the one of the LUTs, and that the other LUT  12  or  11  is provided with a power-supply-control memory and a power-supply-control switch. 
     Thus, if both of the LUTs  11  and  12  are used in the LUT  20 , the delay of the circuit is reduced because the connection directions of the LUTs  11  and  12  with respect to the input wires are made to be opposite to one another. In addition, the power supply to at least one of the LUTs  11  and  12  can be shut off. Thus, the power consumption can be reduced. 
     The memories of the memory groups  21   a  and  21   b  may be either volatile memories or nonvolatile memories. Alternatively, both of volatile and nonvolatile memories may be used as the memories of the memory groups  21   a  and  21   b . However, if nonvolatile memories are used as the memories of the memory groups  21   a  and  21   b , the power supply can be shut off even during operation of the LUT  20 . 
     As illustrated in  FIGS. 13A and 13B , a floating type flash memory, a charge-trap type metal-oxide-nitride-oxide-semiconductor (MONOS) memory, a phase-change memory, MRAM, an ionic memory, and a resistance change type memory such as a resistance random access memory (ReRAM) can be employed as the nonvolatile memory. Further, as illustrated in  FIGS. 13C ,  13 D,  13 E and  13 F, selection transistors, such as NMOS transistors, PMOS transistors and transfer gates, may be used in combination with the above nonvolatile memories. If the drive power of the memory is low, the drive power can be increased by connecting a buffer, such as a complementary metal-oxide semiconductor (CMOS) inverter, to an output terminal of the memory, as illustrated in  FIG. 14 . Incidentally, in the case described with reference to  FIGS. 13A ,  13 B and  14 , the power supply wire is connected to one of two memories, while the ground wire is connected to the other memory. These figures illustrate a state at the time of causing the LUTs to operate. In addition, a programming power supply and a control circuit, which are used to write and erase data to and from each element, are connected to the memories, though this power supply and this control circuit are not illustrated. 
     As an example of interrupting the power supply during operation of the LUT  20 , the following case may be considered. That is, it is obvious or expected that only the LUT  11  is used and the LUT  12  is not used in a certain time period during operation of the LUT  20 . In this case, the power supply to the LUT  12  is interrupted. 
     In addition, after a lapse of the certain time period in which the LUT  12  is not used, the power supply to the LUT  12  is restored. 
     The power supply to a part of an LUT may be interrupted using an input signal, as illustrated in  FIG. 15 . An LUT  201  includes two  3 -input LUTs.  5 . Power-supply-control switches  32   a ,  32   b ,  33   a  and  33   b  are provided on the input wire A of the LUT  201 . Consequently, if an input signal A represents “1”, the power-supply-control switches  32   a  and  33   a  are turned off. Thus, the power supply to memories connected to the power-supply-control switches  32   a  and  33   a  is shut off. At that time, the power-supply-control switches  32   b  and  33   b  are in an on-state. Therefore, power is supplied to memories connected to the power-supply-control switches  32   b  and  33   b . On the other hand, if a signal input from the input wire A represents “0”, the power-supply-control switches  32   a  and  33   a  are turned on, while the power-supply-control switches  32   b  and  33   b  are turned off. The number of memories connected to the power-supply-control switches  32   a  and  33   a  is a half the number of memories provided in the LUT  201 . Memories connected to the power-supply-control switches  32   b  and  33   b  are the remaining half of the memories provided in the LUT  201 . Thus, leakage current can be reduced by half by providing the power-supply-control switches  32   a ,  32   b ,  33   a  and  33   b  in the LUT  201 . 
     In an example of  FIG. 15 , the input wire B functions as a first input wiring, the input wire C functions as a second input wiring, and the input wire A functions as a third input wiring. Although the power-supply-control switches are connected to the input wire A in  FIG. 15 , the power-supply-control switches may be connected to the input wires B, C, and D other than the input wire A. Whichever input-wire the power-supply-control switch is provided on, leakage current can be reduced by half. 
     In addition, the power supply to a part of an LUT can be interrupted using plural input signals, as illustrated in  FIG. 16 . An LUT  202  is configured such that the power-supply-control switches  32   a ,  33   a ,  32   b ,  33   b ,  32   c ,  33   c ,  32   d  and  33   d  are connected to the input wire A and the input wire B. Each of the power-supply-control switches  32   a ,  32   b ,  32   c  and  32   d  is configured by series-connecting a PMOS transistor, whose gate is connected to the input wire A, and a PMOS transistor, whose gate is connected to the input wire B. Each of the power-supply-control switches  33   a ,  33   b ,  33   c  and  33   d  is configured by series-connecting an NMOS transistor, whose gate is connected to the input wire A, and an NMOS transistor, whose gate is connected to the input wire B. 
     Consequently, if the input signal A and the input signal B represent “1”, the power-supply-control switches  32   d  and  33   d  are turned on. Other power-supply-control switches are turned off. Thus, one of a pair of the power-supply-control switches  32   a  and  33   a , a pair of the power-supply-control switches  32   b  and  33   b , a pair of the power-supply-control switches  32   c  and  33   c , and a pair of the power-supply-control switches  32   d  and  33   d  is turned on according to the combination of values respectively represented by the input signal A and the input signal B. Other power-supply-control switches are turned off. Therefore, leakage current of the memory groups included in the LUT  202  may be reduced to ¼. 
     The power-supply-control switches illustrated in  FIG. 16  can be configured using logic gates.  FIG. 17  illustrates an example of an LUT in the case of configuring the power-supply-control switches using NAND-gates to control the power supply according to the combination of values respectively represented by the input signal A and the input signal B. An LUT  203  is configured such that one of the power-supply-control switches  34   a  to  34   d  is turned on according to the combination of values respectively represented by the input signal A and the input signal B, and that other power-supply-control switches are turned off, similarly to the LUT  202 . Therefore, leakage current of the memory groups included in the LUT  203  may be reduced to ¼. 
     Incidentally, in the case of the LUTs respectively illustrated in  FIGS. 16 and 17 , the power supply to the memories is controlled, based on the two input signals. 
     However, the power supply to the memories may be performed, based on three or more input signals. Leakage current may be more reduced using a larger number of input signals in controlling the power supply. 
     The modifications of the first embodiment may be applied to the LUTs according to the second embodiment. For example, the number of inputs to the inner LUTs may vary thereamong. Three or more inner LUTs may be provided. And, the multiplexer  13  for selecting one of outputs from the inner LUTs may be omitted. 
     In the examples of  FIGS. 15-17 , the power-supply-control switch is provided between the power supply wire and the memory group and between the memory group and the ground wire. However, as mentioned above in relation to the example of  FIG. 12 , the power-supply-control switch may be provided only between the power supply wire and the memory group, or between the memory group and the ground wire. 
     According to the configuration of the above embodiments, LUTs with short delay time can be provided. The invention is not limited to the above embodiments. Various changes can be made to the above embodiments without departing from the spirit and scope of the invention.

Technology Category: h