Patent Publication Number: US-6667933-B2

Title: Semiconductor memory and method of operating the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to technologies for low power consumption and high speed of a semiconductor memory. 
     The present invention also relates to a semiconductor memory having memory cells with capacitors, and more particularly to a technology for performing refresh of the memory cells internally and automatically. 
     2. Description of the Related Art 
     Semiconductor memories such as a DRAM typically use common address terminals to receive address signals as a row address and a column address separately, and perform read, write, or other operations. For example, a 64-Mbit DRAM having an 8-bit I/O (address space; 8 Mbits) uses two bank address terminals and twelve address terminals to receive a 12-bit row address signal and a 9-bit column address signal. This DRAM, for example, receives an active command and a row address signal (upper address) in synchronization with a first clock signal, and receives a read command and a column address (lower address) in synchronization with the next clock signal. Here, all the word lines corresponding to the row address are activated to read out the data retained in the memory cells. The read data is amplified by sense amplifiers. In this example, the activation of the word lines selects one out of 16 k memory regions so that data is rewrite of 4 k memory cells. Then, the memory cells to read/write data from/to are selected by the column address signal, followed by a read operation or a write operation. 
     Now, a rise in the number of bits of the row address signal can reduce the size of a memory region to be selected at a time, and decrease the number of word lines to be activated simultaneously. The power consumption during read and write operations depends on the number of word lines to be activated as well as the number of sense amplifiers to be operated in accordance with the activation of the word lines. Thus, the greater the number of bits of the row address signal is, the lower the operating power consumption becomes. 
     For example, if the above-mentioned DRAM has two bank address terminals and thirteen address terminals to receive a 13-bit row address and an 8-bit column address, the number of word lines to be activated at a time will be cut by half, with a reduction in power consumption. 
     The greater number of bits of the row address signal, however, increases the number of terminals on the package, which makes the package larger in outside dimensions. As a result, the system board to mount the DRAM on comes to drop in parts mounting density. The parts mounting density can also be lowered by a rise in the number of lines on an address bus and an increase in the area of patterned wiring on the system board. The increased number of address terminals also leads to a greater chip size. 
     Meanwhile, a great number of DRAMs are used not only in mainframes and personal computers but also in consumer appliances. Generally, most DRAMs to be mounted on consumer appliances have only to operate at several tens of megahertz. Conventional DRAMs, however, have not been developed specifically for consumer appliances. Therefore, the consumer appliances to mount DRAMs on have had to adopt high-speed DRAMs targeted for computers. DRAMs of this type are developed for high-speed operations, and are often high in power consumption. In the fields of battery-driven consumer appliances such as cellular phones and digital cameras, DRAMs of low power consumption have been hoped for. 
     DRAM memory cells store information by having their capacitors charged. This requires that DRAM-mounted systems refresh memory cells at predetermined intervals to maintain the information written in the memory cells. Read and write operations to memory cells cannot be performed during refresh operations. This means the tendency of the above-mentioned systems to drop in I/O bus occupation rate. In particular, concerning DRAMs to be operated at lower frequencies which have been hoped for in the fields of consumer appliances and the like, there has been made no proposal for optimizing refresh operations thereof to improve the I/O bus occupation rate. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce the power consumption of a semiconductor memory. 
     Another object of the present invention is to optimize the refresh operations of memory cells in a DRAM being operative at a low frequency, for the sake of improving an I/O bus occupation rate. 
     Still another object of the present invention is to reduce the time taken for the supply of a command to the completion of a read or write operation in the semiconductor memory being operative at a low frequency. 
     According to one of the aspects of the present invention, a semiconductor memory receives a plurality of commands in succession and performs a memory operation to read/write data from/to memory cells in accordance with the combination of these commands. Here, a word line for controlling transfer switches of the memory cells is activated after the reception of one of the commands except the first command. This allows a control circuit for activating the word line to be operated at a frequency lower than heretofore, which reduces the power consumption. 
     There is a sufficient period between the supply of the first command and the activation of the word line. Therefore, this period can be utilized to operate internal circuits without being recognized from exterior. The internal circuits include, for example, a refresh control circuit for automatically performing a refresh operation of the memory cells, and a self-test circuit for checking circuit functions. 
     According to another aspect of the present invention, address signals for designating the memory cells to be operated are supplied along with the commands. The word line is activated based on the address signal supplied along with the first command as well as part of the address signal supplied along with at least one of the commands except the first command. Activating the word line by using more address signals than heretofore can reduce the number of word lines to be activated simultaneously. In other words, the memory regions to be selected by these address signals become smaller. As a result, the scale of the circuits to be operated for memory operations can be decreased for a reduction in power consumption. 
     According to another aspect of the present invention, the word line includes a main-word line and a plurality of sub-word lines branching off from this main-word line. The sub-word lines are connected to the transfer switches of the memory cells. The main-word line corresponds to, for example, an upper address and is activated during a plurality of memory operations. The sub-word lines correspond to, for example, lower addresses and are activated upon each of the memory operations. Activating the plurality of sub-word lines in succession can operate all the memory cells that are selectable by the activated main-word line. That is, consecutive accesses typically referred to as page operations can be performed in a wider memory region. 
     According to another aspect of the present invention, the word line includes a main-word line and a plurality of sub-word lines branching off from this main-word line, the sub-word lines being connected to the transfer switches of the memory cells. The main-word line and the sub-word lines are activated during a plurality of memory operations. Performing page operations without inactivating the sub-word lines allows a reduction in the power consumption of a control circuit for the sub-word lines. That is, the power consumption during operations can be reduced further. 
     According to another aspect of the present invention, the word line includes a main-word line and a plurality of sub-word lines branching off from this main-word line, the sub-word lines being connected to the transfer switches of the memory cells. The main-word line is activated in accordance with the address signal supplied along with the first command. The sub-word lines are activated in accordance with the address signal supplied along with at least one of the commands except the first command. The pre-activation of the main-word line can reduce the period between the supply of the second command to the activation of the sub-word lines. As a result, page operations can be performed at high speed. Besides, if there is any redundancy circuit for relieving the memory cells and the like, a relief judgment can be performed at high speed. Moreover, if redundancy is given on a main-word-line basis, a relief judgment can be performed in between an active command and a rise of the main-word line so that the judging time of the redundancy circuit is included into the operating time of other circuits. That is, the operation of the redundancy circuit can be prevented from becoming critical. 
     According to another aspect of the present invention, a plurality of bit lines are respectively connected to the plurality of memory cells connected to the sub-word lines. A precharging circuit for setting the bit lines to a predetermined voltage releases a part of the bit lines from a precharge operation in accordance with the address signal supplied along with the first command. Thus, the release of the precharge operation at an earlier time of memory operation can reduce the time taken for the supply of the first command to the completion of a read or write operation. 
     According to another aspect of the present invention, a plurality of memory blocks having the memory cells are composed of a plurality of segments. The precharging circuit releases the bit lines from a precharge operation with respect to each of the segments. This can decrease the number of circuits to be operated in read and write operations, which reduces the power consumption during operation. 
     According to another aspect of the present invention, one of a plurality of memory blocks having the memory cells is selected in accordance with the address signal supplied along with the first command. Selecting a memory block at an earlier time of memory operation can decrease the number of circuits to start operation in response to the second and subsequent commands. As a result, the time taken for the supply of the first command to the completion of a read or write operation can be reduced. That is, it is possible to achieve a speedup while maintaining the characteristic of low power consumption. 
     According to another aspect of the present invention, the memory blocks are composed of a plurality of segments. A plurality of bit lines are respectively connected to the plurality of memory cells connected to the sub-word lines. A plurality of sense amplifiers are formed commonly for a pair of memory blocks, and amplify data on the bit lines within the blocks. That is, the sense amplifiers each are supplied with a single memory block. A plurality of bit line selecting switches establish connections between the bit lines of each memory block and the sense amplifiers, respectively. The bit line selecting switches are selected with respect to each of the segments, in accordance with the address signal supplied along with the first command. Therefore, the connections between the bit lines of a memory block to be operated and sense amplifiers can be established at an earlier time of memory operation. In other words, the bit lines of memory blocks not to be operated and sense amplifiers can be disconnected. As a result, the time taken for the supply of the first command to the completion of a read or write operation can be reduced. 
     Since the bit lines and the sense amplifiers are connected segment by segment, the number of circuits to be operated in read and write operations can be decreased for a reduction in the power consumption during operations. 
     According to another aspect of the present invention, a refresh operation for rewriting data retained in the volatile memory cells is performed by selecting the bit line switches in a plurality of segments at once. More segments can be operated during refresh than in read and write operations, which reduces the number of times of refresh necessary for refreshing all the memory cells. Accordingly, refresh intervals can be extended to increase the ratio of read and write operations within a predetermined period. This means an improvement in I/O bus occupation rate (data transmission rate). 
     According to another aspect of the present invention, a decoder generates a decoding signal for selecting one of the sub-word lines in accordance with the address signals. An activation control circuit activates the decoder in response to the second command alone. The activation control circuit will not respond to the third and subsequent commands. Accordingly, the sub-word lines selected in response to the second command are also valid for the third and subsequent commands. Among the address signals supplied along with the third and subsequent commands, address signals for selecting sub-word lines are ignored. This can surely prevent the semiconductor memory from malfunctions such as multiple selection of word lines. The decoder can be reduced in power consumption since it performs no operation in response to the third and subsequent commands. 
     According to another aspect of the present invention, in a refresh operation for rewriting data retained in the volatile memory cells, the main-word line and the sub-word lines are selected in accordance with the address signal corresponding to the first command. This allows a start of a refresh operation in response to the first command so that the refresh operation can be performed faster than read and write operations. Accordingly, within a predetermined period, the ratios of read and write operations can be increased to improve the I/O bus occupation rate (data transfer rate). 
     According to another aspect of the present invention, the semiconductor memory includes a refresh occurring circuit, a buffer, and a refresh control circuit. The refresh occurring circuit generates at predetermined intervals a refresh request for refreshing the memory cells. The buffer holds the refresh request. The refresh control circuit performs a refresh operation based on the refresh request held by the buffer when the memory operation is not in execution. This semiconductor memory has a sufficient period between the completion of the memory operation and the activation of word lines for the next memory operation. In accordance with the refresh request held by the buffer, a refresh operation can be performed in this period so that the refresh operation is performed without being recognized from exterior. That is, the refresh of the memory cells can be performed internally and automatically. 
     According to another aspect of the present invention, the semiconductor memory includes a plurality of buffers for alternately holding the refresh request. During page operations, no refresh operation can be performed even if refresh requests occur. For this reason, the maximum number of page operations is limited in accordance with a refresh period, or the interval at which a refresh request occur. Since refresh requests can be held as many as the buffers, it is possible to increase the maximum number of page operations. 
     According to another aspect of the present invention, the commands are supplied in synchronization with a clock signal. The refresh control circuit performs a refresh operation in synchronization with the clock signal which is supplied when the memory operation is not in execution. This facilitates the timing design of the circuits necessary for refresh control. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by identical reference numbers, in which: 
     FIG. 1 is a block diagram showing a first embodiment of the present invention; 
     FIG. 2 is a timing chart showing read operations in the first embodiment; 
     FIG. 3 is another timing chart showing read operations in the first embodiment; 
     FIG. 4 is a circuit diagram showing a refresh control circuit according to a second embodiment of the present invention; 
     FIG. 5 is a timing chart showing a read operation in the second embodiment; 
     FIG. 6 is another timing chart showing a read operation in the second embodiment; 
     FIG. 7 is a circuit diagram showing a refresh control circuit according to a third embodiment of the present invention; 
     FIG. 8 is a timing chart showing a read operation in the third embodiment; 
     FIG. 9 is another timing chart showing a read operation in the third embodiment; 
     FIG. 10 is another timing chart showing a read operation in the third embodiment; 
     FIG. 11 is a circuit diagram showing a refresh control circuit according to a fourth embodiment of the present invention; 
     FIG. 12 is a timing chart showing a read operation in the fourth embodiment; 
     FIG. 13 is another timing chart showing a read operation in the fourth embodiment; 
     FIG. 14 is a circuit diagram showing a refresh control circuit according to a fifth embodiment of the present invention; 
     FIG. 15 is a block diagram showing a sixth embodiment of the present invention; 
     FIG. 16 is a block diagram showing the details of the memory arrays shown in FIG. 15; 
     FIG. 17 is a block diagram showing the details of memory blocks shown in FIG. 16; 
     FIG. 18 is an explanatory diagram showing the uses of address signals within the SDRAM; 
     FIG. 19 is a circuit diagram showing the details of sub-word decoders and a sense amplifier shown in FIG. 17; 
     FIG. 20 is a timing chart showing read operations in the sixth embodiment; 
     FIG. 21 is another timing chart showing a read operation in the sixth embodiment; 
     FIG. 22 is a block diagram showing a seventh embodiment of the present invention; and 
     FIG. 23 is a timing chart showing the operations of the SDRAM in the seventh embodiment. 
     FIG. 24 is a timing chart showing read operations of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     FIG. 1 shows a first embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. 
     This semiconductor memory is formed as a 64-Mbit SDRAM (Synchronous DRAM)  10  on a silicon substrate by using CMOS processes. This SDRAM  10  is designed chiefly for consumer appliances, and has a maximum clock frequency of 40 MHz (lower than or equal to a half that of the fastest SDRAMs). For the sake of excellent usability, the SDRAM  10  has the function of refreshing its memory cells internally and automatically. Consequently, users can design system boards to mount the SDRAM on without any consideration of refresh. 
     The SDRAM  10  has the same terminal configuration as that of ordinary 64-Mbit SDRAMs. That is, the SDRAM  10  has twelve address terminals for receiving a 12-bit row address signal and a 9-bit column address signal, two bank select terminals, and eight data input/output terminals. 
     As shown in FIG. 1, the SDRAM  10  has a refresh control circuit  12 , a refresh counter  14 , latches  16  and  18 , address selectors  20  and  22 , a timing generator  24 , and four banks BK. 
     The banks BK each include a timing generator  26 , a row predecoder  28 , a segment selector  30 , a column predecoder  32 , and a memory core  34 . The memory core  34  has a memory array MA which is composed of a plurality of segments S arranged vertically and horizontally. In this example, each single segment S is composed of 256 k memory cells. A row main decoder  36  and a column main decoder  38 , as well as not-shown sense amplifiers, data input/output switches, and the like, are arranged around the memory array MA. The row main decoder  36  selects, through main-word lines MWL, segments S which align in the vertical direction of the diagram. The column main decoder  38  selects, through not-shown column switches and the like, segments S which align in the horizontal direction of the diagram. The main-word lines MWL each are connected to a plurality of sub-word lines SWL via switches such as transistors. The sub-word lines SWL are connected to the gates of the transfer transistors (transfer switches) in the memory cells. 
     The refresh control circuit  12  supplies control signals such as an internal refresh signal IREF and an internal row address strobe signal IRAS to individual circuits, thereby controlling the refresh operations of the memory cells. More specifically, a not-shown refresh occurring circuit (oscillator) generates a refresh request at regular intervals. The refresh request is temporarily held by a buffer (not shown) in the refresh control circuit  12 , and output as the internal refresh signal IREF when the memory cores  34  are not in operation. The internal row address strobe signal IRAS is activated when operating the memory cores  34  (when selecting main-word lines MWL). 
     In this embodiment, the memory cores  34  are designed so that 16 k memory cells (4 k a bank) are refreshed by each single refresh operation. Therefore, generating a refresh request at every 15.6 μs or so can maintain data in all the memory cells (16 k cells×(64 ms/15.6 μs)=64M cells). 
     In order for the refresh request to occur at the regular intervals, the maximum time (timing specification) of tRAS (/RAS active time) is set at 15.6 μs. tRAS is the time that elapses from the reception of an active command ACT to the reception of a precharge command PRE, in which read/write operations can be performed in succession with word lines selected. 
     The refresh counter  14  generates a refresh address REFAD. The latch  16  latches an address signal AD, supplied through the address terminals, as a row address signal RAD in synchronization with an active command signal ACTV. The active command signal ACTV is activated upon the reception of the active command ACT for activating the memory cores  34 . The latch  18  latches the address signal AD as a column address signal CAD in synchronization with a read/write command signal RW. The read/write command signal RW is activated upon the reception of a read command RD for performing a read operation and a write command WR for performing a write operation. 
     The address selector  20  transmits the row address signal RAD as an internal row address signal IRAD in normal operations (IREF; low level). In refresh operations, the address selector  20  transmits the refresh address REFAD (upper address) as the internal row address signal IRAD. The address selector  22  transmits the column address signal CAD as an internal column address signal ICAD in normal operations. In refresh operation, the address selector  22  transmits the refresh address (lower address) as the internal column address signal ICAD. The timing generator  24  receives the read/write command signal RW and the refresh signal IREF, and generates a timing signal for operating the column predecoder  32  and a timing signal for operating the segment selector  30 . 
     The timing generators  26  receive the internal row address strobe signal IRAS, and generate a timing signal for operating the row predecoder  28 . The row predecoder  28  decodes the internal row address signal IRAD, and output the decoded signal to the row main decoder  36 . The segment selector  30  decodes the internal column address signal ICAD to select a segment S and sub-word lines SWL in the segment S. The column predecoder  32  decodes the internal column address signal ICAD, and output the decoded signal to the column main decoder  38 . 
     FIG. 2 shows the timing of normal read operations with no occurrence of refresh requests. 
     Initially, an active command ACT and an address signal AD (RAD 1 ) are supplied in synchronization with the rising edge of a clock signal CLK (FIG.  2 ( a )). The latch  16  shown in FIG. 1 latches the address signal AD in synchronization with the rising edge of the active command signal ACTV, and outputs the latched signal as the row address signal RAD. 
     The address selector  20  receives the refresh signal IREF of low level, and transmits the row address signal RAD as the internal row address signal IRAD (FIG.  2 ( b )). Then, the row predecoder  28  and the row main decoder  36  operate at predetermined timing, activating a main-word line MWL (FIG.  2 ( c )). Here, the row predecoder  28  is activated at timing later than heretofore, so that the main-word line MWL is activated later than heretofore (the broken lines). Because of the operating frequency of 40 MHz, the late activation timing of the main-word line MWL will not hinder the proper execution of subsequent read operations. Since the control circuits for activating the main-word line MWL operate at a frequency lower than heretofore, the power consumption of these control circuits becomes smaller. 
     Unlike heretofore (the broken lines), no sub-word lines SWL are inactivated at this point. That is, the main-word line MWL is activated in accordance with the address signal AD that is supplied along with the first active command ACT. Activating the main-word line MWL based on the active command ACT allows a relief judgment to be performed quickly if there is any redundancy circuit for relieving memory cells and the like. Since the main-word line MWL and the sub-word lines SWL are not activated simultaneously, electric currents arising from word line activation decrease in their peaks. 
     A read command RD and an address signal AD (CAD 1 ) are supplied in synchronization with the rise of the next clock signal CLK (FIG.  2 ( d )). The latch  18  latches the address signal AD in synchronization with the rising edge of the read/write command signal RW, and outputs the latched signal as the column address signal CAD. The address selector  22  receives the refresh signal IREF of low level, and transmits the column address signal CAD as the internal column address signal ICAD (FIG.  2 ( e )). The segment selector  30  selects a segment S according to the internal column address signal ICAD, and activate a plurality of sub-word lines SWL( 1 ) in the segment S (FIG.  2 ( f )). That is, the sub-word lines SWL are activated in accordance with the address signal AD that is supplied along with the second command (read command RD). Then, the column predecoder  32  and the column main decoder  38  operate to output read data (FIG.  2 ( g )). 
     Conventionally, all the sub-word lines SWL corresponding to the row address signal RAD were activated. In this embodiment, the sub-word lines SWL are activated by using not only the row address signal RAD but also the column address signal CAD. Therefore, it is possible to reduce the number of sub-word lines SWL to be activated in a read operation. This consequently reduces the power necessary for the word line activation and decreases the number of sense amplifiers to operate, thereby lowering the power consumption in a read operation. For SDRAMs with boosted word lines, their boosters may have lower capacities than heretofore. This means a particularly higher effect of reducing the power consumption. Power consumption is similarly reduced in write operations. 
     In synchronization with the rising edge of the next clock signal CLK, a read command RD and an address signal AD (CAD 2 ) are supplied (FIG.  2 ( h )). The segment selectors  30  select a segment S according to the internal column address signal ICAD, and activate a plurality of sub-word lines SWL in the segment S. Then, read operations (page operations) are performed based on the read commands RD supplied in succession. Since the main-word line MWL is kept activated here, data can be read from the segments S which align in the vertical direction of FIG.  1 . That is, the read operations can be performed over a wide region of the memory array MA. Moreover, the pre-activation of the main-word line MWL shortens the period between the supply of the read commands RD and the activation of sub-word lines SWL. As a result, the page operations can be performed at high speed. The main-word line MWL is inactivated after the reception of the precharge command PRE (FIG.  2 ( i )). 
     FIG. 3 shows the timing of read operations for situations where a refresh request occurs internally just before the reception of the active command ACT. 
     Initially, the refresh control circuit  12  shown in FIG. 1 activates the refresh signal IREF in response to the refresh request (FIG.  3 ( a )). Then, the active command ACT and an address signal AD (RAD 2 ) are supplied in synchronization with the rising edge of the clock signal CLK (FIG.  3 ( b )). The latch  16  latches the address signal AD in synchronization with the rising edge of the active command signal ACTV. The refresh control circuit  12  activates the internal row address strobe signal IRAS with some delay after the activation of the refresh signal IREF. 
     The address selectors  20  and  22  receive the refresh signal IREF of high level, and output the refresh address REFAD as the internal row address signal IRAD (upper address) and the internal column address signal ICAD (lower address), respectively (FIG.  3 ( c )). 
     The timing generators  24  and  26  receive the refresh signal IREF and the internal row address strobe signal IRAS, and output timing signals to the row predecoder  28  and the segment selector  30 , respectively. The row predecoder  28  and the segment selector  30  activate the main-word lines MWL and sub-word lines SWL that correspond to the refresh address REFAD 1 , thereby performing a refresh operation (FIG.  3 ( d )). 
     That is, this SDRAM  10  performs a refresh operation by utilizing the period between the completion of the previous memory operation and the activation of word lines MWL and SWL for the next memory operation. As a result, the refresh operation is performed automatically without any recognition from exterior. Therefore, SDRAM users can design system boards to mount the SDRAM on without consideration of refresh operations. 
     After the completion of the refresh operation, the refresh control circuit  12  inactivates the refresh signal IREF and the internal row address strobe signal IRAS (FIG.  3 ( e )). The main-word lines MWL and the sub-word lines SWL are inactivated in response to this inactivation (FIG.  3 ( f )). 
     The address selector  20  receives the inactivation of the refresh signal IREF, and outputs the address signal AD (RAD 2 ) as the internal row address signal IRAD (FIG.  3 ( g )). The refresh control circuit  12  reactivates the internal row address strobe signal IRAS. The row predecoder  28  and the row main decoder  36  operate in almost the same timing as in FIG. 2, activating a main-word line MWL (FIG.  3 ( h )). Then, as in FIG. 2, read commands RD and address signals AD (CAD 3 , CAD 4 ) are supplied in succession so that read operations are performed. 
     Incidentally, if a refresh request occurs during the operation of the memory cores  34 , the refresh control circuit  12  completes the operation of the memory cores  34  before the activation of the refresh signal IREF. 
     As has been described, in the semiconductor memory of the present embodiment, the main-word lines MWL are activated at timing later than heretofore. Besides, the sub-word lines SWL for controlling the transfer transistors of the memory cells are activated not after the reception of the active command ACT but after the reception of the read command RD that is supplied second. Therefore, the control circuits for activating the word lines MWL and SWL can be operated at a frequency lower than heretofore, with a reduction in power consumption. 
     The use of more address signals than heretofore reduces the number of word lines SWL to be activated at the same time. As a result, the circuits to operate for the sake of memory operations can be reduced in scale, with a reduction in power consumption. 
     Since the main-word lines MWL are kept activated over a plurality of memory operations, it is possible to perform page operations on wider memory regions. 
     The activation of the main-word lines MWL prior to that of the sub-word lines SWL can shorten the period between the supply of a read command RD to the activation of the sub-word lines SWL. As a result, page operations can be performed at high speed. 
     Since the main-word lines MWL are activated in advance, a relief judgment can be performed quickly if there is any redundancy circuit for relieving the memory cells and the like. 
     There is a sufficient period between the supply of the active command ACT and the activation of the main-word lines MWL. Using this period, a refresh operation can be performed without being recognized from exterior. That is, the refresh of the memory cells can be performed internally and automatically. 
     The provision of the buffer for holding a refresh request makes it possible to perform a refresh operation securely in the period between the completion of a memory operation and the activation of word lines for the next memory operation. 
     FIG. 4 shows a second embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. The same signals as those described in the first embodiment will be designated by identical symbols. 
     This embodiment includes a refresh control circuit  40  that differs from the refresh control circuit  12  of the first embodiment. The other configuration is identical to that of the first embodiment. That is, the sub-word lines SWL are activated by using not only a row address signal but also a column address signal which is supplied along with a read command RD (or write command). This SDRAM has a maximum clock frequency of 40 MHz, and has the function of refreshing its memory cells internally and automatically. 
     The refresh control circuit  40  comprises flip-flops  42 ,  44 ,  46 ,  48 , and  50 , delay circuits  52 ,  54 , and  56 , pulse generators  58  and  60 , a plurality of logic gates, and a not-shown refresh occurring circuit. 
     The flip-flop  42  is set in response to the activation of a refresh request signal REFRQ, turning a refresh latched signal REFL to high level. The flip-flop  42  is reset in response to the activation of a refresh precharge signal PRER, turning the refresh latched signal REFL to low level. The flip-flop  42  functions as a buffer for holding the refresh request signal REFRQ. The refresh precharge signal PRER is a signal to be automatically generated to perform a precharge operation after the completion of a refresh operation. 
     The flip-flop  44  is set in response to the activation of the active command signal ACTV, turning an active latched signal ACTL to high level. The flip-flop  44  is reset in response to the activation of a delayed precharge signal PRED, turning the active latched signal ACTL to low level. The delayed precharge signal PRED is generated from a precharge signal PREC to be activated in accordance with a precharge command PRE supplied from exterior, or an auto-precharge signal PREA to be activated in an auto-precharge operation. The delay circuit  52  delays the timing of activation of the delayed precharge signal PRED by tRP (/RAS Precharge time) after the activation of the precharge signal PREC or the auto-precharge signal PREA. The delay circuit  52  thereby inhibits a refresh request from being received during precharge. tRP is the time that elapses from the reception of the PRE command to the reception of the next ACTV command, and is necessary for a precharge operation. 
     The flip-flop  46  resets its output node in response to the activation of the refresh latched signal REFL, and sets the output node in response to the activation of the active latched signal ACTL. The pulse generator  58  generates a refresh pulse REFP of high level when the refresh latched signal REFL is activated to change the output node of the flip-flop  46  to low level. 
     The flip-flop  48  is set in response to the generation of the refresh pulse REFP or the activation of a delayed active signal ACTD, thereby activating the internal row address strobe signal IRAS. The flip-flop  48  is reset in response to the activation of the refresh precharge signal PRER or the activation of an internal precharge signal IPRE, thereby inactivating the internal row address strobe signal IRAS. 
     The delayed active signal ACTD is activated with as much delay as the delay time of the delay circuit  54  after the activation of the active command signal ACTV. The delay time of the delay circuit  54  is set to a value required for a single refresh operation and a following precharge operation. This allows a refresh cycle to be inserted into the period between the reception of the active command ACT and the activation of the internal row address strobe signal IRAS. The internal precharge signal IPRE is activated in response to the activation of the precharge signal PREC or the auto-precharge signal PREA. 
     The flip-flop  50  is set in response to the occurrence of the refresh pulse REFP, activating the internal refresh signal IREF. The flip-flop  50  is reset in response to the activation of the refresh precharge signal PRER, inactivating the internal refresh signal IREF. 
     The pulse generator  60  generates the refresh precharge signal PRER (a pulse of high level) when the signal output from the delay circuit  56  changes to high level. 
     The refresh precharge signal PRER is generated in a refresh operation, with as much delay as the delay time of the delay circuit  56  after the flip-flop  48  is set. The delay time of the delay circuit  56  is set to a time necessary for a single refresh operation. On this account, a refresh operation can be surely performed before a precharge operation. 
     FIG. 5 shows the operation timing of the refresh control circuit  40  in a read operation when a refresh request occurs internally just before the reception of the active command ACT. 
     Initially, the occurrence of the refresh request activates the refresh request signal REFRQ (FIG.  5 ( a )). The flip-flop  42  shown in FIG. 4 activates the refresh latched signal REFL in synchronization with the rising edge of the refresh request signal REFRQ (FIG.  5 ( b )). That is, the refresh request is held by the flip-flop  42 . Here, the active command ACT is yet to be supplied and the active latched signal ACTL is at low level. The flip-flop  46  is therefore reset in response to the activation of the refresh latched signal REFL. The pulse generator  58  outputs the refresh pulse REFP (FIG.  5 ( c )). 
     The flip-flop  48  is set in accordance with the refresh pulse signal REFP, activating the internal row address strobe signal IRAS (FIG.  5 ( d )). The flip-flop  50  is set in accordance with the refresh pulse signal REFP, activating the internal refresh signal IREF (FIG.  5 ( e )). Then, a refresh operation is performed in the period over which the internal row address strobe signal IRAS and the internal refresh signal IREF are activated. The internal refresh signal IREF is transmitted to the delay circuit  56  and the pulse generator  60  so that the refresh precharge signal PRER is activated upon the completion of the refresh operation (FIG.  5 ( f )). A precharge operation is performed by the activation of the refresh precharge signal PRER. 
     The flip-flops  42 ,  48 , and  50  are reset in response to the activation of the refresh precharge signal PRER, thereby inactivating the refresh latched signal REFL, the internal row address strobe signal IRAS, and the internal refresh signal IREF, respectively (FIGS.  5 ( g, h, i )). 
     Meanwhile, the supply of the active command ACT activates the active command signal ACTV (FIG.  5 ( j )). The flip-flop  44  is set in response to the activation of the active command signal ACTV, activating the active latched signal ACTL (FIG.  5 ( k )). The delay circuit  54  activates the delayed active signal ACTD in response to the activation of the active command signal ACTV (FIG.  5 ( l )). 
     The flip-flop  48  is set in response to the activation of the delayed active signal ACTD, thereby reactivating the internal row address strobe signal IRAS (FIG.  5 ( m )). Next, the read command RD is supplied to perform a read operation. Even in this embodiment, the number of sub-word lines to be activated becomes smaller than heretofore, with a reduction in power consumption. 
     Then, the supply of the precharge command PRE activates the precharge signal PREC (FIG.  5 ( n )). The flip-flop  48  is reset in response to the activation of the precharge signal PREC, inactivating the internal row address strobe signal IRAS (FIG.  5 ( o )). The delay circuit  52  delays the internal precharge signal IPRE, and outputs the resultant as the delayed precharge signal PRED (FIG.  5 ( p )). The flip-flop  44  is reset in response to the activation of the delayed precharge signal PRED, inactivating the active latched signal ACTL (FIG.  5 ( q )). Then, the precharge operation is completed. 
     The refresh operation is performed by utilizing the period between the completion of the previous memory operation and the activation of word lines MWL and SWL for the next memory operation. Consequently, as in the first embodiment, the refresh operation is performed automatically without any recognition from exterior. 
     FIG. 6 shows the operation timing of the refresh control circuit  40  for situations where a refresh request occurs internally after the reception of the active command ACT. 
     Initially, the active command ACT is supplied to activate the active command signal ACTV (FIG.  6 ( a )). The flip-flop  44  and the delay circuit  54  activate, as in FIG. 5, the active latched signal ACTL and the delayed active signal ACTD, respectively (FIGS.  6 ( b, c )). 
     The flip-flop  48  is set in response to the activation of the delayed active signal ACTD, thereby reactivating the internal row address strobe signal IRAS (FIG.  6 ( d )). Then, the read command RD is supplied so that predetermined word lines are selected for a read operation. Here, no refresh operation is performed yet. 
     After the reception of the active command ACT, a refresh request occurs to activate the refresh request signal REFRQ (FIG.  6 ( e )). The flip-flop  42  is set in response to the activation of the refresh request signal REFRQ, thereby activating the refresh latched signal REFL (FIG.  6 ( f )). That is, it holds the refresh request. The flip-flop  46 , however, has received the activation of the active latched signal ACTL before the activation of the refresh latched signal REFL. The flip-flop  46  is therefore kept set, not reset. 
     After the execution of the read operation, the precharge command PRE is supplied to activate the precharge signal PREC (FIG.  6 ( g )). The flip-flop  48  is reset in response to the activation of the precharge signal PREC, thereby inactivating the internal row address strobe signal IRAS (FIG.  6 ( h )). The delay circuit  52  delays the internal precharge signal IPRE, and outputs the resultant as the delayed precharge signal PRED (FIG.  6 ( i )). The flip-flop  44  is reset in response to the activation of the delayed precharge signal PRED, inactivating the active latched signal ACTL (FIG.  6 ( j )). 
     The flip-flop  46  is reset in response to the inactivation of the active latched signal ACTL, turning its output node to low level. The pulse generator  58  outputs the refresh pulse REFP in response to the reset of the flip-flop  46  (FIG.  6 ( k )). 
     Then, as in FIG. 5, the internal precharge signal IPRE is reactivated (FIG.  6 ( l )) and the internal refresh signal IREF is activated (FIG.  6 ( m )) to perform a refresh operation. Moreover, the refresh precharge signal PRER is activated to perform a precharge operation (FIG.  6 ( n )), whereby the refresh latched signal REFL and the internal refresh signal IREF are inactivated (FIGS.  6 ( o, p )). 
     The precharge operation is performed immediately after the supply of the next command (for example, the active command ACT). Therefore, even if a refresh operation is performed after the read operation, this refresh operation has no effect on the memory operation according to the next command. That is, the refresh operation is performed automatically without any recognition from exterior. Incidentally, the worst timing of a refresh request is one in which the refresh request occurs internally just before the reception of the active command ACT (FIG.  5 ). 
     This embodiment can offer the same effects as those obtained from the first embodiment described above. 
     FIG. 7 shows a third embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. The same circuits and signals as those described in the second embodiment will be designated by identical reference numbers or symbols. Detailed description of these circuits and signals will be omitted here. 
     This embodiment includes a refresh control circuit  62  that differs from the refresh control circuit  12  of the first embodiment. The other configuration is identical to that of the first embodiment. That is, the SDRAM of this embodiment activates its main-word lines MWL by using not only a row address signal RAD but also a column address signal CAD which is supplied along with a read command RD (or write command). The SDRAM has a maximum clock frequency of 40 MHz, and has the function of refreshing its memory cells internally and automatically. 
     The refresh control circuit  62  has registers REG 1  and REG 2  composed of flip-flops, toggle flip-flops  64  and  66 , and a delay circuit  68 . The refresh control circuit  62  also has flip-flops  44 ,  46 ,  48 , and  50 , delay circuits  52 ,  54 , and  56 , and pulse generators  58  and  60  which are the same as those of the second embodiment, along with a plurality of logic gates. The delay circuit  68  is the same as the delay circuit  52 . That is, the delay circuit  68  delays the refresh precharge signal PRER by tRP. 
     The toggle flip-flop  64  inverts the levels of select signals SEL 1  and /SEL 1  in synchronization with the falling edge of the refresh request signal REFRQ. The toggle flip-flop  66  inverts the levels of select signals SEL 2  and /SEL 2  in synchronization with the falling edge of the refresh latched signal REFL. The toggle flip-flops  64  and  66  are reset by a power-on resetting signal POR which is activated when the power supply is turned on. 
     The register REG 1  enables its setting function when the select signal SEL 1  is at high level, and enables its resetting function when the select signal SEL 2  is at high level. That is, the register REG 1  activates a request signal REQ 1  in response to the rising edge of the refresh request signal REFRQ when the select signal SEL 1  is at high level. It inactivates the request signal REQ 1  in response to the rising edge of the refresh precharge signal PRER when the select signal SEL 2  is at high level. 
     Similarly, the register REG 2  activates a request signal REQ 2  in response to the rising edge of the refresh request signal REFRQ when the select signal /SEL 1  is at high level. It inactivates the request signal REQ 2  in response to the rising edge of the refresh precharge signal PRER when the select signal /SEL 2  is at high level. Since the select signals SEL 1 , /SEL 1  and the select signals SEL 2 , /SEL 2  are both complementary signals, the registers REG 1  and REG 2  function as buffers for holding the refresh request signal REFRQ alternately. That is, in the present embodiment, the refresh control circuit  62  can hold two internally-occurring refresh requests. 
     The request signals REQ 1  and REQ 2  are transmitted to the flip-flop  46  and the toggle flip-flop  66  as the refresh latched signal REFL when the select signals SEL 2  and /SEL 2  are at high level, respectively. 
     The flip-flops  44 ,  46 ,  48 , and  50 , the delay circuits  52 ,  54 , and  56 , the pulse generators  58  and  60 , and the logic gates are connected as in the second embodiment. 
     FIG. 8 shows the operation timing of the refresh control circuit  62  in a read operation when a refresh request occurs internally just before the reception of the active command ACT. The timing of the signals except the select signals SEL 1 , SEL 2  and the request signals REQ 1 , REQ 2  is the same as in FIG. 5 described above. In this example, the timing chart starts with the select signals SEL 1  and SEL 2  at high level, the register REG 1  enabled, and the register REG 2  disabled. 
     Initially, the register REG 1  shown in FIG. 7 is set in synchronization with the rising edge of the refresh request signal REFRQ, changing the request signal REQ 1  to high level (FIG.  8 ( a )). The toggle flip-flop  64  turns the select signal SELL to low level in synchronization with the falling edge of the refresh request signal REFRQ (FIG.  8 ( b )). Due to the change of the select signal SEL 1 , the register REG 1  disables the reception of a refresh request and the register REG 2  enables the reception of a refresh request. 
     The high level of the request signal REQ 1  turns the refresh latched signal REFL to high level (FIG.  8 ( c )). Then, the refresh control circuit  62  operates as in FIG. 5, thereby performing a refresh operation and a post-refresh precharge operation. 
     The register REG 1  changes the request signal REQ 1  to low level in accordance with the refresh precharge signal PRER which is activated after the refresh operation (FIG.  8 ( d )). The refresh latched signal REFL is turned to low level in accordance with the change of the request signal REQ 1  (FIG.  8 ( e )). The toggle flip-flop  66  turns the select signal SEL 2  to low level in synchronization with the falling edge of the refresh latched signal REFL (FIG.  8 ( f )). The refresh request to occur subsequently will be held by the register REG 2 . 
     Next, the read command RD and the precharge command PRE are supplied in succession so that a read operation and a precharge operation are performed. That is, even in this embodiment, the refresh operation is performed automatically without any recognition from exterior. 
     FIG. 9 shows the operation timing of the refresh control circuit  62  for situations where a refresh request occurs internally after the reception of the active command ACT. The timing of the signals except the select signals SEL 1 , SEL 2  and the request signals REQ 1 , REQ 2  is the same as in FIG. 6 described above. In this example, the timing chart starts with the select signals SEL 1  and SEL 2  at high level, the register REG 1  enabled, and the register REG 2  disabled. 
     Initially, the active command ACT and the read command RD are supplied in succession so that a read operation is performed as in FIG.  6 . 
     When a refresh request occurs after the supply of the active command ACT, the request signal REQ 1  is turned to high level in synchronization with the rising edge of the refresh request signal REFRQ (FIG.  9 ( a )). The select signal SEL 1  is turned to low level in synchronization with the falling edge of the refresh request signal REFRQ (FIG.  9 ( b )). The refresh request to occur subsequently will be held by the register REG 2 . The refresh latched signal REFL is turned to high level in response to the high level of the request signal REQ 1  (FIG.  9 ( c )). 
     Next, as in FIG. 6, a read operation and a precharge operation are performed. After the precharge operation is performed, the refresh operation is performed, and then the precharge operation is performed after the refresh operation. Then, as in FIG. 8, the request signal REQ 1  is changed to low level in response to the rise of the refresh precharge signal PRER (FIG.  9 ( d )). The refresh latched signal REFL is turned to low level in accordance with the change of the request signal REQ 1  (FIG.  9 ( e )). The select signal SEL 2  is turned to low level in synchronization with the falling edge of the refresh latched signal REFL (FIG.  9 ( f )). 
     Like FIG. 6, even in this example, the refresh operation is performed automatically without any recognition from exterior. 
     FIG. 10 shows the operation timing of the refresh control circuit  62  in read operations when two refresh requests occur internally during the read operations (page operations) which follow the reception of the active command ACT. In this example, the timing chart also starts with the select signals SEL 1  and SEL 2  at high level, the register REG 1  enabled, and the register REG 2  disabled. 
     After receiving the active command ACT, the timing from a first refresh request to the performance of the read operation, is the same as in FIG. 9 described above. Thus, description thereof will be omitted here. 
     After the page operation, a second refresh request occurs before the reception of the precharge command PRE, so that the refresh request signal REFRQ is turned to high level (FIG.  10 ( a )). The second refresh request is not limited to this timing. For example, it may occur during the page operation. Here, the select signals SEL 1  and /SEL 1  are at low level and high level, respectively. Therefore, the refresh request is held by the register REG 2 . 
     When refresh requests are to occur at predetermined intervals, some memory cells can delay in refresh and cause a data loss if the page operation period is longer than the interval of occurrence of the refresh requests (refresh period). For this reason, the maximum number of page operations is restricted in accordance with the refresh period. In this embodiment, the refresh control circuit  62  can hold two refresh requests. Thus, the maximum number of page operations may be approximately twice that of the second embodiment. 
     The register REG 2  turns the request signal REQ 2  to high level in synchronization with the rising edge of the refresh request signal REFRQ. The toggle flip-flop  64  turns the select signals /SEL 1  and SEL 1  to low level and high level, respectively, in synchronization with the falling edge of the refresh request signal REFRQ (FIG.  10 ( b )). That is, the register REG 2  receives the refresh request before the register REG 1  re-enables the reception of a refresh request and the register REG 2  disables the reception of a refresh request. 
     Then, the precharge command PRE is supplied to perform a precharge operation, followed by a first refresh operation as in FIG.  9 . After the refresh operation, the refresh latched signal REFL is inactivated so that the select signals SEL 2  and /SEL 2  change to low level and high level, respectively (FIG.  10 ( c )). Here, the register REG 2  holds the second refresh request, outputting the request signal REQ 2  of high level. On this account, the refresh latched signal REFL is reactivated in response to the changes of the select signals SEL 2  and /SEL 2  (FIG.  10 ( d )). 
     In this example, a no-operation command NOP is supplied after the precharge command PRE. Since the active latched signal ACTL is at low level, the flip-flop  46  is reset in synchronization with the rising edge of the refresh latched signal REFL. Then, the refresh pulse REFP is generated (FIG.  10 ( e )), and a second refresh operation is performed as the first refresh operation was. 
     The activation of the refresh precharge signal PRER that follows the refresh operation resets the register REG 2 , changing the request signal REQ 2  to low level (FIG.  10 ( f )). The refresh latched signal REFL is inactivated in accordance with the change of the request signal REQ 2  (FIG.  10 ( g )), so that the select signals SEL 2  and /SEL 2  change to high level and low level, respectively (FIG.  10 ( h )). That is, the registers REG 1  and REG 2  restore their states to before the reception of the active command ACT. 
     Incidentally, if an active command ACT is supplied after the precharge command PRE, the second refresh operation is performed after the memory operation as in FIG.  9 . 
     This embodiment can offer the same effects as those obtained from the first embodiment described above. Moreover, in this embodiment, the registers REG 1  and REG 2  can hold two refresh requests. This means an increase in the number of accesses that can be made within a single page operation. 
     FIG. 11 shows a fourth embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. The same circuits and signals as those described in the second embodiment will be designated by identical reference numbers or symbols. Detailed description of these circuits and signals will be omitted here. 
     This embodiment includes a refresh control circuit  70  that differs from the refresh control circuit  12  of the first embodiment. The other configuration is identical to that of the first embodiment. 
     The refresh control circuit  70  has latches  72  and  74 , flip-flops  42 ,  44 ,  48 , and  50 , delay circuits  52 ,  54 , and  56 , a pulse generator  60 , a plurality of logic gates, a latch  76 , a delay circuit  78 , and an OR gate  80 . The latches  72  and  74  latch the active command signal ACTV and the precharge signal PREC, respectively, in synchronization with the rising edge of the clock signal CLK. The flip-flops  42 ,  44 ,  48 , and  50 , the delay circuits  52 ,  54 , and  56 , and the pulse generator  60  are the same as those of the second embodiment. The latch  76 , the delay circuit  78 , and the OR gate  80  substitute as the flip-flop  46  of the second embodiment. 
     The output of the latch  72  is connected to the set terminal of the flip-flop  44 . The output of the latch  74  is connected to an input of the OR gate that generates the internal precharge signal IPRE. The latch  76  latches the refresh latched signal REFL in synchronization with the rising edge of the clock signal CLK, and outputs the latched signal as the refresh pulse REFP. The refresh pulse REFP is fed back to the reset terminal R of the latch  76  through the delay circuit  78  and the OR gate  80 . The reset terminal R of the latch  76  is also supplied with the active latched signal ACTL through the OR gate  80 . 
     The flip-flops  42 ,  44 ,  48 , and  50 , the delay circuits  52 ,  54 , and  56 , the pulse generator  60 , and the logic gates are connected as in the second embodiment. 
     FIG. 12 shows the operation timing of the refresh control circuit  70  in a read operation when a refresh request occurs internally just before the reception of the active command ACT. In this example, the timing of the signals is almost the same as in FIG. 5 described above except that a refresh operation is performed in synchronization with the clock signal CLK. 
     Initially, the reception of the refresh request turns the refresh latched signal REFL to high level (FIG.  12 ( a )). The active command ACT is supplied in synchronization with the rise of the clock signal CLK. Here, the active latched signal ACTL and the refresh pulse REFP are at low level (non-reset state), and the latch  76  is in operation. The latch  76  latches the refresh latched signal REFL of high level in synchronization with the rising edge of the clock signal CLK, and turns the refresh pulse REFP to high level. The latch  76  is reset by the refresh pulse REFP that is fed back through the delay circuit  78  and the OR gate  80 . This consequently generates the refresh pulse REFP of pulse form (FIG.  12 ( b )). Then, the refresh control circuit  70  operates as in FIG. 5, thereby performing a refresh operation, a read operation, and a post-refresh precharge operation in succession. 
     FIG. 13 shows the operation timing of the refresh control circuit  70  in a read operation when a refresh request occurs internally after the reception of the active command ACT. In this example, the timing of the signals is almost the same as in FIG. 6 described above except that a refresh operation is performed in synchronization with the clock signal CLK. 
     The latch  76  is kept reset over the high-level period of the active latched signal ACTL, and thus incapable of latching the refresh latched signal REFL (FIGS.  13 ( a, b )). Therefore, the latch  76  generates the refresh pulse REFP in synchronization with the rise of the clock signal CLK that comes after the inactivation of the active latched signal ACTL (FIG.  13 ( c )). Here, for example, a DESL command, a NOP command, or an active command ACT is supplied in synchronization with the clock signal CLK. Then, a refresh operation is performed. The refresh pulse REFP is activated at predetermined timing after the rise of the clock signal CLK, so that the refresh operation is performed at the same timing as in FIG. 12 described above. 
     This embodiment can offer the same effects as those obtained from the first and second embodiments described above. Moreover, in this embodiment, commands such as the active command ACT are supplied in synchronization with the clock signal CLK. The refresh control circuit  70  performs refresh operations in synchronization with the clock signal CLK. This facilitates the timing design of the circuits necessary for refresh control. 
     FIG. 14 shows a fifth embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. The same circuits and signals as those described in the third and fourth embodiments will be designated by identical reference numbers or symbols. Detailed description of these circuits and signals will be omitted here. 
     This embodiment includes a refresh control circuit  82  that differs from the refresh control circuit  12  of the first embodiment. The other configuration is identical to that of the first embodiment. 
     The refresh control circuit  82  has latches  72 ,  74 , and  76 , a delay circuit  78 , and an OR gate  80  which are the same as those of the fourth embodiment. It also has registers REG 1  and REG 2 , flip-flops  44 ,  48 , and  50 , delay circuits  52 ,  54 ,  56 , and  68 , and a pulse generator  60  which are the same as those of the third embodiment, along with a plurality of logic gates. 
     The registers REG 1  and REG 2 , the flip-flops  44 ,  48 , and  50 , the delay circuits  52 ,  54 ,  56 , and  68 , the pulse generator  60 , and the logic gates are connected as in the third embodiment. The latches  72 ,  74 , and  76 , the delay circuit  78 , and the OR gate  80  are connected as in the fourth embodiment. 
     In this embodiment, as in the third embodiment, two refresh requests are held individually by the registers REG 1  and REG 2 . As in the fourth embodiment, the refresh pulse REFP is generated in synchronization with the rising edge of the clock signal CLK. Then, memory operations are performed in almost the same timing as that shown in FIGS. 8 through 10 described above. 
     This embodiment can offer the same effects as those obtained from the third and fourth embodiments described above. 
     FIG. 15 shows a six embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. The same circuits and signals as those described in the first embodiment will be designated by identical reference numbers or symbols. Detailed description of these circuits and signals will be omitted here. 
     This semiconductor memory is formed as a 64-Mbit SDRAM  84  on a silicon substrate by using CMOS processes. For the sake of excellent usability, the SDRAM  84  has the function of refreshing its memory cells internally and automatically. On this account, users can design system boards to mount the SDRAM on without any consideration of refresh. 
     The SDRAM  84  has the same terminal configuration as that of ordinary 64-Mbit SDRAMs. That is, the SDRAM  84  has twelve address terminals for receiving a 12-bit row address signal and an 8-bit column address signal, two bank select terminals, and sixteen data input/output terminals. 
     The SDRAM  84  has a refresh control circuit  12 , a refresh counter  14 , latches  16  and  18 , and four banks BK. The banks BK each include an address selector  20 , an address latch  86 , timing generators  88 ,  90 , and  92 , row predecoders  94  and  96 , a segment selector  98 , a column predecoder  32 , and a memory core  34 A. The refresh control circuit  12 , the refresh counter  14 , the latches  16  and  18 , the address selectors  20 , and the column predecoders  32  are identical to those of the first embodiment. The memory cores  34 A each have a memory array MA which includes a row main decoder  36 , a column main decoder  38 , and a plurality of segments S arranged vertically and horizontally. In this example, each single segment S consists of 512 k memory cells. Main-word lines MWL each are electrically connected to a plurality of sub-word lines SWL via transistors. The sub-word lines SWL are connected to the gates of the transfer transistors in the memory cells. 
     The address latch  86  latches a column address signal CAD output from the latch  18  when the internal refresh signal IREF is inactivated, and output the latched signal as an internal column address signal ICAD. 
     The timing generator  88  receives a read/write command signal RW and an internal refresh signal IREF, and generate timing signals for operating the column predecoder  32  and timing signals for operating the row predecoder  96 . That is, when a read command RD, a write command WR, and a refresh command (a refresh request occurring internally) are supplied, the row predecoder  96  and the column predecoder  32  are put into operation. 
     The timing generator  90  receives an active command signal ACTV, and generate timing signals for operating the row predecoder  94  and the segment selector  98 . That is, when the active command ACT is supplied, the row predecoder  94  and the segment selector  98  are put into operation. 
     The timing generator  92  receives an internal row address strobe signal IRAS, and generate timing signals for operating the row predecoders  94  and  96 . That is, as will be described later, the row predecoders  94  and  96  are put into operation when a refresh request occurs. 
     The row predecoder  94  decodes internal row address signals IRAD (IRAD 0 - 9  shown in FIG. 18 to be described later) when supplied with the active command ACT, and decode an internal row address signal IRAD (IRAD 0 - 8  shown in FIG. 18) when a refresh request occurs. The row predecoder  94  outputs the decoding signal to the row main decoder  36 . The row predecoder  96  decodes an internal row address signal IRAD (IRAD 9 - 11  shown in FIG. 18) in a refresh operation, and decode an internal column address signal ICAD (ICAD 5 - 7  shown in FIG. 18) in read and write operations. The row predecoder  96  outputs the decoding signal to the memory array MA. 
     The segment selector  98 , when supplied with the active command ACT, decodes an internal row address signal IRAD (IRAD 10 - 11  shown in FIG. 18) and select a segment S. Moreover, the segment selector  98 , at the time of occurrence of a refresh request, select all the segments which align in the vertical direction of the diagram irrespective of the internal row address signal IRAD. The column predecoder  32 , in read and write operations, decodes the internal column address signal ICAD (ICAD 0 - 4  shown in FIG. 18) and outputs the decoded signal to the column main decoder  38 . 
     FIG. 16 shows the details of the memory arrays MA shown in FIG.  15 . On each bank BK, the memory array MA has eight memory blocks MBLK and four sense amplifier rows composed of a plurality of sense amplifiers SA. Each sense amplifier is shared between two memory blocks MBLK on both sides. 
     As shown on the top left bank BK in the diagram, each single memory block MBLK has 1024 word lines WL which are arranged in the horizontal direction of the diagram and 2048 bit line pairs which are arranged in the vertical direction of the diagram. The word lines WL are composed of 128 main-word lines MWL and eight sub-word lines SWL that branch off from every main-word line MWL. 
     As shown on the top right bank BK in the diagram, each memory array MA has two memory blocks MBLK astride a single memory block MBLK that are selected by the internal row address signal IRAD 7 - 8  (row block address BRAA in FIG.  18 ). That is, as shown shadowed in the diagram, two memory blocks on the same sides of sense amplifier rows (for example, the left sides in the diagram) are selected. Moreover, in each of the memory blocks MBLK selected, a single main-word line MWL is selected by the internal row address signal IRAD 0 - 6 . 
     From between the two memory blocks MBLK selected by the internal row address signal IRAD 7 - 8 , either one is further selected by the internal row address signal IRAD 9  (row block address BRAB in FIG.  18 ). The selection of the memory blocks MBLK and the selection of the main-word lines MWL through the use of the internal row address signal IRAD 0 - 9  are effected by the row predecoder  94  shown in FIG.  15 . Incidentally, in refresh operations, the row predecoder  94  ignores the internal row address signal IRAD 9 . Therefore, a refresh operation is performed on the two main-word lines in the two memory blocks MBLK shown shadowed in the diagram. 
     In read and write operations, the internal row address signal IRAD 10 - 11  (column block address BCAA in FIG. 18) selects a quarter of the area from the single memory block MBLK selected. That is, within the quarter area of the memory block MBLK, the internal row address signal IRAD 10 - 11  releases the bit lines from a precharge operation and activates the sense amplifiers SA. 
     FIG. 17 shows the details of memory blocks MBLK shown in FIG.  16 . Each memory block MBLK includes 128 main-word lines MWL 0 - 127  and  1024  sub-word lines SWL 0 - 1023  which are laid along the vertical direction of the diagram, and 2048 bit line pairs BL and /BL which are laid along the horizontal direction of the diagram. Memory cells (small circles in the diagram) are formed on the intersections of the sub-word lines SWL and the bit lines BL (or /BL). For example, when a sub-word line SWL is activated to select memory cells in a read operation, data is read out to either ones of bit line pairs BL and /BL. The others of the bit line pairs BL and /BL function as references for amplifying the data. 
     Each single main-word line MWL is connected to four sub-word decoders SWD arranged on either side (top, bottom in the diagram) of the memory block MBLK. Then, each main-word line MWL branches into eight sub-word lines SWL through the sub-word decoders SWD. The main-word line MWL is selected, as mentioned above, by the internal row address signal IRAD 0 - 6  (row addresses RAA, RAB, and RAC). The sub-word line SWL is selected by the internal column address signal ICAD 5 - 7  (sub-word select SWLS). 
     The sense amplifiers SA are connected to the bit line pairs BL and /BL in the memory blocks MBLK on both sides, the right and left in the diagram. The connections between the sense amplifiers SA and the bit line pairs BL and /BL on both sides, and the activation of the sense amplifiers SA are controlled, as mentioned above, by the internal row address signal IRAD 10 - 11  with respect to each of the segments S which are the quarter areas of the memory blocks MBLK. From among the 512 bit line pairs BL, /BL and 512 sense amplifiers SA that are activated by the internal row address signal IRAD 10 - 11 , sixteen each are selected by the internal column address ICAD 0 - 4  (column addresses CAA and CAB). That is, data corresponding to sixteen data input/output terminals is input/output to/from the memory cells. 
     FIG. 18 shows the uses of address signals within the SDRAM. The SDRAM receives a bank address signal BA 0 - 1  through the bank select terminals, and receives a row address signal RAD 0 - 11  (internal row address signal IRAD 0 - 11 ) and a column address signal CAD- 07  (internal column address signal ICAD 0 - 7 ) through the address terminals, to perform a read or write operation. The SDRAM also performs a refresh operation under a refresh address REFAD (internal row address signal IRAD 0 - 11 ) which is generated by the refresh counter  14  shown in FIG.  15 . 
     The row address signal RAD 0 - 11  (internal row address signal IRAD 0 - 11 ) to be supplied along with the active command ACT consists of IRAD 0 - 6  (MWDEC), IRAD 7 - 9  (RBLKS), and IRAD 10 - 11  (CBLKS). IRAD 0 - 6  (MWDEC) and IRAD 7 - 9  (RBLKS) are supplied to the row predecoder  94  shown in FIG. 15, and IRAD 10 - 11  (CBLKS) to the segment selector  98 . 
     As mentioned above, the internal row address signal IRAD 0 - 6  (MWDEC) selects one out of 128 main-word lines in a memory block MBLK. The internal row address signal IRAD 7 - 8  (BRAA of RBLKS) selects two out of eight memory blocks MBLK. The internal row address signal IRAD 9  (BRAB of RBLKS) selects one out of two memory blocks MBLK selected. The internal row address signal IRAD 10 - 11  (CBLKS) selects a segment S which is a quarter area of the memory block MBLK selected. 
     The column address signal CAD 0 - 7  (internal column address signal ICAD 0 - 7 ) to be supplied along with the read command RD or the write command WR consists of the internal column address signal ICAD 0 - 4  (CDEC) and the internal column address signal ICAD 5 - 7  (SWLS). The internal column address signal ICAD 0 - 4  (CDEC) is supplied to the column predecoder  32  shown in FIG. 15, and the internal column address signal ICAD 5 - 7  (SWLS) to the column predecoder  96 . 
     As mentioned above, the internal column address signal ICAD 0 - 4  (CDEC) selects sixteen out of both 512 bit line pairs BL, /BL and 512 sense amplifiers SA in a segment S selected. The internal column address signal ICAD 5 - 7  (SWLS) selects one out of eight sub-word lines SWL corresponding to a main-word line MWL selected. 
     This embodiment is characterized in that the row address signal RAD to be supplied along with the active command ACT selects a segment S, and the column address signal CAD to be supplied along with the read command RD (or the write command WR) selects a sub-word line SWL. In particular, since a segment S is selected in response to the row address signal RAD, the operation of releasing the connections between unused bit line pairs BL, /BL and sense amplifiers SA and the operation of releasing the precharge of the bit line pairs BL, /BL can be started before the supply of the read command RD (or write command WR). As a result, read and write operations can be performed faster than in the first embodiment. 
     FIG. 19 shows the details of sub-word decoders SWD and a sense amplifier SA shown in FIG. 17, along with their peripheral circuits. The sub-word decoders SWD (SWD 0 ,  1 ) each include two nMOS transistors connected in parallel and a pMOS transistor connected in series with these nMOS transistors. The gate of one of the nMOS transistors and the gate of the pMOS transistor are connected to a main-word line MWL. The gate of the other nMOS transistor and the source of the pMOS transistor receive decoding signals CDEC 0 ,  1  of the internal column address signal ICAD 5 - 7  (in this example, negative-logic internal row address signal /ICAD 5 - 7 ), respectively, which are generated by a decoding circuit  98   a . The decoding circuit  98   a  is activated when a decoding signal of the internal row address signal IRAD 10 - 11 , generated by a decoding circuit  96   a , is at high level. The sources of the nMOS transistors are connected to a ground line. The connection node between the nMOS transistors and the pMOS transistor is connected to a sub-word line SWL (SWL 0 ,  1 ). 
     In each of the sub-word decoders SWD described above, the pMOS transistor turns on and either one of the nMOS transistors turns off when the main-word line MWL is selected and turned to low level. Then, the decode signal CDEC 0  (or CDEC 1 ) of high level, output from the decoding circuit  98   a , is transmitted to the sub-word line SWL 0  (or SWL 1 ). That is, the column address signal CAD selects the sub-word line SWL. The selection of the sub-word line SWL turns on the transfer transistor of the memory cell MC so that the charge retained in the capacitor is transmitted onto the bit line BL (or /BL). 
     The sense amplifier SA includes a latch  99   a , a bit line control circuit  99   b , nMOS transistors  99   c  and  99   d , and bit line selecting switches  99   e  and  99   f . The latch  99   a  has two CMOS inverters whose inputs and outputs are connected to each other. The latch  99   a  is activated in response to activation signals PSA and NSA. The bit line control circuit  99   b  is composed of three nMOS transistors for equalizing the bit line pair in the sense amplifier SA, or supplying the same with a precharging voltage VPR. The nMOS transistor  99   c  equalizes the bit line pair BL, /BL of the memory block MBLK on the left in the diagram. The nMOS transistor  99   d  equalizes the bit line pair BL, /BL of the memory block MBLK on the right in the diagram. The bit line selecting switch  99   e  is composed of two nMOS transistors for connecting the bit line pair BL, /BL of the memory block MBLK on the left in the diagram to the latch  99   a . The bit line selecting switch  99   f  is composed of two nMOS transistors for connecting the bit line pair BL, /BL of the memory block MBLK on the right in the diagram to the latch  99 . 
     The bit line control circuit  99   b  sets the bit line pair BL, /BL to the precharging voltage VPR when a sub-bit line reset signal SBRSC is at high level. The sub-bit line reset signal SBRSC turns to high level in response to a main-bit line reset signal MBRSC of low level, thereby precharging the bit line pair BL, /BL. 
     The nMOS transistor  99   c  equalizes the bit line pair BL, /BL when a sub-bit line reset signal SBRSL is at high level. The sub-bit line reset signal SBRSL turns to high level in response to a main-bit line reset signal MBRSL of low level, thereby equalizing the bit line pair BL, /BL. 
     Similarly, the nMOS transistor  99   d  equalizes the bit line pair BL, /BL when a sub-bit line reset signal SBRSR is at high level. The sub-bit line reset signal SBRSR turns to high level in accordance with a main-bit line reset signal MBRSR of low level, thereby equalizing the bit line pair BL, /BL. 
     The bit line selecting switch  99   e  turns on in response to the activation (high level) of a sub-bit line transfer signal SBTL, thereby establishing connection between the bit line pair BL, /BL and the latch  99   a . The sub-bit line transfer signal SBTL changes to high level in accordance with a main-bit line transfer signal MBTL of low level, turning on the bit line selecting switch  99   e.    
     Similarly, the bit line selecting switch  99   f  turns on in response to the activation (high level) of a sub-bit line transfer signal SBTR, thereby establishing connection between the bit line pair BL, /BL and the latch  99   a . The sub-bit line transfer signal SBTR changes to high level in response to a main-bit line transfer signal MBTR of low level, turning on the bit line selecting switch  99   f.    
     The main-bit line reset signal MBRSC is activated in accordance with the row address signal RAD 7 - 8 . The main-bit line reset signals MBRSL, MBRSR and the main-bit line transfer signals MBTL, MBTR are activated in accordance with the row address signal RAD 7 - 9 . More specifically, when the main-bit line reset signal MBRSC and the main-bit line reset signal MBRSL (or MBRSR) corresponding to the memory block MBLK to operate are activated, the equalization of the bit lines BL and /BL is released. Then, the main-bit line transfer signal MBTL (or MBTR) that corresponds to the not-operating one out of the pair of memory blocks MBLK astride the sense amplifier SA is activated to release the connection between the bit lines BL, /BL and the latch  99   a.    
     The sub-bit line reset signals SBRSC, SBRSL, and SBRSR, and the sub-bit line transfer signals SBTL and SBTR are selected in accordance with the row address signal IRAD 10 - 11 . That is, the signal lines of these signals are laid with respect to each of the segments S which are the quarter areas of the memory blocks MBLK. Incidentally, all the decoding signals of the internal row address signal IRAD 10 - 11 , generated by the decoding circuit  96   a , are turned to high level in refresh operations. Therefore, during refresh operations, all the segments S in the memory block MBLK are selected. This allows all the memory cells to be refreshed by fewer refresh requests. 
     FIG. 20 shows read operations at a lower clock frequency. In this example, two read commands RD are supplied in succession after the active command ACT. Then, the precharge command PRE is supplied. Detailed description will be omitted of the same operations as those of FIG. 2 above. 
     Initially, the active command ACT and a row address signal RAD( 1 ) are supplied in synchronization with the rising edge of the clock signal CLK (FIG.  20 ( a )). In accordance with the internal row address signal IRAD 0 - 9 , main-word lines MWL are selected (turned to low level). The main-bit line transfer signals MBT (MBTL, MBTR) and the main-bit line reset signals MBRS (MBRSC, MBRSL, MBRSR) change to low level (FIG.  20 ( b )). 
     In this embodiment, the sub-bit line transfer signals SBT (SBTL, SBTR) and the sub-bit line reset signals SBRS (SBRSC, SBRSL, SBRSR) are also selected in accordance with the internal row address signal IRAD 10 - 11  (FIG.  20 ( c )). Accordingly, the bit line control circuit  99   b , the nMOS transistors  99   c  and  99   d , and the bit line selecting switches  99   e  and  99   f  shown in FIG. 19 start their operations earlier than heretofore, or before the reception of the read command RD. That is, the memory block MBLK and sense amplifiers SA not performing the read operation (or write operation) are disconnected to each other. 
     The read command RD and a column address signal CAD( 1 ) are supplied in synchronization with the rise of the next clock signal CLK (FIG.  20 ( d )). Sub-word lines SWL are selected in accordance with the internal column address signal ICAD 5 - 7  (FIG.  20 ( e )). That is, even in this embodiment, the sub-word lines SWL are activated by using not only the row address signal RAD but also the column address signal CAD. Then, the activation signals PSA and NSA in the segment S selected by the internal row address signal IRAD 7 - 11  are activated to activate the sense amplifiers SA (FIG.  20 ( f )). 
     Due to the selection of the sub-word lines SWL, data is read from the memory cells MC to the bit lines BL (or /BL), and amplified by the sense amplifiers SA. Furthermore, a column line select signal CL is selected in accordance with the internal column address ICAD 0 - 4  (FIG.  20 ( g )) so that column selecting switches (not shown) are turned on to establish connections between 16 bit line pairs BL, /BL and common data bus lines (16 bits). Then, the read data is output to exterior (FIG.  20 ( h )). 
     The read command RD and a column address signal CAD( 2 ) are supplied in synchronization with the rising edge of the next clock signal CLK (FIG.  20 ( i )). Sub-word lines SWL are selected in accordance with the internal column address signal ICAD 5 - 7  (FIG.  20 ( j )). Then, a so-called page read operation is performed as described above. The page operation is effected by the column address signal CAD 0 - 4  selecting 16 bits out of the data that is amplified by the 512 sense amplifiers SA activated. 
     The precharge command PRE is supplied in synchronization with the rising edge of the next clock signal CLK (FIG.  20 ( k )). The precharge command PRE inactivates the main-word lines MWL, the sub-word lines SWL, the main-bit line transfer signal MBT, the main-bit line reset signal MBRS, the sub-bit line transfer signal SBT, the sub-bit line reset signal SBRS, and the activation signals PSA and NSA of the sense amplifiers SA, and precharges the bit lines BL and /BL. 
     In write operations, although not particularly shown in the drawings, main-word lines MWL and sub-word lines SWL are selected in the same timing as in read operations, followed by the selection of the bit line transfer signals MBT, SBT and the bit line reset signals MBRS, SBRS. 
     In refresh operations, two main-word lines MWL in two memory blocks MBLK on each of the four banks BK are activated. The sub-word lines SWL in the corresponding eight segments S are activated at the same time. That is, 16 kbits of memory cells MC (512 bits×8 segments×4 banks) are refreshed at a time. Incidentally, refresh may be performed segment by segment or bank BK by bank BK if a reduction is intended of the peak current in refresh operations and as long as the period necessary for refreshing all the memory cells (typically, refresh time tREF) is met. 
     FIG. 21 shows read operations at a higher clock frequency. In this example, the read command RD and the precharge command PRE are supplied in succession after the active command ACT. Then, at the next clock but one, the active command ACT is supplied again. 
     The basic operation timing of FIG. 21 is identical to that of FIG.  20 . That is, the bit line transfer signals MBT, SBT and the bit line reset signals MBRS, SBRS are selected in accordance with the row address signal RAD 1  which is supplied along with the active command ACT (FIGS.  21 ( a ), ( b )). Sub-word lines SWL are selected in accordance with the column address signal CAD 1  which is supplied along with the read command RD (FIG.  21 ( c )). 
     Since higher clock frequencies make the operation of the internal circuits relatively slower, the read data cannot be output within the clock cycle at which the read command RD is supplied. In this example, the read data is output in synchronization with the rising edge of the third clock signal CLK from the reception of the read command RD (FIG.  21 ( d )). 
     As described above, this embodiment can offer the same effects as those obtained from the first embodiment described above. Moreover, in this embodiment, some of the circuits for controlling memory operations (read operations and write operations) are operated in accordance with the row address signal RAD which is supplied along with the active command ACT. Accordingly, the circuits which used to start operation in response to the read command RD or the write command WR can be operated in advance, to perform a read operation or write operation at higher speed. That is, it is possible to achieve a speedup while maintaining the low-power-consumption characteristic. 
     More specifically, any of a plurality of memory blocks MBLK is selected in accordance with the row address signal RAD 7 - 9  which is supplied along with the active command ACT to be supplied at the beginning of a memory operation. Selecting a memory block MBLK at an earlier time of memory operation can decrease the circuits to start operation in response to the subsequent read command RD (or write command WR). As a result, it is possible to reduce the time that elapses from the supply of the active command ACT to the completion of the read operation (or write operation). 
     In accordance with the row address signal RAD 10 - 11  which is supplied along with the active command ACT, the bit line selecting switches  99   e  and  99   f  are selected with respect to each segment S. Therefore, the connections between the bit lines BL, /BL of not-operating memory blocks MBLK and sense amplifiers SA can be released at an earlier time of memory operation. As a result, it is possible to reduce the time that elapses from the supply of a first command to the completion of a read or write operation. Besides, since the connections between the bit lines BL, /BL and the sense amplifiers SA are established by segment S, the number of circuits to be operated in read and write operations can be decreased in number, with a reduction in the operating power consumption. 
     In refresh operations, the bit line selecting switches  99   e  and  99   f  in all the segments in a memory block MBLK are selected at the same time. Since the number of segments S to operate during refresh is made greater than that of segments to operate in read and write operations, it is possible to reduce the number of times of refresh necessary for refreshing all the memory cells MC. Accordingly, the refresh intervals can be extended so that read and write operations increase in ratio within a predetermined period. This means an improved I/O bus occupation rate (data transmission rate). 
     In refresh operations, the main-word lines MWL and the sub-word lines SWL are selected in accordance with the refresh address REFAD which corresponds to the active command ACT. Unlike read and write operations, refresh operations can be started in response to the active command ACT. This allows refresh operations to be performed faster than read and write operations. Accordingly, within a predetermined period, the ratios of read and write operations can be increased to improve the I/O bus occupation rate (data transmission rate). 
     FIG. 22 shows a seventh embodiment of the semiconductor memory and the method of operating a semiconductor memory in the present invention. The same circuits and signals as those described in the first and sixth embodiments will be designated by identical reference numbers or symbols. Detailed description of these circuits and signals will be omitted here. 
     In FIG. 22, memory cores are omitted. The semiconductor is constituted as an SDRAM, the basic operations of which are the same as those of the foregoing sixth embodiment except that refresh operations are performed in accordance with refresh commands from exterior. 
     The SDRAM has a peripheral circuit and a bank BK to be laid out on the periphery of a memory core (the outer periphery or central portion of the chip). The memory core in the bank BK is omitted from the diagram. The dashed line in the diagram indicates the boundary between the peripheral circuit and the bank BK. 
     The peripheral circuit includes a refresh counter  14 , a command decoder  100 , a refresh address latch  102 , an RAS generator  104 , a CAS generator  106 , and a latch  108 . 
     The command decoder  100  generates a refresh command signal REF, an active command signal ACTV, a precharge command signal PRE, and a read/write command signal RW in accordance with a command signal CMD which is supplied from exterior. The refresh address latch  102  outputs a refresh address received from the refresh counter  14  as a refresh address signal REFAD in accordance with the refresh command signal REF. 
     The RAS generator  104  outputs, in accordance with the active command signal ACTV, a timing signal RASZ for controlling circuits corresponding to a row address signal RAD. The CAS generator  106  outputs, in accordance with a read command RD and a write command WR, a read/write timing signal RWT for controlling a read operation and a write operation. The latch  108  latches a column address signal CAD in synchronization with the AND logic (operation control signal RWC) of a bank signal BANK and the read/write timing signal RWT. 
     The bank BK includes a BRAS generator  110 , a PRE generator  112 , a timing generator  114 , latches  116 ,  118 , and  120 , an AND gate  121 , a switch  122 , a row predecoder  124 , a switch  126 , an SA generator  128 , a row predecoder  130 , and column predecoders  132  and  134 . The latch  118  and the AND gate  121  function as an activation control circuit for activating the row predecoder  130  in response to only the first read command RD (or write command WR) that follows an active command ACT. 
     The BRAS generator  110  activates a basic timing signal BRASZ in accordance with the refresh command REF and the read/write command signal RW, and inactivates the basic timing signal BRASZ in accordance with the precharge command signal PRE. The PRE generator  112  outputs a timing signal to the BRAS generator  110  in accordance with the precharge command signal PRE. 
     The timing generator  114  generates timing signals T 1 , T 2 , T 3 , and T 4  in accordance with the basic timing signal BRASZ. The latch  116  latches the row address signal RAD in synchronization with the AND logic (operation control signal ACTC) of the bank signal BANK and the active command signal ACTV, and outputs the resultant as an internal row address signal IRAD. 
     The latch  118  latches the timing signal T 4  in synchronization with the falling edge of the operation control signal RWC, which is the AND logic of the bank signal BANK and the read/write timing signal RWT, and outputs the resultant as complementary timing signals TS and /T 5 . The latch  118  is reset when the basic timing signal BRASZ is at low level. Here, the basic timing signal BRASZ is activated from the supply of the active command ACT to the supply of the precharge command PRE (operation completion command). The latch  118  latches the level of the basic timing signal BRASZ each time the read command RD (or write command WR) is supplied. 
     The latch  120  latches the internal row address signal IRAD in synchronization with the rising edge of the timing signal T 2 , and outputs the resultant to the switch  122 . The switch  122  selects the internal row address signal IRAD and the refresh address signal when the timing signal T 3  is at low level and high level, respectively. The row predecoder  124  decodes the address signal transmitted from the switch  122 , and outputs decoding signals RAA 0 - 7 , RAB 0 - 7 , and RAC 0 - 1  for selecting main-word lines MWL, and decoding signals BRAA 0 - 3  and BRAB 0 - 1  for selecting memory blocks MBLK. The decoding signals RAA 0 - 7 , RAB 0 - 7 , RAC 0 - 1 , BRAA 0 - 3 , and BRAB 0 - 1  are signals generated from the internal address signal IRAD 0 - 2 , IRAD 3 - 5 , IRAD 6 , IRAD 7 - 8 , and IRAD 9  shown in FIG. 18, respectively. 
     The latch  126  selects the timing signal T 1  in refresh operations, and the timing signal T 5  in read and write operations. The SA generator  128  receives the output of the switch  126 , and generates activation signals PSA and NSA for sense amplifiers SA. 
     The AND gate  121  outputs the operation control signal RWC as a sub-word latched signal SWLL when the timing signal /T 5  is at high level. That is, the operation control signal RWC is gated by an output of the latch  118  (timing signal /T 5 ). The timing signal /T 5 , as will be described later, remains at high level only when the first read command RD (or write command WR) is supplied after the supply of the active command ACT. It changes to low level by the time when the second read command RD (or write command WR) is supplied. 
     The row predecoder  130  has a latch  130   a  and a switch  130   b . The latch  130   a  latches an internal column address signal ICAD in synchronization with the sub-word latched signal SWLL. The switch  130   b  selects the internal column address ICAD when the timing signal T 3  is at low level, and selects the refresh address signal REFAD when the timing signal T 3  is at high level. The switch  130   b  decodes the selected signal in a not-shown decoding circuit, and outputs the resultant as a decoding signal RADS 0 - 7 . The decoding signal RADS 0 - 7  is a signal generated from ICAD 5 - 7  (in read and write operations) or IRAD 9 - 11  (in refresh operations) shown in FIG.  18 . 
     The column predecoder  132  decodes the internal column address signal ICAD, and outputs decoding signals CAA 0 - 3  and CAB 0 - 7 . The decoding signals CAA 0 - 3  and CAB 0 - 7  are signals generated from the internal row addresses ICAD 0 - 1  and ICAD 3 - 6  shown in FIG. 18, respectively. 
     The column predecoder  134  has a latch  134   a  and a switch  134   b . The latch  134   a  latches the internal row address signal IRAD in synchronization with the timing signal T 2 . The switch  134   b  outputs a low level in refresh operations. In read and write operations, the switch  134   b  selects the internal row address signal IRAD 10 - 11 , decodes the selected signal in a not-shown decoding circuit, and outputs the resultant as a decoding signal BCAA 0 - 3 . 
     FIG. 23 shows read operations of the SDRAM described above. In this example, as in FIG. 20 described above, two read commands RD (or write commands WR) are supplied in succession after the active command ACT. Then, the precharge command PRE is supplied. Since the generation timing of the main-word lines MWL, the sub-word lines SWL, the bit line transfer signals MBT and SBT, and the bit line reset signals MBRS and SBRS is the same as in FIG. 20, description thereof will be omitted. FIG. 23 provides detailed description up to the generation of the decoding signals to be supplied to the memory cores. 
     Initially, the active command ACT is supplied in synchronization with the clock signal CLK (FIG.  23 ( a )). The command decoder  100  shown in FIG. 22 activates the active command signal ACTV in accordance with the active command ACT (FIG.  23 ( b )). The activation of the active command signal ACTV also activates the operation control signal ACTC (FIG.  23 ( c )). The latch  116  shown in FIG. 22 latches a row address signal RAD( 1 ) in synchronization with the operation control signal ACTC, and outputs the resultant as the internal row address signal IRAD (FIG.  23 ( d )). 
     The BRAS generator  110  activates the basic timing signal BRASZ in response to the active command signal ACTV (FIG.  23 ( e )). The timing generator  114  activates the timing signal T 2  in synchronization with the rising edge of the basic timing signal BRASZ (FIG.  23 ( f )). The latch  116  latches the internal row address signal IRAD 0 - 11  in synchronization with the rising edge of the timing signal T 2 . The switch  122  receives the timing signal T 3  of low level, and transmits the internal row address signal IRAD 0 - 9  to the row predecoder  124 . The row predecoder  124  decodes the internal row address signal IRAD 0 - 9 , and outputs any of the decoding signals RAA 0 - 7 , RAB 0 - 7 , RAC 0 - 7 , BRAA 0 - 3 , and BRAB 0 - 1  (FIG.  23 ( g )). The activation of the decoding signals RAA 0 - 7 , RAB 0 - 7 , RAC 0 - 7 , BRAA 0 - 3 , and BRAB 0 - 1  selects main-word lines MWL, the bit line transfer signal MBTL, and the bit line reset signals MBRS as in the sixth embodiment. 
     The column predecoder  134  receives the row address signal IRAD 10 - 11  in synchronization with the rising edge of the timing signal T 2 , decodes the received signal, and outputs any of the decoding signal BCAA 0 - 3  (FIG.  23 ( h )). The activation of the decoding signal BCAA 0 - 3  selects the bit line transfer signal SBTL and the bit line reset signals SBRS as in the sixth embodiment. That is, the row address signal RAD supplied along with the active command ACT releases the precharge of bit lines BL, /BL and establishes connection between a memory block MBLK and sense amplifiers SA. 
     The timing generator  114  activates the timing signal T 4  with a predetermined time of delay after the rising edge of the basic timing signal BRASZ (FIG.  23 ( i )). The high level of the basic timing signal BRASZ releases the latch  118  from a reset state, turning the timing signals T 5  and /T 5  to low level and high level, respectively (FIG.  23 ( j )). 
     A read command RD (or write command WR) is supplied in synchronization with the next clock signal CLK (FIG.  23 ( k )). The command decoder  100  activates the read/write command signal RW in accordance with the read command RD. The CAS generator  106  activates the read/write timing signal RWT in response to the read/write command signal RW. The activation of the read/write timing signal RWT activates the operation control signal RWC for a predetermined period (FIG.  23 ( l )). 
     The latch  108  latches the column address signal CAD in synchronization with the rising edge of the operation control signal RWC, and outputs the resultant as the internal column address signal ICAD 0 - 7  (not shown). The high levels of the timing signal /T 5  and the operation control signal RWC activate the sub-word latched signal SWLL for a predetermined period (FIG.  23 ( m )). The row predecoder  130  latches the internal column address signal ICAD 5 - 7  in synchronization with the rising edge of the sub-word latched signal SWLL, and outputs any of the decoding signal RADS 0 - 7  (FIG.  23 ( n )). The decoding signal RADS 0 - 7  selects sub-word lines SWL as in the sixth embodiment. That is, the sub-word lines SWL are selected by the column address signal CAD that is supplied along with the read command RD (or write command WD). Then, read data is output from the memory cells MC to the sense amplifiers SA. 
     The latch  118  latches the high level of the timing signal T 4  in synchronization with the falling edge of the operation control signal RWC, and turns the timing signals T 5  and /T 5  to high level and low level, respectively (FIG.  23 ( o )). The timing signal T 4  remains at high level over the active period. Therefore, the timing signal /T 5  remains at low level until the basic timing signal BRASZ is turned to low level. The low level of the operation control signal RWC also changes the sub-word latched signal SWLL to low level (FIG.  23 ( p )). The switch  126  transmits the timing signal T 5  of high level to the SA generator  128 . The SA generator  128  receives the timing signal T 5  of high level, and activates the activation signals PSA and NSA (FIG.  23 ( q )). Then, sense amplifiers SA operate to amplify the read data on the bit lines BL (or /BL). The column predecoder  132  receives the internal column address ICAD 0 - 4 , and outputs any of the decoding signals CAA 0 - 3  and CAB 0 - 7  (not shown). Then, the read data is output. 
     A read command RD (or write command WR) is supplied in synchronization with the next clock signal CLK (FIG.  23 ( r )). The command decoder  100  activates the read/write command signal RW in accordance with the read command RD. The CAS generator  106  activates the read/write timing signal RWT in response to the read/write command signal RW. The activation of the read/write timing signal RWT activates the operation control signal RWC for a predetermined period (FIG.  23 ( s )). The latch  108  latches the column address signal CAD in synchronization with the rising edge of the operation control signal RWC, and outputs the resultant as the internal column address signal ICAD 0 - 7  (not shown). 
     Since the timing signal T 5  is at low level, the sub-word latched signal SWLL will not activated in response to the activation of the operation control signal RWC. This precludes the operation of the row predecoder  130 . Thus, the sub-word lines SWL are selected only by the read command RD (or write command WR) that is supplied next to the active command ACT. 
     The precharge command PRE is supplied in synchronization with the next clock signal CLK, turning the basic timing signal BRASZ to low level (FIG.  23 ( t )). The latch  118  is reset by the basic timing signal BRASZ of low level, thereby turning the timing signals T 5  and /T 5  both to low level (FIG.  23 ( u )). Then, the decoding signals RAA, RAB, RAC, BRAA, BRAB, RADS, CAA, CAB, and BCAA, and the activation signals PSA and NSA are inactivated to put the memory blocks MBLK into an idle state. 
     Although not shown in FIG. 23, the timing generator  114  activates the timing signals T 1  and T 3  in a refresh operation. In accordance with the activation of the timing signal T 3 , the switches  122 ,  126 ,  130   b , and  134   b  select the refresh address REFAD 0 - 8 , the timing signal T 1 , the refresh address REFAD 9 - 11 , and the ground voltage, respectively. Then, the refresh operation is performed. 
     As has been described, this embodiment can offer the same effects as those obtained from the first and sixth embodiments described above. Moreover, in this embodiment, the row predecoder  130  which generates the decoding signal for selecting sub-word lines SWL is operated only by the first read command RD (or write command WR) after the active command ACT. Among the column address signals CAD that are supplied along with the second and subsequent read commands RD (or write commands WR), the column address signals CAD 5 - 7  for selecting sub-word lines SWL are ignored. This can surely prevent the SDRAM from malfunctions such as multiple selection of word lines. Because of not operating in response to the subsequent read commands RD (or write commands WR), the row predecoder  130  can be reduced in power consumption. 
     Incidentally, the first embodiment described above has dealt with the case where the sub-word lines SWL are activated each time the read command RD is supplied, and the main-word lines MWL are kept activated until the precharge command PRE is supplied. However, the present invention is not limited to such an embodiment. For example, as shown in FIG. 24, the main-word lines MWL and the sub-word lines SWL both may be kept activated until the supply of the precharge command PRE. Performing page operations without inactivating the sub-word lines SWL allows a further reduction of power consumption during operation. 
     The embodiments described above have dealt with the operating timing in read operations. However, the present invention is not limited to such embodiments. For example, write operations can be performed in the same timing as that of the read operations. 
     The above-described embodiments have dealt with the cases where a no-read-operation period is utilized to perform a refresh operation. However, the present invention is not limited to such embodiments. For example, this period may be used to perform a self test or the like inside the chip. Based on the result of the self test, control signal timing or a control voltage may be adjusted to operate the semiconductor memory at optimum timing in accordance with voltage variations and/or temperature variations. 
     The above-described embodiments have dealt with the cases where the present invention is applied to an SDRAM. The present invention is not limited thereto, and may be applied to such semiconductor memories as a DRAM or an SRAM. The present invention may also be applied to system LSIs that implement DRAM memory cores. 
     The invention is not limited to the above embodiments and various modifications may be made without departing from the spirit and the scope of the invention. Any improvement may be made in part or all of the components.