Abstract:
A semiconductor memory device having a self-refresh operation includes a detection circuit generating a detection signal when detecting a change of a given input signal, and a comparator circuit comparing the detection signal with a refresh request signal internally generated and generating a control signal indicative of a circuit operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to semiconductor memory devices, and more particularly to a semiconductor memory device of a DRAM (Dynamic Random Access Memory) type in which a self-refresh operation is constantly performed in the device. 
     2. Description of the Related Art 
     Recently, a compact mobile terminal such as a cellular phone has collaborated with the Internet and handled a large amount of data. This has stimulated a large-capacity memory. Nowadays, an SRAM (Static Random Access Memory) is employed in the cellular phones because of its low power consumption. However, the SRAM does not have a high integration density. The larger the SRAM capacity, the more expensive the cost. In contrast, the DRAM is a low-cost, high-capacity memory. The DRAM and SRAM do not have different command systems. This does not allow the SRAM to be simply interchanged with the DRAM. In this case, a major problem arises from a refresh operation of the DRAM. Data stored in memory cells of the DRAM will be lost unless the DRAM is periodically refreshed. The periodic refresh can be implemented by supplying a refresh command to the DRAM from a controller provided outside of the DRAM. However, this would apply a considerable load to the controller. 
     This needs a periodic refresh that is spontaneously performed within the DRAM. Such a periodic refresh is called self-refresh. If the DRAM is of asynchronous type and operates independently of a clock supplied thereto, a refresh request signal that is internally generated may collide with a request for an active operation (such as a data write or read operation) that is supplied from the outside of the DRAM. 
     FIG. 1 is a block diagram of a part of a core peripheral part of a conventional asynchronous DRAM. FIG. 2 is a timing chart of an operation of the configuration shown in FIG.  1 . 
     Referring to FIG. 1, the DRAM includes a filter  10 , a command control circuit  11 , a refresh (REF) control circuit  12 , a refresh-active (REF-ACT) comparator circuit  13 , and a core control circuit  14 . The REF control circuit  12  periodically supplies the REF-ACT comparator circuit  13  and the core control circuit  14  with a refresh (REF) request signal refpz. A read or write command that is supplied from the outside of the DRAM is applied to the command control circuit  11  via the filter  10 . The read or write command is defined by a combination of control signals (command signals) such as a chip enable signal /CE, a write enable signal /WE, and an output enable signal /OE. The symbol “/” denotes an active-low signal. The filter  10  filters the command signals /CE, /WE and /OE in order to avoid malfunction of the asynchronous type DRAM due to noise. The command control circuit  11  decodes the command received via the filter  10 ,ad an active (ACT) request signal (that requests activation of the core) actpz to the REF-ACT comparator circuit  13  and the core control circuit  14 . 
     The REF-ACT comparator circuit  13  selects one of the ACT request signal actpz and the REF request signal refpz that has been received earlier, and outputs a resultant REF-ACT selection signal refz to the core control circuit  14 . If the refresh operation is selected, the REF-ACT selection signal refz is at a high (H) level (FIG.  2 ( b )). In contrast, if the ACT request signal is selected, the REF-ACT selection signal refz is at a low (L) level (FIG.  2 ( a )). The core control circuit  14  receives either the ACT request signal actpz or the REF request signal refpz, and operates a core (not shown for the sake of simplicity). While the core is operating, the core control circuit  14  outputs a busy signal busyz to the REF-ACT comparator circuit  13  to thus prevent the REF-ACT selection signal from switching over. If the REF request signal refpz is applied during the active operation (read or write operation), of if the ACT request signal actpz is input during the refresh operation, the later input operation is caused to wait for completion of the former input operation, and is then allowed when the former input operation is completed, that is, the busy signal busyz is switched to the L level. 
     FIG. 3 is circuit diagram of the REF-ACT comparator circuit  13 . The circuit  13  includes inverters  15  and  16 , NAND gates  17  and  18 , a transfer switch  19 , a latch  20  and an inverter  21 . The NAND gates  17  and  18  form a flip-flop. When the ACT request signal actpz is applied to the comparator circuit  13 , an output n 1  of the NAND gate  17  is switched to L and an output n 2  of the NAND gate  18  are both switched to H. In contrast, when the REF request signal refpz is input to the circuit  13 , the output n 1  is H, and the output n 2  is L. When the busy signal busyz from the core control circuit  14  is L, the transfer switch  19  is ON, and n 1 =n 3  (the output of the switch  19 )=refz. In contrast when the busy signal busyz is H, the transfer switch  19  is OFF, and the output refz of the inverter  21  does not change. 
     FIG. 4 is a circuit diagram of a configuration of the core control circuit  14 . The circuit  14  includes inverters  22 ,  27 ,  28  and  29 , and NAND gates  23 ,  24 ,  25 ,  26 ,  30 ,  31 ,  32 ,  33  and  34 . The NAND gates  23  and  24  form a flip-flop FF 1 , and the NAND gates  30  and  31  form a flip-flop FF 2 . A core control signal out, which is the output of the NAND gate  34 , is output to a circuit part (not shown) involved in control of the core. When the core control signal out is H, the control of the core is started, and the busy signal busyz is switched to H. The NAND gates  26  and  32  receive the busy signal busyz. When the ACT request signal actpz and the REF request signal refpz are applied, the outputs n 2  and n 1  of the flip-flops FF 2  and FF 1  become H. When the busy signal busyz is L, the core control signal out is H. When the busy signal busyz is H, the output control signal out is L. 
     When the active operation is initiated (busyz=H, refz=L), the flip-flop FF 2  is reset and N 2 =L. When the refresh operation is initiated (busyz=H, refz=H), the flip-flop FF 1  is reset and N 1 =L. If the ACT request command actpz is applied during the refresh operation, the circuit waits for completion of the refresh operation with n 2 =H. When the refresh operation is completed and the busy signal busyz is switched to L, the H level of the node n 2  acts as the core control signal out, and the active operation is initiated. The circuit operates in the same manner as described above when the refresh request signal refpz is applied during the active operation. 
     The access time in the above-mentioned control is the longest in a case where the ACT request signal is output immediately after the REF request signal refpz is applied. The longest access time needs the longest time it takes to output data. FIG. 5 is a timing chart of the above case. The access time shown in FIG. 5 is the total of the time it takes for the ACT request signal actpz to be output from an access command is input (the chip enable signal /CE switches to L, the time of the refresh operation, and the time it takes for data to be output after the ACT request signal actpz is applied. 
     The asynchronous type DRAM independent of the external clock needs the filter  10  to prevent the DRAM from malfunctioning due to noise that may be superimposed on the control signals such as /CE, /WE and /OE and the address signal. The signals that have passed through the filter  10  are applied to the internal circuits of the DRAM. For example, the ACT request signal actpz is generated from the signals that have passed through the filter  10 . It is necessary to delay the signals by at least 1 ns in order to avoid noise equal to 1 ns. Thus, if it is attempted to eliminate noise having a relatively long width, the filter  10  is required to have a long delay. This lengthens the access time until requested data is read out from the DRAM after the corresponding command is applied thereto. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a semiconductor memory device in which the above disadvantages are eliminated. 
     A more specific object of the present invention is to provide a semiconductor memory device having a reduced access time. 
     The above objects of the present invention are achieved by a semiconductor memory device having a self-refresh operation comprising: a detection circuit generating a detection signal when detecting a change of a given input signal; and a comparator circuit comparing the detection signal with a refresh request signal internally generated and generating a control signal indicative of a circuit operation. When a change of the given signal is detected, an instruction about a circuit operation of the semiconductor memory device may be applied thereto from the outside of the device. It takes a certain time to analyze and identify the instruction because it is necessary to eliminate noise from a signal describing the instruction and decode this signal. The detection signal is generated by detecting a change of the given input signal without performing the process for interpreting an instruction described by the given input signal. The detection signal is compared with the refresh request signal. Hence, a circuit operation that is to be executed can be selected promptly. 
     The above objects of the present invention are also achieved by a semiconductor memory device having a self-refresh operation comprising: a detection circuit generating a detection signal when detecting a change of a given input signal; and a refresh control circuit generating a refresh request signal that requests the self-refresh operation responsive to the detection signal in a test mode. The self-refresh operation can be instructed from the outside of the device. Hence, it is possible to easily detect the access time, which corresponds to the time until data is actually read from the device after a change of the given input signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a part of a core peripheral circuit of a conventional DRAM; 
     FIG. 2 is a timing chart of an operation of the configuration shown in FIG. 1; 
     FIG. 3 is a circuit diagram of a REF-ACT selection circuit shown in FIG. 1; 
     FIG. 4 is a circuit diagram of a core control circuit shown in FIG. 1; 
     FIG. 5 is a timing chart for explaining the longest access time; 
     FIG. 6 is a timing chart of a principle of the present invention; 
     FIG. 7 is a block diagram of a first embodiment of the present invention; 
     FIG. 8 is a timing chart of an operation of the semiconductor memory device shown in FIG. 7; 
     FIG. 9 is a circuit diagram of a transition detection circuit shown in FIG. 7; 
     FIG. 10 is a circuit diagram of a configuration of a REF-ACT comparator circuit shown in FIG. 7; 
     FIG. 11 is a circuit diagram of a configuration of a core control circuit shown in FIG. 7; 
     FIG. 12 is a block diagram of a second embodiment of the present invention; 
     FIG. 13 is a timing chart of an operation of the semiconductor memory device shown in FIG. 12; 
     FIG. 14 is a circuit diagram of a configuration of a REF control circuit shown in FIG. 12; 
     FIG. 15 is a circuit diagram of a configuration of a pulse width expansion circuit shown in FIG. 12; 
     FIG. 16 is a circuit diagram of a configuration of a REF-ACT comparator circuit shown in FIG. 12; and 
     FIG. 17 is a block diagram of an example of the entire structure of the semiconductor memory device of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 6 is a timing chart illustrating the principle of the present invention. The timing chart of FIG. 6, which is associated with the timing chart of FIG. 5, principally shows an operation of a semiconductor memory device. According to one aspect of the present invention, a transition detection signal stdpz is newly introduced. The transition detection signal stdpz is generated when a transition is detected in any of the signals applied to the filter  10  shown in FIG.  1 . As has been described with reference to FIG. 1, the filter  10  is supplied with the control signals /CE, /WE and /OE and the address signal. For example, the transition detection signal stdpz is generated when a transition is detected in any of the control signals and the address signal. In the example shown in FIG. 6, the chip enable signal /CE has a transition in which it changes from H to L. 
     The transition detection signal stdpz is compared with the REF request signal refpz in order to select an earlier one of these signals. In the example shown in FIG. 6, the REF request signal refpz slightly leads to the transition detection signal stdpz. Thus, the refresh operation is selected, and the active operation is started after the refresh operation is completed. It can be seen from comparison between the access time shown in FIG.  5  and that shown in FIG. 6 that the semiconductor memory device of the present invention has a shorter access time than the conventional device and operates faster. It is to be noted that the conventional circuit is designed so that the command control circuit  11  generates the ACT request signal actpz from the signals that have passed through the filter  10 . In contrast, the transition detection signal stdpz is generated from the signals that do not pass through the filter  10 . Hence, the present invention is capable of outputting data faster. 
     As has been described, the access time is the total of the time it takes for the transition detection signal stdpz to be output from the time when the access command is applied to the filter  10 , the time of the refresh operation, and the time it takes for data to be output after the ACT request signal actpz is applied. The data outputting shown in FIG. 6 is faster than that shown in FIG. 5 by the time necessary for the ACT request signal to be output (equal to the delay of time by the filter  10 ) after the transition detection signal stdpz is output. 
     FIG. 7 is a block diagram of a semiconductor memory device according to an embodiment of the present invention. In FIG. 7, parts that are the same as those shown in FIG. 1 are given the same reference numbers as previously. The semiconductor memory device shown in FIG. 7 includes the filter  10 , the command control circuit  11 , the REF control circuit  12 , a transition detection circuit  41 , a pulse width expansion circuit  42 , a REF-ACT comparator circuit  43 , and a core control circuit  44 . The transition detection circuit  41  and the pulse width expansion circuit  42  are newly introduced by the present embodiment. The REF-ACT comparator circuit  43  and the core control circuit  44  have configurations different from those of the REF-ACT comparator circuit  13  and the core control circuit  14  shown in FIG.  1 . 
     The transition detection circuit  41  generates the transition detection signal stdpz shown in FIG.  6 . The detection subject signals are the input signals of the filer  10 , namely, the control signals including the chip enable signal /CE, the write enable signal /WE and the output enable signal /OE, and the address signal. The transition detection circuit  41  detects a transition of a given bit in the control signals and the address signals. For example, the transition detection circuit  41  detects a change from H to L in any of the control signals /CE, /WE and /OE and a change from L to H or vice versa in the address signal. When such a transition is detected, the transition detection circuit  41  generates the transition detection signal stdpz. The transition detection signal stdpz is applied to the REF-ACT comparator circuit  44  and the pulse width expansion circuit  42 . The REF-ACT comparator circuit  43  compares the transition detection signal stdpz with the REF request signal refpz, and outputs the resultant REF-ACT selection signal refz to the core control circuit  44 . If the transition detection signal stdpz leads to the REF request signal refpz in terms of timing, the REF-ACT selection signal refz is L. In contrast, if the transition detection signal stdpz lags behind the REF request signal refpz, the REF-ACT selection signal refz is H. The ACT request signal actpz is generated from the signals that have passed through the filter  10  as in the case of FIG.  1 . 
     FIG. 8 is a timing chart of an operation of the circuit shown in FIG.  7 . More particularly, FIG. 8 illustrates a case where the REF request signal refpz leads to the ACT request signal actpz. The transition detection signal stdpz generated by the transition detection circuit  41  leads to the REF request signal refz. Hence, the REF-ACT comparator circuit  43  sets the REF-ACT selection signal refz to L to thus select the active operation, and then sets the REF-ACT selection signal to H to thereby select the refresh operation. It will be noted that the conventional configuration selects the refresh operation first and the active operation send if the situation shown in FIG. 8 occurs in the conventional configuration. 
     The mere replacement of the input of the REF-ACT comparator circuit  43  from the ACT request signal actpz to the transition detection signal stdpz would select the refresh operation if the REF request signal is output until the ACT request signal actpz is output after the transition detection signal stdpz is output. Thus, it is necessary to disable the refresh operation until the busy signal busyz switches to H after the transition detection signal stdpz is output. The above control is implemented by a signal stdpwz, which may be obtained by expanding the pulse width of the transition detection signal stdpz. The pulse width expansion circuit  42  expands to the pulse width of the transition detection signal stdpz to thus generate the signal stdpwz. 
     The REF-ACT comparator circuit  43  receives the pulse-width-expanded signal stdpwz from the pulse width expansion circuit  42 , and does not accept (invalidates) the REF request signal refpz as long as the signal stdpwz is ON (H). The pulse-width-expanded signal stdpwz is applied to the core control circuit  44 . The refresh operation is not executed until the core control circuit  44  ends the active operation. 
     FIG. 9 is a circuit diagram of a configuration of the transition detection circuit  41 . The circuit  41  includes inverters  50  and  51 , a delay element  52 , a NAND gate  53  and an inverter  54 . A plurality of detection circuits, each having the configuration shown in FIG. 9, are respectively provided to the control signals /CE, /WE and /OE. For the sake of simplicity of illustration in FIG. 9, the control signals /CE, /WE and /OE are commonly applied to the transition detection circuit. The transition detection circuit  41  includes a detection circuit made up of inverters  55  and  56 , a delay element  57 , and a NAND gate  58 . Similarly, the circuit  41  includes another detection circuit made up of an inverter  59 , a delay element  60 , and a NAND gate  61 . The two detection circuits related to the address signal are paired and provided for each address bit. That is, the pair of detection circuits processes a single bit of the address signal. 
     For example, when the chip enable signal /CE changes from H to L, the output of the inverter  50  is changed from L to H, which is received by the NAND gate  53 . The output of the delay element  52  is initially H and changes to L when the delay time of the delay element lapses after the output of the inverter  50  changes to H. Thus, the output of the NAND gate  53  changes from H to L at the same time as the chip enable signal /CE changes from H to L, and returns to H after the delay time of the delay element  52 . The transition detection signal stdpz is the inverted version of the output of the NAND gate  63 . In short, the transition detection signal stdpz is a high-level pulse signal generated when the chip enable signal /CE changes from H to L. 
     FIG. 10 is a circuit diagram of a configuration of the REF-ACT comparator circuit  43 . In FIG. 10, parts that are the same as those shown in FIG. 3 are given the same reference numbers as previously. The REF-ACT comparator circuit  43  shown in FIG. 10 is obtained by adding a NOR gate  65  to the circuit configuration shown in FIG.  3 . The NOR gate  65  performs a NOR operation on the busy signal busyz and the pulse-width-expanded signal stdpwz, and controls the transfer switch  19  on the basis of the result of the NOR operation. When either the busy signal busyz or the pulse-width-expanded signal stdpwz is H, the transfer switch  19  is OFF. Hence, the REF-ACT selection signal refz is maintained. That is, the refresh operation is not accepted until the busy signal busyz changes to H after the transition detection signal stdpz is detected and during the busy period. 
     FIG. 11 is a circuit diagram of a configuration of the core control circuit  44 . In FIG. 11, parts that are the same as those shown in FIG. 4 are given the same reference numerals as previously. The core control circuit  44  shown in FIG. 11 can be configured by modifying the circuit shown in FIG. 4 so that an inverter  66  is added thereto and a three-input NAND gate  67  is substituted for the NAND gate  25 . The NAND gate  67  is closed as long as the pulse-width-expanded signal stdpwz is H. Hence, even if the REF request signal refpz is applied, there is no change of the output out of the NAND gate  34 . That is, the refresh operation is not performed until the active operation ends after the transition detection signal stdpz is output. 
     A description will now be given of a second embodiment of the present invention. FIG. 12 is a block diagram of a semiconductor memory device according to the second embodiment of the present invention. In FIG. 12, parts that are the same as those shown in FIG. 7 are given the same reference numbers as previously. FIG. 13 is a timing chart of an operation of the semiconductor memory device shown in FIG.  12 . 
     The second embodiment of the present invention is configured taking the following into consideration. It is not possible to detect, from the outside of the memory device, the time at which the REF control circuit  12  generates the REF request signal refpz. When the read or write command is applied to the memory device, the corresponding active operation is performed if the refresh operation is in progress. If the refresh operation and the active operation overlap each other in term of timings, the refresh operation is executed first, and the active operation is executed second. Hence, the access time is not constant but varies. For example, as shown in FIG. 5, when the refresh operation and the active operation are almost concurrently requested, the access time is the longest. It is required to know the longest access time in order to evaluate the semiconductor memory device. However, the timing relationship as shown in FIG. 5 cannot be known from the outside of the memory device, and therefore the longest access time cannot be known. 
     According to the second embodiment of the present invention, a test signal is applied to the first embodiment of the present invention to set the device in a test mode. In the test mode, the refresh operation is necessarily executed in advance of an active operation requested from the outside of the device. 
     Referring to FIGS. 12 and 13, a test signal tesz is applied to a REF control circuit  72  and a pulse width expansion circuit  74  from the outside of the DRAM. The REF control circuit  72  generates the REF request signal refpz when the transition detection signal stdpz output by the transition detection circuit  41  switches to H in a state in which the test signal tesz has been applied to the circuit  72 . A REF-ACT comparator circuit  73  receives the REF request signal refpz, and switches the REF-ACT selection signal refz to H in order to cause the core control circuit  44  to select the refresh operation. In response to a falling edge of the busy signal busyz after the refresh operation ends, the REF-ACT comparator circuit  73  sets the REF-ACT selection signal to L, and the core control circuit  44  instructs the core to execute an operation responsive to the ACT request signal actpz. 
     As described above, the refresh operation is executed immediately after the transition detection signal stdpz is detected in the test mode, and the active operation is then executed. Hence, it is possible to easily measure the access time (the time it takes for data to be output after the signal /CE falls) shown in FIG.  6 . 
     FIG. 14 is a circuit diagram of a configuration of the REF control circuit  72 . The circuit  72  includes a control circuit  76 , NAND gates  77 ,  78  and  79 , and an inverter  80 . The control circuit  76  generates an internal refresh request signal srtz, which is supplied to the NAND gate  78 . The refresh request signal refpz shown in FIG. 1 or FIG. 7 is the internal refresh signal itself. When the circuit is not in the test mode, the test signal tesz is L, and the internal refresh request signal srtz acts as the refresh request signal refpz. When the circuit is in the test mode, the test signal tesz is H. The refresh request signal refpz switches to H immediately in response to the transition detection signal stdpz. 
     FIG. 15 is a circuit diagram of a configuration of the pulse width expansion circuit  74 . The circuit  74  is made up of a pulse width expanding part  80 , inverters  81  and  82 , and a NAND gate  83 . The pulse width expanding part  80  corresponds to the pulse width expansion circuit  42  shown in FIG.  7 . In the test mode, the refresh operation is executed in advance. Thus, the transition detection signal stdpz is not applied to the pulse width expanding part  80 . When the circuit is not in the test mode, the transition detection signal stdpz is applied to the pulse width expanding part  80  via the NAND gate  83  and the inverter  82 . 
     FIG. 16 is a circuit diagram of a configuration of the REF-ACT comparator circuit  73 . In FIG. 16, parts that are the same as those shown in FIG. 10 are given the same reference numerals. The circuit shown in FIG. 16 is configured by substituting a NAND gate  84  for the inverter  16  shown in FIG.  10 . The NAND gate  84  is supplied with the busy signal busyz and the REF-ACT request signal refz. In the configuration shown in FIG. 10, the flip-flop composed of the NAND gates  17  and  18  is reset by the transition detection signal stdpz, and the REF-ACT selection signal refz is switched to L in the active operation. Since the transition detection signal stdpz is output in advance of the REF request signal refpz in the test mode in the test mode, the REF-ACT selection signal refz cannot be switched to L using the transition detection signal stdpz after the refresh operation. With the above in mind, the circuit configuration shown in FIG. 16 is designed so that the flip-flop is set when the circuit enters the refresh operation. 
     FIG. 17 is a block diagram of an example of the entire structure of the semiconductor memory device of the present invention. 
     The memory device shown in FIG. 17 includes an address terminal  171 , command input terminals  172  through  174 , a data input/output terminal  175 , input buffers  176  through  179  respectively connected to the terminals  171  through  174 , a refresh control circuit  180  for controlling the refresh operation, an input buffer/output buffer  181 , an address register  182 , a control circuit  183 , a data control circuit  184 , a memory cell array (core)  185 , and a write amplifier/sense buffer  186 . The refresh control circuit  180  corresponds to the REF control circuit  12  shown in FIG.  7  and the REF control circuit  72  shown in FIG.  12 . An external address is received via the address terminal  171  and the input buffer  176 , and decoded addresses in the row and column systems are applied to the memory cell array  185 . The control signals /CE, /WE and /OE are applied to the control circuit  183  via the input buffers  177 ,  178  and  179 , respectively. The data input/output circuit  184  controls inputting/outputting of data under the control of the control circuit  183 . 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application no. 2000-054882 filed on Feb. 29, 2000, the entire contents of which are hereby incorporated by reference.