Abstract:
The semiconductor memory includes a memory cell which handles a clock signal, an address fetch and a command circuit. The memory cell is designated by an address signal and stores data. The clock signal is supplied thereto so as to provide timing for an access to the memory cell, and the clock signal has a leading edge and a trailing edge. The address fetch circuit fetches the address signal for designating the memory cell in synchronism with both of the leading edge and trailing edge of the clock signal. The command circuit fetches a command signal for instructing the access to the memory cell in synchronism with both of the leading edge and the trailing edge of the clock signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-200253, filed Jun. 30, 2000, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory, and more specifically to a semiconductor memory having a DDR (double data rate) transfer technique. 
     2. Description of the Related Art 
     Recently, there are greatly increasingly demands on improving the processing speed of computer systems. In order to meet such demands, a synchronous DRAM (DDR-SDRAM) is presently being developed. The synchronous DRAM uses a DDR (double data rate) transfer technique, which transfers data between the memory and the CPU effectively at double the speed of a conventional technique, by synchronizing the transfer of data in synchronism with both of the leading edge and trailing edge of clock signals supplied for controlling the operation timing. 
     In the conventional synchronous DRAM (DDR-SDRAM), data is input/output by a DDR operation; however, various signals such as a row address strobe signal bRAS, a column address strobe signal bCAS, a chip select signal bCS, write enable signal bWE, bank select signals (BS 0  and BS 1 ), and row (column) address signals (A 0  to A 11 ) are not handled by the DDR operation. It should be noted that the first letter of each reference symbol, that is, b, indicates that the signal is of an inverted type. 
     Further, signals of row addresses and column addresses share address basses, input pins, input buffers and the like, and therefore they cannot be inputted at the same time. 
     Due to the two points described above, there is the following problem. That is, gaps are created while transferring data, thereby deteriorating the effective data transfer rate, particularly in the case where random row accessing is carried out. 
     FIG. 1 is a diagram illustrating a read-out operation in an interleave where the latency of the CAS signal is 2, the burst length is 4 and the number of banks is 4 and FIG. 2 is a diagram illustrating a write operation in an interleave where the latency of the CAS signal is 2, the burst length is 4 and the number of banks is 4. Further, FIG. 3 is a diagram illustrating a read-out operation in an interleave where the latency of the CAS signal is 2, the burst length is 2 and the number of banks is 4 and FIG. 4 is a diagram illustrating a write operation in an interleave where the latency of the CAS signal is 2, the burst length is 2 and the number of banks is 4. 
     In order for avoiding such a problem that the data transfer rate described above is deteriorated, there is a technique which prepares address buses, input pins, input buffers and the like are provided separately and exclusively for row address and column address. According to this technique, row address and column address are input at the same time, and therefore the above-described problem can be easily solved. However, with this technique, the necessary area for forming address buses, input pins, input buffers and the like will become twice as large, thereby increasing the chip area. Therefore, it is difficult to put it into practice. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been proposed in consideration of the above-described problems entailed to the conventional technique, and the object thereof is to provide a semiconductor memory capable of enhancing the data transfer efficiency and increasing the circuit operation speed without increasing its chip area. 
     In order to achieve the above-described object, there is provided, according to the first aspect of the present invention, a semiconductor memory comprising: a memory cell for storing data, the memory cell being designated by an address signal; an address fetch circuit for fetching the address signal for designating the memory cell in synchronism with both of a leading edge and a trailing edge of a clock signal, the clock signal for providing timing for access to the memory cell; and a command circuit for fetching a command signal for instructing the access to the memory cell in synchronism with both of the leading edge and the trailing edge of the clock signal. 
     In order to achieve the above-described object, there is provided according to the second aspect of the present invention, a semiconductor memory comprising: a memory cell for storing data, the memory cell being designated by an address signal; a clock generating circuit for generating a clock signal used for providing timing for an access to the memory cell, the clock signal having a leading edge and a trailing edge; a first holding circuit for holding the address signal for designating the memory cell in synchronism with either one of a leading edge and a trailing edge of the clock signal; a second holding circuit for holding the address signal in synchronism with an other edge which comes after the one of edges, which is different from the one of the edges, used by the first holding circuit to hold the address signal; a first decoding circuit for decoding the address signal held by the first holding circuit; and a second decoding circuit for decoding the address signal held by the second holding circuit. 
     In order to achieve the above-described object, there is provided, according to the third aspect of the present invention, a semiconductor memory comprising: a memory cell for storing data; a clock generating circuit for generating a clock signal used for providing timing for an access to the memory cell, the clock signal having a leading edge and a trailing edge; 
     a first holding circuit for holding a command signal for instructing the access to the memory cell in synchronism with either one of the leading edge and the trailing edge of the clock signal; a second holding circuit for holding the command signal in synchronism with an other edge which comes after the one of edges, which is different from the one of the edges, used by the first holding circuit to hold the command; a first decoding circuit for decoding the command signal held by the first holding circuit; and a second decoding circuit for decoding the command signal held by the second holding circuit. 
     With the semiconductor memory having the above-described structure, not only input/output of data, but also the command signals, that is, the address signal, bank selection signal, row address strobe signal bRAS, column address strobe signal bCAS, chip select signal bCS and write enable signal bWE, are fetched in synchronism with both of the leading edge (the rising edge) and the trailing edge (the falling edge) of the clock signal. In this manner, it becomes possible to improve the effective data transfer rate. Further, not simply fetching these signals in synchronism with both of the leading edge and the trailing edge of the clock signal, but each signal is handled in accordance with its role of the signal, that is, for example, the handling of some signals is limited such that they are fetched in synchronism with leading edges of external clock signals, while others are fetched in synchronism with trailing edges of external clock signals. In this manner, the number of signals input from outside can be reduced. Thus, the circuit can be simplified in structure, and the chip area can be reduced and the circuit operation speed can be increased. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a time chart indicating an example of the read operation in a conventional synchronous DRAM; 
     FIG. 2 is a time chart indicating an example of the write operation in a conventional synchronous DRAM; 
     FIG. 3 is a time chart indicating an example of the read operation in another conventional synchronous DRAM; 
     FIG. 4 is a time chart indicating an example of the write operation in another conventional synchronous DRAM; 
     FIG. 5 is a block diagram showing a structure of a semiconductor memory according to an embodiment of the present invention; 
     FIGS. 6A to  6 E are circuit diagrams illustrating a structure of a clock generating portion in the semiconductor memory; 
     FIGS. 7A to  7 E are circuit diagrams illustrating a structure of a command portion in the semiconductor memory; 
     FIGS. 8A to  8 C are circuit diagrams illustrating a structure of an address portion in the semiconductor memory; 
     FIG. 9 is a time chart indicating operations of the clock generating portion, command portion and address portion in the semiconductor memory; 
     FIG. 10 is a time chart indicating an example of the read operation in the semiconductor memory; 
     FIG. 11 is a time chart indicating an example of the write operation in the semiconductor memory; 
     FIG. 12 is a time chart indicating another example of the read operation in the semiconductor memory; and 
     FIG. 13 is a time chart indicating another example of the write operation in the semiconductor memory. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described with reference to accompanying drawings. 
     FIG. 5 is a block diagram showing a structure of a semiconductor memory according to an embodiment of the present invention. 
     As can be seen in FIG. 5, an address strobe signal bRASCAS, a write enable signal bWE, and a chip select signal bCS are input to a command signal buffer  11 . An output from the command signal buffer  11  is input to a command decoder  12 B via a command latch  12 A, as well as to a command decoder  13 B via a command latch  13 A. 
     Outputs from the command decoders  12 B and  13 B are both input to a control signal generator  14 . An output from the control signal generator  14  is input to a bank block  15  including banks BK 0  to BKn. Here, an example case where the bank block  15  includes banks BK 0  to BK 3 , that is, the number of banks is 4, will be discussed. A bank is the minimum unit of a set of a plurality of memory cells which can be accessed to at the same time. 
     Address signals ADDRESS (A 0  to A 11 ) and bank select signals BSn (BS 0  and BS 1 ) are input to an address buffer  16 . An output from the address buffer  16  is input to each of the row address latch  17 A and column address latch  18 A. An output from the row address latch  17 A is input to the mode register  19  as well as to the bank block  15  via a row address decoder  17 B. Similarly, an output from the column address latch  18 A is input to the mode register  19  as well as to the bank block  15  via a column address decoder  18 B. 
     From an outside member, a clock signal CLK is input to a clock generating portion  20 . A clock signal bCMDLTC is output from the clock generating portion  20  to the command latch  12 A. Also, a clock signal CMDLTC is output from the clock generating portion  20  to a command latch  13 A. Further, a clock signal ADDDLTC is output from the clock generating portion  20  to the row address latch  17 A. A clock signal bADDDLTC is output from the clock generating portion  20  to the column address latch  18 A. Further, a clock signal output from the clock generating portion  20  is input to each of the control signal generator  14  and the memory cell array portion (bank block)  15 . 
     An output from a column counter  21  is input to the column address latch  18 A. An output from a refresh counter  22  is input to the row address latch  17 A. Meanwhile, data DQ 0  to DQn are output from the memory cell array portion  15  via a DQ buffer  23 . The memory cell array portion  15  has banks BK 0  to BK 3 . 
     It should be noted that the command signal buffer  11 , the command latch  12 A, the command decoder  12 B, the command latch  13 A and the command decoder  13 B constitute a command portion  30 . Further, the address buffer  16 , the row address latch  17 A, the column address latch  18 A, the row address decoder  17 B and the column address decoder  18 B constitute an address portion  40 . 
     The clock generating portion  20  generates a clock signal bCMDLTC and a signal CMDLTC, which are used in the command portion  30 , and a clock signal bADDDLTC and a signal ADDDLTC, which are used in the address portion  40 . The clock signal bCMDLTC and signal CMDLTC are used to synchronize the start of operations in the command portion  30 , and the clock signal bADDDLTC and signal ADDDLTC are used the start of operations in the address portion  40 . 
     The command portion  30  latches each of the address strobe signal bRASCAS, write enable signal bWE, chip select signal bCS, which are input to the command signal buffer  11 , in the command latch  12 A in synchronism with the clock signal bCMDLTC, and then outputs each of the latched signals to the command decoder  12 B. The command decoder  12 B decodes each of the input signals, and outputs a signal ROW-COMMAND for carrying out an arbitrary row-series circuit operation. 
     Further, the command portion  30  latches each of the address strobe signal bRASCAS, write enable signal bWE, chip select signal bCS, which are input to the command signal buffer  11 , in the command latch  13 A in synchronism with the clock signal CMDLTC, and then outputs each of the latched signals to the command decoder  13 B. The command decoder  13 B decodes each of the input signals, and outputs a signal COLUMN-COMMAND for carrying out an arbitrary column-series circuit operation. 
     The address portion  40  latches the address signal ADDRESS input to the Gommand buffer  16 , in the row address latch  17 A in synchronism with the clock signal bADDDLTC, and then outputs the latched signal ROW-ADDRESS to the row address decoder  17 B. The row address decoder  17 B decodes the input signal ROW-ADDRESS, and activates an arbitrary row-address selection line. 
     Further, the address portion  40  latches the address signal ADDRESS input to the command buffer  16 , in the column address latch  18 A in synchronism with the clock signal ADDDLTC, and then outputs the latched signal COLUMN-ADDRESS to an address decoder (not shown). The address decoder decodes the input signal COLUMN-ADDRESS, and activates an arbitrary column-address selection line. 
     Furthermore, the address portion  40  latches the bank select signal BSn input to the command buffer  16 , in a latch circuit (not shown) in synchronism with the clock signal bADDDLTC and signal ADDDLTC. Then, it decodes the latched signal in a decoder (not shown), and outputs a signal BS&lt;0:3&gt; for selecting a bank. 
     Next, the circuit structures of the clock generating portion  20 , the command portion  30  and the address portion  40  will now be described in detail with reference to FIGS. 6A to  6 E, FIGS. 7A to  7 E and FIGS. 8A to  8 C. FIG. 9 is a time chart illustrating operations of the clock generating portion  20 , the command portion  30  and the address portion  40 . 
     FIGS. 6A to  6 E are circuit diagrams showing a structure of the clock generating portion  20 . 
     As shown in FIG. 6A, the clock signal CLK input to the input buffer IB 1  passes through inverters I 1 , I 2 , I 3  and I 4 , and is output from an output portion of the inverter I 4  as a clock signal ACP. Further, the signal ACP passes through an inverter I 5 , and then is output as a clock signal bACP. Waveforms of these clock signals CLK, clock signal ACP and clock signal bACP are as shown in FIG.  9 . 
     Further, as shown in FIG. 6C, the clock signal ACP is input to a first terminal of an NAND circuit NA 1  and also to a second terminal of the NAND circuit NA 1  via inverters I 6 , I 7  and I 8 . A clock signal bCMDLTC is output from an output portion of the NAND circuit NA 1 . As shown in FIG. 6B, the clock signal bACP is input to a first terminal of an NAND circuit NA 2  and also to a second terminal of the NAND circuit NA 2  via inverters I 9 , I 10  and I 11 . A clock signal CMDLTC is output from an output portion of the NAND circuit NA 2 . Waveforms of these clock signals bCMDLTC and clock signal CMDLTC are as shown in FIG.  9 . 
     Furthermore, as shown in FIG. 6E, the clock signal ACP is input to a first terminal of an NAND circuit NA 3  and also to a second terminal of the NAND circuit NA 3  via inverters I 12 , I 13  and I 14 . A clock signal bADDDLTC is output from an output portion of the NAND circuit NA 3 . As shown in FIG. 6D, the clock signal bACP is input to a first terminal of an NAND circuit NA 4  and also to a second terminal of the NAND circuit NA 4  via inverters I 15 , I 16  and I 17 . A signal ADDDLTC is output from an output portion of the NAND circuit NA 4 . Waveforms of these clock signals bADDDLTC and clock signal ADDDLTC are as shown in FIG.  9 . 
     FIGS. 7A to  7 E are circuit diagrams showing a structure of the command portion  30 . 
     First, circuit structures of a command signal buffer  11  and command latches  12 A and  13 A in the command portion  30  will be described. 
     As shown in FIG. 7A, the address strobe signal bRASCAS input to an input buffer IB 11  is input to a driver DR 1  via an inverter I 21 . An output from the driver DR 1  is input to an inverter I 22  and an inverter I 23 , which constitute a latch circuit, via a clocked inverter CI 1 , and a signal RASLTC is output from an output portion of the latch circuit. An output from the driver DR 1  is input via a clocked inverter CI 2  to an inverter I 24  and an inverter I 25  which constitute a latch circuit. A signal CASLTC is output from an output portion of this latch circuit. It should be noted that a signal bCMDLTC is input to a control terminal of the clocked inverter CI 1  and a signal CMDLTC is input to a control terminal of the clocked inverter CI 2 . 
     In the circuit shown in FIG. 7A, when an “L” level of a signal bRASCAS is input to the input buffer IB 11 , the signal is inverted by the inverter I 21  to an “H” level, which is then supplied to an input portion of the clocked inverter CI 1  via the driver DR 1 . The clocked inverter CI 1  is activated only when the signal bCMDLTC input to the control terminal is at an “L” level, and an “H” level signal supplied to the input portion is inverted to an “L” level, which is then supplied to the latch circuit consisting of the inverters I 22  and I 23 . This latch circuit inverts the “L” level signal supplied there to an “H” level, which is output as a signal RASLTC. It should be noted that the signal bCMDLTC input to the control terminal of the clocked inverter CI 1  is at an “H” level, an output from the clocked inverter CI 1  is in a high impedance state, and no signal is output from the output portion. Therefore, the signal latched in the latch circuit is output directly as a signal RASLTC. 
     The “H” level signal output from the inverter I 21  is supplied to the input portion of the clocked inverter CI 2  via the driver DR 1 . The clocked inverter CI 2  is activated only when the signal CMDLTC input to the control terminal is at an “L” level, and an “H” level signal supplied to the input portion is inverted to an “L” level, which is then supplied to the latch circuit consisting of the inverters I 24  and I 25 . This latch circuit inverts the “L” level signal supplied there to an “H” level, which is output as a signal CASLTC. It should be noted that the signal CMDLTC input to the control terminal of the clocked inverter CI 2  is at an “H” level, no signal is output from the output portion of the clocked inverter CI 2 . Therefore, the signal latched in the latch circuit is output directly as a signal CASLTC. 
     As shown in FIG. 7B, the signal bWE input to an input buffer IB 12  is input to a driver DR 2  via an inverter I 26 . An output from the driver DR 2  is input to an inverter I 27  and an inverter I 28 , which constitute a latch circuit, via a clocked inverter CI 3 , and a signal WELTC-o is output from an output portion of the latch circuit. An output from the driver DR 2  is input via a clocked inverter CI 4  to an inverter I 29  and an inverter I 30  which constitute a latch circuit. A signal WELTC-e is output from an output portion of this latch circuit. It should be noted that a signal bCMDLTC is input to a control terminal of the clocked inverter CI 3  and a signal CMDLTC is input to a control terminal of the clocked inverter CI 4 . 
     In the circuit shown in FIG. 7B, when an “L” level of a signal bWE is input to the input buffer IB 12 , the signal is inverted by the inverter I 26  to an “H” level, which is then supplied to an input portion of the clocked inverter CI 3  via the driver DR 2 . The clocked inverter CI 3  is activated only when the signal bCMDLTC input to the control terminal is at an “L” level, and an “H” level signal supplied to the input portion is inverted to an “L” level, which is then supplied to the latch circuit consisting of the inverters I 27  and I 28 . This latch circuit inverts the “L” level signal supplied there to an “H” level, which is output as a signal WELTC-o. It should be noted that the signal bCMDLTC input to the control terminal of the clocked inverter CI 3  is at an “H” level, no signal is output from the clocked inverter CI 3 . Therefore, the signal latched in the latch circuit is output directly as a signal WELTC-o. 
     The “H” level signal output from the inverter I 26  is supplied to the input portion of the clocked inverter CI 4  via the driver DR 2 . The clocked inverter CI 4  is activated only when the signal CMDLTC input to the control terminal is at an “L” level, and an “H” level signal supplied to the input portion is inverted to an “L” level, which is then supplied to the latch circuit consisting of the inverters I 29  and I 30 . This latch circuit inverts the “L” level signal supplied there to an “H” level, which is output as a signal WELTC-e. It should be noted that the signal CMDLTC input to the control terminal of the clocked inverter CI 4  is at an “H” level, no signal is output from the output portion of the clocked inverter CI 4 . Therefore, the signal latched in the latch circuit is output directly as a signal WELTC-e. 
     As shown in FIG. 7C, the signal bCS input to an input buffer IB 13  is input to a driver DR 3  via an inverter I 31 . An output from the driver DR 3  is input to an inverter I 32  and an inverter I 33 , which constitute a latch circuit, via a clocked inverter CI 5 , and a signal CSLTC-o is output from an output portion of the inverter I 32 . An output from the driver DR 3  is input via a clocked inverter CI 6  to an inverter I 34  and an inverter I 35  which constitute a latch circuit. A signal CSLTC-e is output from an output portion of the inverter I 34 . It should be noted that a signal bCMDLTC is input to a control terminal of the clocked inverter CI 5  and a signal CMDLTC is input to a control terminal of the clocked inverter CI 6 . 
     In the circuit shown in FIG. 7C, when an “L” level of a signal bCS is input to the input buffer IB 13 , the signal is inverted by the inverter I 31  to an “H” level, which is then supplied to an input portion of the clocked inverter CI 5  via the driver DR 3 . The clocked inverter CI 5  is activated only when the signal bCMDLTC input to the control terminal is at an “L” level, and an “H” level signal supplied to the input portion is inverted to an “L” level, which is then supplied to the latch circuit consisting of the inverters I 32  and I 33 . This latch circuit inverts the “L” level signal supplied there to an “H” level, which is output as a signal CSLTC-o. It should be noted that the signal bCMDLTC input to the control terminal of the clocked inverter CI 5  is “H”, no signal is output from the clocked inverter CI 5 . Therefore, the signal latched in the latch circuit is output directly as a signal CSLTC-o. 
     The “H” level signal output from the inverter I 31  is supplied to the input portion of the clocked inverter CI 6  via the driver DR 3 . The clocked inverter CI 6  is activated only when the signal CMDLTC input to the control terminal is at an “L” level, and an “H” level signal supplied to the input portion is inverted to an “L” level, which is then supplied to the latch circuit consisting of the inverters I 34  and I 35 . This latch circuit inverts the “L” level signal supplied there to an “H” level, which is output as a signal CSLTC-e. It should be noted that the signal CMDLTC input to the control terminal of the clocked inverter CI 6  is at an “H” level, no signal is output from the output portion of the clocked inverter CI 6 . Therefore, the signal latched in the latch circuit is output directly as a signal CSLTC-e. 
     In the circuits shown in FIGS. 7A to  7 C, when “H” levels of a signal bRASCAS, a signal bWE and a signal bCS are input to each of the input buffers IB 11  to IB 13 , the signals of these portions each will have an opposite polarity, and “L” levels of a signal RASLTC, a signal CASLTC, a signal WELTC-o, a signal WELTC-e, a signal CSLTC-o and a signal CSLTC-e are output. It should be noted that the operation of each of the clocked inverters CI 1  to CI 6  to which the signal bCMDLTC or signal CMDLTC is input is the same as that described above. 
     Next, the circuit structures of the command decoders  12 B and  13 B in the command portion  30  will now be described. 
     As shown in FIG. 7D, a signal CSLTC-o is input to the first terminal of a NAND circuit NA 5 . A signal xRASLTC is input to the second terminal thereof and a signal xWELTC-o is input to the third terminal thereof. An output from the NAND circuit NA 5  is output as a signal ROW-COMMAND via a driver DR 4 . Signals xRASLTC and xWELTC-o input to the second and third terminal of the NAND circuit NA 5  indicates that they are of one of those types which are logically inverting each of the signals RASLTC and WELTC-o by an inverter or not logically inverting each of these signals. 
     There are provided the corresponding number of command decoders having the above-described structure, to that of the number of the row commands. Only in the case where all of the signals CSLTC-o, xRASLTC and xWELTC-o are at an “H” level, the signal ROW-COMMAND becomes an “L” level, and commands such as “BANK ACTIVE” and “BANK PRECHARGE” become active. 
     Further, as shown in FIG. 7E, a signal CSLTC-e is input to the first terminal of a NAND circuit NA 6 . A signal xCASLTC is input to the second terminal thereof and a signal xWELTC-e is input to the third terminal thereof. An output from the NAND circuit NA 6  is output as a signal COLUMN-COMMAND via a driver DR 5 . Signals xCASLTC and xWELTC-e input to the second and third terminal of the NAND circuit NA 6  indicates that they are of one of those types which are logically inverting each of the signals CASLTC and WELTC-e by an inverter or not logically inverting each of these signals. 
     There are provided the corresponding number of command decoders having the above-described structure, to that of the number of the column commands. Only in the case where all of the signals CSLTC-e, xCASLTC and xWELTC-e are at an “H” level, the signal COLUMN-COMMAND becomes an “L” level, and commands such as “READ” and “WRITE” become active. 
     It should be noted that, in the above-description, in the case where all of the signals CSLTC-o, RASLTC and WELTC-o input to the NAND circuit NA 5  are at an “H” level, commands are selected; however alternatively, it is also possible to select commands when all of these signals are at an “L” level. Here, it is necessary to replace the NAND circuit NA 5  with a NOR circuit. Similarly, in the above-description, in the case where all of the signals CSLTC-e, CASLTC and WELTC-e input to the NAND circuit NA 6  are at an “H” level, commands are selected; however alternatively, it is also possible to select commands when all of these signals are at an “L” level. Here, it is necessary to replace the NAND circuit NA 6  with a NOR circuit. 
     FIGS. 8A to  8 C are circuit diagrams showing a structure of the address portion  40 . 
     As shown in FIG. 8A, the address signal ADDRESS input to the input buffer IB 21  from outside is input to a driver DR 11  via an inverter I 51 . An output from the driver DR 11  is input to the input section of the clocked inverter CI 11 . The clocked inverter CI 11  is activated only when the signal bADDDLTC input to the control terminal is at an “L” level. The signal ADDRESS supplied to the input portion is inverted and the inverted signal is then supplied to the latch circuit consisting of inverters I 41  and I 42 . This latch circuit inverts the signal supplied there, and then outputs a signal ROW-ADDRESS. It should be noted that the signal bADDDLTC input to the control terminal of the clocked inverter C 11  is at an “H” level, no signal is output from the clocked inverter CI 11 . Therefore, the signal latched in the latch circuit is output directly as a signal ROW-ADDRESS. 
     The signal ADDRESS supplied from the driver DR 11  is supplied to the input portion of the clocked inverter CI 12 . The clocked inverter CI 12  is activated only when the signal ADDRESS input to the control terminal is at an “L” level, and the signal ADDRESS input to the input portion is inverted and then supplied to the latch circuit consisting of the inverters I 43  and I 44 . This latch circuit inverts the signal supplied there, and outputs a signal COLUMN-ADDRESS. It should be noted that the signal ADDDLTC input to the control terminal of the clocked inverter CI 12  is at an “H” level, no signal is output from the output portion of the clocked inverter CI 12 . Therefore, the signal latched in the latch circuit is output directly as a signal COLUMN-ADDRESS. 
     Further, as shown in FIG. 8B, the bank select signal BSn input to the input buffer IB 22  from outside is input to a driver DR 12  via an inverter I 52 . An output from the driver DR 12  is input to the input section of the clocked inverter CI 13 . The clocked inverter CI 13  is activated only when the signal bADDDLTC input to the control terminal is at an “L” level. The signal supplied to the input portion is inverted and the inverted signal is then supplied to the latch circuit consisting of inverters I 45  and I 46 . This latch circuit inverts the signal supplied there, and then outputs a signal BSILTCn-e. It should be noted that the signal bADDDLTC input to the control terminal of the clocked inverter CI 13  is at an “H” level, no signal is output from the clocked inverter CI 13 . Therefore, the signal latched in the latch circuit is output directly as a signal BSILTCn-e. 
     The signal supplied from the driver DR 12  is supplied to the input portion of the clocked inverter CI 14 . The clocked inverter CI 14  is activated only when the signal ADDRESS input to the control terminal is at an “L” level, and the signal supplied to the input portion is inverted and then supplied to the latch circuit consisting of the inverters I 47  and I 48 . This latch circuit inverts the signal supplied there, and outputs a signal BSILTCn-o. It should be noted that the signal ADDDLTC input to the control terminal of the clocked inverter CI 14  is at an “H” level, no signal is output from the output portion of the clocked inverter CI 14 . Therefore, the signal latched in the latch circuit is output directly as a signal BSILTCn-o. 
     Further, as shown in FIG. 8C, a clock signal xBSILTC 0 -e is input to a first terminal of an NAND circuit NA 11  and a clock signal xBSILTC 1 -e is input to a second terminal thereof. A signal output from an output portion of the NAND circuit NA 11  is input to a first terminal of an OR circuit RR 1  via an inverter I 49 . A clock signal xBSILTC 0 -o is input to a first terminal of an NAND circuit NA 12  and a clock signal xBSILTC 1 -o is input to a second terminal thereof. A signal output from an output portion of the NAND circuit NA 12  is input to a second terminal of the OR circuit RR 1  via an inverter I 50 . Then, a signal BS&lt;0:3&gt; is output from an output portion of the OR circuit RR 1 . It should be noted that signals xBSILTC 0 -e, xBSILTC 1 -e, xBSILTC 0 -o and xBSILTC 1 -o indicate respectively that they are of one of those types which are logically inverting the signals BSILTC 0 -e, BSILTC 1 -e, BSILTC 0 -o and BSILTC 1 -o, or those which are not logically inverting these signals. 
     Next, the operation of the semiconductor memory will be described with reference to FIGS. 10 to  13 . 
     FIG. 10 is a time chart illustrating a read-out operation in an interleave where the latency of the CAS signal is 2, the burst length is 4 and the number of banks is 4 and FIG. 11 is a time chart illustrating a write operation in an interleave where the latency of the CAS signal is 2, the burst length is 4 and the number of banks is 4. Further, FIG. 12 is a time chart illustrating a read-out operation in an interleave where the latency of the CAS signal is 2, the burst length is 2 and the number of banks is 4 and FIG. 13 is a time chart illustrating a write operation in an interleave where the latency of the CAS signal is 2, the burst length is 2 and the number of banks is 4. 
     First, the following is an explanation of the read-out operation in an interleave shown in FIG. 10 where the latency of the CAS signal is 2, the burst length is 4 and the number of banks is 4. 
     That is, as shown in FIG. 10, during a period where an address strobe signal bRASCAS becomes an “L” level, a row address RA is fetched in synchronism with a rising edge (a leading edge) of a clock signal CLK. Then, when the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RB is fetched in synchronism with the rising edge of the clock signal, and then a column address CA is fetched in synchronism with a falling edge (a trailing edge) of the clock signal CLK. 
     Similarly, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RC is fetched in synchronism with a rising edge of the clock signal CLK, and then a column address CB is fetched in synchronism with the falling edge of the clock signal CLK. Similarly, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RD is fetched in synchronism with a rising edge of the clock signal CLK, and then a column address CC is fetched in synchronism with the falling edge of the clock signal CLK. Further, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CD is fetched in synchronism with a falling edge of the clock signal CLK. 
     With the above-described operations, once the column address CA is fetched, immediately after the second rise of the clock signal CLK, data DA 0 , DA 1 , DA 2  and DA 3  are output. Further, without a gap, data DB 0 , DB 1 , DB 2  and DB 3  are output to follow DA 3 . Then, similarly, without a gap, data DC 0 , DC 1 , DC 2  and DC 3  are output to follow DB 3 . Then, data DD 0 , DD 1 , DD 2  and DD 3  are output to follow DC 3 . 
     It should be noted that data DA 0 , DA 1 , DA 2  and DA 3  are data read in accordance with the row address RA and column address CA. Further, data DB 0 , DB 1 , DB 2  and DB 3  are those read in accordance with the row address RB and column address CB. Data DC 0 , DC 1 , DC 2  and DC 3  are data read in accordance with the row address RC and column address CC. Further, data DD 0 , DD 1 , DD 2  and DD 3  are those read in accordance with the row address RD and column address CD. 
     As described above, in one cycle of the clock signal CLK, the row address signal is fetched in synchronism with a rising edge, and the column address signal is fetched in synchronism with a falling edge. In this manner, it becomes possible to transfer data without a gap in random row accessing. As a result, the effective data transfer rate can be improved. 
     Next, the following is an explanation of the write operation in an interleave shown in FIG. 11 where the latency of the CAS signal is 2, the burst length is 4 and the number of banks is 4. 
     That is, as shown in FIG. 11, during a period where an address strobe signal bRASCAS becomes an “L” level, a row address RA is fetched in synchronism with a rising edge of a clock signal CLK. Then, during the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RB is fetched in synchronism with the rising edge of the clock signal CLK, and then a column address CA is fetched in synchronism with the falling edge of the clock signal CLK. 
     Similarly, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RC is fetched in synchronism with a rising edge of the clock signal CLK, and then a column address CB is fetched in synchronism with the falling edge of the clock signal CLK. Similarly, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RD is fetched in synchronism with a rising edge of the clock signal CLK, and then a column address CC is fetched in synchronism with the falling edge of the clock signal CLK. Further, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CD is fetched in synchronism with a falling edge of the clock signal CLK. 
     With the above-described operations, from immediately after the column address CA is fetched, data DA 0 , DA 1 , DA 2  and DA 3  are written. Further, without a gap, data DB 0 , DB 1 , DB 2  and DB 3  are written to follow DA 3 . Then, similarly, without a gap, data DC 0 , DC 1 , DC 2  and DC 3  are written to follow DB 3 . Then, data DD 0 , DD 1 , DD 2  and DD 3  are written to follow DC 3 . 
     It should be noted that data DA 0 , DA 1 , DA 2  and DA 3  are written successively in four addresses whose leading address is selected in accordance with the row address RA and column address CA. Further, data DB 0 , DB 1 , DB 2  and DB 3  are written successively in four addresses whose leading address is selected in accordance with the row address RB and column address CB. Data DC 0 , DC 1 , DC 2  and DC 3  are written successively in four addresses whose leading address is selected in accordance with the row address RC and column address CC. Further, data DD 0 , DD 1 , DD 2  and DD 3  are written successively in four addresses whose leading address is selected in accordance with the row address RD and column address CD. 
     As described above, in one cycle of the clock signal CLK, the row address signal is fetched in synchronism with a rising edge, and the column address signal is fetched in synchronism with a falling edge. In this manner, it becomes possible to transfer data without a gap in random row accessing. As a result, the effective data transfer rate can be improved. 
     Next, the following is an explanation of the read-out operation in an interleave shown in FIG. 12 where the latency of the CAS signal is 2, the burst length is 2 and the number of banks is 4. 
     That is, as shown in FIG. 12, during a period where an address strobe signal bRASCAS becomes an “L” level, a row address RA is fetched in synchronism with a rising edge of a clock signal CLK. Then, during the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RB is fetched in synchronism with the rising edge of the clock signal CLK. Further, during the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RC is fetched in synchronism with the rising edge of the clock signal. 
     Furthermore, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CA is fetched in synchronism with a falling edge of the clock signal CLK, then a row address RD is fetched in synchronism with the rising edge of the clock signal CLK, and then a column address CB is fetched in synchronism with the falling edge of the clock signal CLK. Further, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CC is fetched in synchronism with the falling edge of the clock signal CLK. Then, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CD is fetched in synchronism with a falling edge of the clock signal CLK. 
     With the above-described operations, once the column address CA is fetched, immediately after the second rise of the clock signal, data DA 0  and DA 1  are output. Further, without a gap, data DB 0  and DB 1  are output to follow DA 1 . Then, similarly, without a gap, data DC 0  and DC 1  are output to follow DB 1 . Then, data DD 0  and DD 1  are output to follow DC 1 . 
     It should be noted that data DA 0  and DA 1  are data read in accordance with the row address RA and column address CA. Further, data DB 0  and DB 1  are those read in accordance with the row address RB and column address CB. Data DC 0  and DC 1  are data read in accordance with the row address RC and column address CC. Further, data DD 0  and DD 1  are those read in accordance with the row address RD and column address CD. 
     As described above, the row address signal is fetched in synchronism with a rising edge of the clock signal CLK, and the column address signal is fetched in synchronism with a falling edge of the clock signal CLK. In this manner, it becomes possible to transfer data without a gap in random row accessing. As a result, the effective data transfer rate can be improved. 
     Next, the following is an explanation of the read-out operation in an interleave shown in FIG. 13 where the latency of the CAS signal is 2, the burst length is 2 and the number of banks is 4. 
     That is, as shown in FIG. 13, during a period where an address strobe signal bRASCAS becomes an “L” level, a row address RA is fetched in synchronism with a rising edge of a clock signal CLK. Then, during the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RB is fetched in synchronism with the rising edge of the clock signal CLK. Further, during the address strobe signal bRASCAS becomes an “L” level for the next time, a row address RC is fetched in synchronism with the rising edge of the clock signal CLK. 
     Furthermore, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CA is fetched in synchronism with a falling edge of the clock signal CLK, then a row address RD is fetched in synchronism with the rising edge of the clock signal CLK, and then a column address CB is fetched in synchronism with the falling edge of the clock signal CLK. Further, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CC is fetched in synchronism with the falling edge of the clock signal CLK. Then, during a period where the address strobe signal bRASCAS becomes an “L” level for the next time, a column address CD is fetched in synchronism with a falling edge of the clock signal CLK. 
     With the above-described operations, immediately after the column address CA is fetched, data DA 0  and DA 1  are written. Further, without a gap, data DB 0  and DB 1  are written to follow DA 1 . Then, similarly, without a gap, data DC 0  and DC 1  are written to follow DB 1 . Then, data DD 0  and DD 1  are written to follow DC 1 . 
     It should be noted that data DA 0  and DA 1  are written successively in two addresses whose leading address is selected in accordance with the row address RA and column address CA. Further, data DB 0  and DB 1  are written successively in two addresses whose leading address is selected in accordance with the row address RB and column address CB. Data DC 0  and DC 1  are written successively in two addresses whose leading address is selected in accordance with the row address RC and column address CC. Further, data DD 0  and DD 1  are those read in accordance with the row address RD and column address CD. 
     As described above, the row address signal is fetched in synchronism with a rising edge of the clock signal CLK, and the column address signal is fetched in synchronism with a falling edge of the clock signal CLK. In this manner, it becomes possible to transfer data without a gap in random row accessing. As a result, the effective data transfer rate can be improved. 
     According to the embodiment discussed above, not only input/output of data, but also the command signals, that is, the address signal, bank selection signal, row address strobe signal bRAS, column address strobe signal bCAS, chip select signal bCS and write enable signal bWE, are handled by DDR. In this manner, it becomes possible to transfer data without a gap in random row accessing. As a result, the effective data transfer rate can be improved. 
     Further, when such a limitation is set that a row address is fetched at a rising edge of a clock signal and a column address is fetched at a falling edge of the clock signal, it becomes possible to reduce the number of signals RAS and CAS, that is, 2 in the conventional SDRAM case, to only 1, or reduce the 4-bit signal conventionally used in decoding of a command to a 3-bit signal. In this manner, the structure of the circuit of the command decoder can be simplified, and it becomes possible to reduce the layout area for the circuit, increase the processing speed and realize a low-consumption power. 
     In a conventional synchronous DRAM (DDR-SDRAM), a row address and a column address cannot be given at the same time, and therefore gaps are created while transferring data especially when row addresses are input at random. In the embodiment, the fetching of address signals are done in DDR, and the row address signal is fetched in synchronism with a rising signal of a clock and the column address signal is fetched in synchronism with a falling signal of a clock. With these settings, a gap created while transferring data by the conventional synchronous DRAM can be erased, thereby making it possible to enhance the efficiency of data transfer. 
     Further, according to the embodiment, such a limitation is set that a row address is fetched at a rising edge of a clock signal and a column address is fetched at a falling edge of a clock signal, and therefore it becomes possible to reduce the number of signals bRAS and bCAS input to the conventional SDRAM to only 1. In this manner, the structure of the circuit of the command decoder can be simplified, and the layout area for the circuit can be reduced, and a low-consumption power can be realized. 
     As described in the above embodiment, when such setting is made that the row address signal is fetched in synchronism with a rising edge of a clock signal and the column address signal is fetched in synchronism with a falling edge of a clock, a gap created while transferring data by the conventional synchronous DRAM can be erased, thereby making it possible to enhance the efficiency of data transfer without preparing address pins, address buses and address buffers provided separately and exclusively for row address and column address. 
     To summarize, according to the present invention, in a semiconductor memory which carries out a DDR (double data rate) operation, especially, DRAM, not only input/output of data, but also the command signals, that is, the address signal, bank selection signal, row address strobe signal bRAS, column address strobe signal bCAS, chip select signal bCS and write enable signal bWE, are handled by DDR. In this manner, it becomes possible to improve the effective data transfer rate. Further, not simply handling these signals in the DDR mode, but each signal is handled in accordance with its role of the signal, that is, for example, some signals are limited such that they are fetched in synchronism with rising edges of external clock signals, while others are fetched in synchronism with falling edges of external clock signals. In this manner, the number of signals input from outside can be reduced. Thus, the circuit can be simplified in structure, and the chip area can be reduced and the circuit operation speed can be increased. 
     As described above, according to the present invention, it is possible to provide a semiconductor memory capable of enhancing the data transfer efficiency and increasing the circuit operation speed without increasing its chip area. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.