Abstract:
The present invention for preventing a data error by satisfying specifications of tHD and tCBPH is provided. The semiconductor memory device having an enough margin for a write/read operation includes a pre-charging block for performing a pre-charging operation based on a chip selection control signal; a write/read strobe generating block for performing a write/read operation based on the chip selection control signal and a chip selection signal; and a chip selection buffering block for generating the chip selection control signal based on the chip selection signal to control a timing of the pre-charging operation and a timing of the write/read operation.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a semiconductor memory device; and, more particularly, to a semiconductor memory device, which can prevent a data error generally occurring due to an enabling timing of a chip selection signal in a pseudo static random access memory device.  
       DESCRIPTION OF THE RELATED ART  
       [0002]     Generally, dynamic random access memory (DRAM) devices store information into capacitors in the form of charges. The stored charges of the capacitors are distributed to corresponding bit lines through transistors, and sense amplifiers amplify voltages to read data. Since a memory cell includes one transistor and one capacitor, a memory device with large capacitance can be implemented in a smaller area.  
         [0003]     Memory devices have currently been scaled down to achieve several intended functions such as high operation speed, decreased power consumption and minimized processing systems. As memory devices have been minimized, the capacitor areas of the memory cells have been reduced, resulting in a decreased capacitance level. Due to the decreased capacitance level, an amount of charges that can be retained is reduced even if data are inputted at the same voltage level with respect to the capacitors.  
         [0004]     A refresh operation is generally performed periodically to compensate the decreased amount of charges of the capacitors that can be retained. The refresh operation reads data stored into the capacitors of the memory cells through employing bit lines and then, the read data are amplified by sense amplifiers. The amplified data are rewritten onto the capacitors of the original memory cells.  
         [0005]     Therefore, if a data retention characteristic is degraded in micronized devices, it is often required to shorten a period of the refresh operation to compensate the degradation of the data retention characteristic. However, if the refresh operation period is shortened, external processing devices cannot access DRAM devices during the refresh operation and thus, the processing systems may perform poorly.  
         [0006]     Also, if the period of the refresh operation is shortened, current for the refresh operation may be consumed more highly. Particularly, it may be difficult to satisfy a low level of standby current required for a data retention mode in portable devices that operate by batteries. Thus, DRAM devices cannot be applied to those portable devices requiring low power consumption.  
         [0007]     As one method of resolving the above limitation, a pseudo static random access memory (PSRAM) device has been introduced. The PSRAM device operates similarly to a static random access memory (SRAM) device. In the PSRAM device, among memory access cycles, a cycle of a read operation and a write operation and a cycle of a refresh operation run consecutively. That is, since the refresh operation is executed within one memory access cycle, it is possible to hide the refresh operation with respect to an external access operation, and thus, a DRAM device can operate as a SRAM device.  
         [0008]      FIG. 1  is a timing diagram showing an operation of a conventional PSRAM device for controlling a chip selection signal.  
         [0009]     A period from a rising edge of a clock CLK to a rising edge of a chip selection signal CSB is called ‘tHD’. However, the conventional PSRAM device has a high value of a tHD specification. That is, the chip selection signal CSB is disabled at the next clock CLK after the last data d 3  is inputted. However, the conventional tHD specification cannot satisfy the minimum period of 2 ns.  
         [0010]     In the conventional tHD specification, after the tHD period, the chip selection signal CSB is enabled immediately as a period of ‘tCBPH’ representing a width of a high pulse of the chip selection signal CSB is passed by. Herein, the tCBPH period is generally 5 ns. If the chip selection signal CSB is enabled immediately after the state that the tHD period is passed by and the tCBPH period is satisfied, the chip selection signal CSB is specifically set to be enabled before a rising edge of the next clock CLK. Therefore, when the tHD period is 2 ns, it may be difficult to write the data d 3 , which is inputted as being synchronized with the clock CLK, on a memory cell.  
       SUMMARY OF THE INVENTION  
       [0011]     It is, therefore, an object of the present invention to provide a semiconductor memory device, which can prevent a data error by satisfying specifications of tHD and tCBPH and writing a lastly inputted data at a tHD interval on a memory cell of a synchronous pseudo static random access memory (PSRAM) device.  
         [0012]     In accordance with an aspect of the present invention, there is provided a semiconductor memory device, including a pre-charging block for performing a pre-charging operation based on a chip selection control signal; a write/read strobe generating block for performing a write/read operation based on the chip selection control signal and a chip selection signal; and a chip selection buffering block for generating the chip selection control signal based on the chip selection signal to control a timing of the pre-charging operation and a timing of the write/read operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  is a timing diagram illustrating an operation of a conventional semiconductor memory device;  
         [0015]      FIG. 2  is a configuration diagram illustrating a semiconductor device in accordance with an embodiment of the present invention;  
         [0016]      FIG. 3  is a detailed circuit diagram of a clock buffering block illustrated in  FIG. 2 ;  
         [0017]      FIG. 4  is a detailed circuit diagram of a first pulse generating unit illustrated in  FIG. 3 ;  
         [0018]      FIG. 5  is a detailed circuit diagram of a chip selection buffering block illustrated in  FIG. 2 ;  
         [0019]      FIGS. 6 and 7  are waveform diagrams of exemplary operations of the chip selection buffering block illustrated in  FIG. 5 ;  
         [0020]      FIG. 8  is a detailed circuit diagram of a page controlling block illustrated in  FIG. 2 ; and  
         [0021]      FIG. 9  is a detailed circuit diagram of a write/read strobe generating block illustrated in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     A semiconductor memory device in accordance with exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0023]      FIG. 2  is a configuration diagram of a semiconductor memory device in accordance with an embodiment of the present invention.  
         [0024]     The semiconductor memory device includes a clock buffering block  100 , a chip selection buffering block  200 , a pre-charging block  300 , a page controlling block  400 , and a write/read strobe generating block  500 .  
         [0025]     The clock buffering block  100  buffers a clock CLK and generates a clock transition detection signal CTDB for every rising edge of the clock CLK. The chip selection buffering block  200  buffers a power-up signal PWRUP, the clock transition detection signal CTDB and a chip selection signal CSB and outputs a chip selection controlling signal CSB 4  for determining a pre-charge operation of a word line. The pre-charging block  300  controls a pre-charge operation according to the chip selection control signal CSB 4 .  
         [0026]     The page buffering block  400  controls a page address according to the chip selection signal CSB, an address transition control signal ADVB and the power-up signal PWRUP and outputs a write/read strobe control signal WR_STB_C. The write/read strobe generating block  500  controls a strobe operation according to the chip selection control signal CSB 4 , the clock transition detection signal CTDB and the write/read strobe control signal WR_STB_C and outputs a write/read strobe signal WR_STB.  
         [0027]      FIG. 3  is a detailed circuit diagram of the clock buffering block  100  illustrated in  FIG. 2 .  
         [0028]     The clock buffering block  100  includes first to fourth inverters IV 1  to IV 4 , and a first pulse generating unit  110 . The first inverter IV 1  and the second inverter IV 2  delay the clock CLK without an inversion and output an input signal IN. The first pulse generating unit  110  outputs an output signal OUT with a certain pulse width according to the input signal IN. The third inverter IV 3  and the fourth inverter IV 4  delay the output signal OUT without an inversion and output the clock transition detection signal CTDB. On the basis of these sequential steps, the clock buffering block  100  generates the clock transition detection signal CTDB for every rising edge of the clock CLK.  
         [0029]      FIG. 4  is a detailed circuit diagram of the first pulse generating unit illustrated in  FIG. 3 .  
         [0030]     The first pulse generating unit  110  includes fifth to ninth inverters IV 5  to IV 9  and a first NAND gate ND 1 . The fifth to the ninth inverters IV 5  to IV 9  delay the input signal IN with an inversion. The first NAND gate ND 1  performs a NAND operation on the input signal IN and an output signal of the ninth inverter IV 9  and outputs the output signal OUT with the certain pulse width. Hence, the first pulse generating unit  110  outputs the output signal OUT of a low pulse when the input signal IN is transited from a low level to a high level.  
         [0031]      FIG. 5  is a detailed circuit diagram of the chip selection buffering block illustrated in  FIG. 2 .  
         [0032]     The chip selection buffering block  200  includes tenth to thirteenth inverters IV 10  to IV 30 , a second pulse generating unit  220 , a first P-channel metal oxide semiconductor (PMOS) transistor, P 1 , a first N-channel metal oxide semiconductor (NMOS) transistor N 1 , and a second NMOS transistor N 2 , and second to fourth NAND gates ND 2  to ND 4 .  
         [0033]     The tenth inverter IV 10  and the eleventh inverter IV 11  delay the chip selection signal CSB. The twelfth inverter IV 12  inverts an output of the eleventh inverter IV 11 . A first delaying unit  210  includes the thirteenth to sixteenth inverters IV 13  to IV 16  and delay an output of the twelfth inverter IV 12  for a predetermined time. The second pulse generating unit  220  generates a signal with a certain pulse width according to an output of the first delaying unit  210 . A second delaying unit  230  including the eighteenth inverter IV 18  to the twenty-first inverter IV 21  and a third delaying unit  240  including the twenty-second inverter IV 22  to the twenty-fifth inverter IV 25  delay the output of the eleventh inverter IV 11  for a predetermined time and transmits the output to a first node B.  
         [0034]     The first PMOS transistor P 1  is connected between a power supply terminal and a second node A and is supplied with the clock transition detection signal CTDB through a gate. The first NMOS transistor N 1  is connected between the first node A and a ground voltage terminal and is supplied with an output of the seventeenth inverter IV 17  through a gate. The twenty-seventh inverter IV 27  and the twenty-eighth inverter IV 28  serve as a latching unit, which latches an output of the second node A. The second NMOS transistor N 2  is connected between the second node A and a ground voltage terminal and is supplied with the power-up signal PWRUP through a gate, wherein the power-up signal PWRUP is inverted by the twenty-sixth inverter IV 26 .  
         [0035]     The second NAND gate ND 2  performs a NAND operation to the output of the first node B and the output of the eleventh inverter IV 11  and outputs the NAND operation result to a third node C. The third NAND gate ND 3  performs a NAND operation to an output of the twenty-ninth inverter IV 29  and an output of the third node C and outputs the NAND operation result to a fourth node D. The fourth NAND gate ND 4  performs a NAND operation to an output of the fourth node D and the output of the eleventh inverter IV 11 . The thirtieth inverter IV 30  inverts an output of the forth NAND gate ND 4  and outputs the chip selection control signal CSB 4 .  
         [0036]      FIG. 6  is a waveform diagram illustrating an operation of the chip selection buffering block  200  when a tCBPH period is shorter than one cycle of the clock CLK.  FIG. 7  is a waveform diagram illustrating an operation of the chip selection buffering block  200  when a tCBPH period is longer than one cycle of the clock CLK.  
         [0037]     As illustrated in  FIG. 6 , when the chip selection signal CSB at the tCBPH period with a narrow pulse width is inputted from a pad, the chip selection control signal CSB 4  retains a low level. The pre-charge operation is not carried out by continuously retaining the low level of the chip selection control signal CSB 4 . Hence, it is possible to write a data, which is inputted lastly during an interval of tHD, on a memory cell.  
         [0038]     On the other hand, as illustrated in  FIG. 7 , the chip selection signal CSB at a tCBPH period with a wide pulse width is inputted from a pad, the chip selection control signal CSB 4  is in a high level. As the chip selection control signal CSB 4  is transited from a low level to a high level, the pre-charge operation that disables a currently enabled word line is carried out to terminate a write/read operation. That is, since the chip selection control signal CSB 4  retains the high level due to a certain delay time, the pre-charge operation with respect to the word line is carried out after the last input data is written on the memory cell.  
         [0039]      FIG. 8  is a detailed circuit diagram of the page controlling block  400  illustrated in  FIG. 2 .  
         [0040]     The page controlling block  400  includes thirty-first to forty-first inverters IV 31  to IV 41 , a third pulse generating unit  410 , a fourth pulse generating  430 , a fourth delaying unit  420 , a second PMOS transistor P 2 , third and fourth NMOS transistors N 3  and N 4 , a page control signal generating unit  440 , and a first NOR gate NOR 1 .  
         [0041]     The thirty-first to thirty-third inverters IV 31 , IV 32  and IV 33  invert and delay the address transition control signal ADVB and, output the inverted address transition control signal. The third pulse generating unit  4010  outputs a signal with a certain width according to the inverted address transition control signal ADV. The fourth delaying unit  420  delays the chip selection signal CSB without an inversion. The fourth pulse generating unit  430  delays an output of the fourth delaying unit  420  and outputs a signal with a certain width.  
         [0042]     The second PMOS transistor P 2  and the third NMOS transistor N 3  are connected between a power supply terminal and a ground voltage terminal. The second PMOS transistor P 2  is supplied with an output of the third pulse generating unit  410  through a gate. The third NMOS transistor N 3  is supplied with an output of the third eighth inverter IV 38  through a gate.  
         [0043]     The fortieth inverter IV 40  and the forty-first inverter IV 41  serving as a latching unit latch an output of the second PMOS transistor P 2  and output another chip selection control signal CS_CON. The fourth NMOS transistor N 4  is connected in parallel with the third NMOS transistor N 3 , and the power-up signal PWRUP inverted by the thirty-ninth inverter IV 39  is inputted to the fourth NMOS transistor N 4  through a gate.  
         [0044]     The page control signal generating unit  440  generates a page control signal P_CON for controlling a page operation according to the address transition control signal ADV. The first NOR gate NOR 1  performs a NOR operation to the other chip selection control signal CS_CON and the page control signal P_CON and outputs the write/read strobe control signal WR_STB_C.  
         [0045]      FIG. 9  is a detailed circuit diagram of the write/read strobe generating block  500  illustrated in  FIG. 2 .  
         [0046]     The write/read strobe generating block  500  includes forty-second to forty-eighth inverters IV 42  to IV 48 , a fifth NAND gate ND 5 , and a second NOR gate NOR 2 . The forty-second inverter to forty-seventh to inverters IV 42  to IV 47  delay the chip selection control signal CSB 4  for a predetermined time. The second NOR gate NOR 2  performs a NOR operation to an output of the forty-seventh inverter IV 47  and the clock transition detection signal CTDB. The fifth NAND gate ND 5  performs a NAND operation to the write/read strobe control signal WR_STB_C and an output the NOR gate NOR 2 . The forty-eighth inverter IV 48  inverts an output of the fifth NAND gate ND 5  and outputs the write/read strobe signal WR_STB.  
         [0047]     Hereinafter, operation of the semiconductor memory device (i.e., the PSRAM device) configured as above will be described in detail.  
         [0048]     The power-up signal PWRUP is transited from a low level to a high level while the PSRAM device is supplied with the initial power. At this time, values of internal latch circuits are determined. Thus, the second node A of the chip selection buffering block  200  latches a low level according to the power-up signal PWRUP.  
         [0049]     If the PSRAM device operates asynchronously, the clock CLK is not toggled and thus the clock transition detection signal CTDB retains a high level continuously. As a result, the second node A is in a low level because of the power-up signal PWRUP and the fourth node D is in a high level, so that the chip selection control signal CSB 4  becomes identical to a signal inputted to a chip selection signal CSB pad.  
         [0050]     On the other hand, in the case of a synchronous PSRAM device, the input of the chip selection signal CSB satisfies the tHD specification between rising edges of the clock CLK. As illustrated in  FIG. 6 , operation of the PSRAM device when the tCBPH period is shorter than one cycle of the clock CLK will be described hereinafter.  
         [0051]     When the clock CLK is transited from a low level to a high level, the clock buffering block  100  generates the clock transition detection signal CTDB, which is a low pulse, for every rising edge of the clock CLK. An output of the second pulse generating unit  220  is in a low level when the chip selection signal CSB is enabled in a low level.  
         [0052]     As a result, the second node A latches a signal in a low level by the twenty-seventh inverter IV 27  and the twenty-eighth inverter IV 28  (i.e., the latching unit) and then latches a signal in a high level when the clock transition detection signal CTDB is inputted in a low level at the rising edge of the clock CLK.  
         [0053]     The first node B is inputted with a signal obtained as the second delaying unit  230  and the third delaying unit  240  delay the chip selection signal CSB. That is, as illustrated in  FIG. 6 , when the chip selection signal CSB at the tCBPH period with the pulse width shorter than the delay time of the first node B is inputted from the pad, the chip selection signal CSB is in a low level as passing through the second NAND gate ND 2  to the fourth NAND gate ND 4 . Hence, the chip selection control signal CSB 4  retains a low level.  
         [0054]     As the chip selection control signal CSB 4  retains the low level, the pre-charging block  300  does not pre-charge a word line. Therefore, the lastly inputted data during the tHD interval can be written on a memory cell.  
         [0055]     As illustrated in  FIG. 7 , when the chip selection signal CSB at the tCBPH period with the pulse width larger than the delay time of the first node B is inputted from the pad, the chip selection control signal CSB 4  is transited to a high level and enabled in a low level again.  
         [0056]     There is a predetermined delay time before the chip selection control signal CSB 4  is transited to the high level. Thus, a data inputted at the tHD interval can be written on a memory cell for the predetermined delay time of the chip selection control signal CSB 4 . Next, when the chip selection control signal CSB 4  is transited from a low level to a high level, the pre-charging block  300  performs a pre-charge operation that disables a currently enabled word line, thereby terminating the write/read operation.  
         [0057]     Herein, the pre-charging block  300  disables the word line after the write/read operation is terminated. In accordance with the embodiment of the present invention, the pre-charge operation is set to be carried out when the chip selection control signal CSB 4  is disabled.  
         [0058]     The address transition control signal ADVB of the page controlling block  400  is synchronized with the clock CLK at the moment that an external instruction is inputted to the synchronous PSRAM device, whereby the address transition control signal ADVB is in a low level. Therefore, the page control signal generating unit  400  generates the page control signal P_CON at the moment that among the synchronous write/read operations, the write/read strobe signal WR_STB is enabled according to the address transition control signal ADVB.  
         [0059]     When the address transition control signal ADVB is in a low level as an external instruction is inputted, the inverted address control signal ADV is in a high level and the output of the third pulse generating  410  is in a low level. As a result, the chip selection control signal CS_CON of a low level is latched. Under this state, the write/read strobe control signal WR_STB_C is outputted according to the page control signal P_CON.  
         [0060]     At this time, if the chip selection signal CSB is in a high level, it is unnecessary to generate the write/read strobe signal WR_STB. Therefore, when the chip selection signal CSB is in a high level, the chip selection control signal CS_CON of a high level is latched to thereby prevent a generation of the write/read strobe signal WR_STB.  
         [0061]     Meanwhile, at the write/read strobe generating block  500 , when the chip selection control signal CSB 4  is in a low level as the PSRAM device is enabled, the clock CLK is toggled, whereby the clock transition detection signal CTDB is continuously inputted in a low level. At this time, when the write/read strobe control signal WR_STB_C is inputted in a high level, the write/read strobe signal WR_STB is enabled in a high level for every clock, so that the PSRAM device can carry out the write/read operation.  
         [0062]     In accordance with the embodiment of the present invention, the write/read strobe signal WR_STB necessary for the pre-charge operation and the write/read operation is controlled by the chip selection control signal CSB 4  generated at the chip selection buffering block  200  in order for a data inputted at the tHD interval to be written on a memory cell. Thus, when the chip selection signal CSB having a low value of the tCBPH period is disabled right after the tHD period, the pre-charge operation for disabling a currently enabled word line is not carried out. After the tHD period, if the chip selection signal CSB is disabled, the write/read strobe signal WR_STB is generated to write a data on a memory cell.  
         [0063]     Also, after the tHD period, if the chip selection signal CSB with a high value of the tCBPH period is inputted, the chip selection control signal CSB 4  is disabled after a predetermined delay time. On the basis of the chip selection control signal CSB 4 , the word line is pre-charged to write a data, which is inputted lastly during the tHD period, on a memory cell.  
         [0064]     On the basis of the embodiment of the present invention, it is possible to prevent a generation of a data error caused by a certain timing of a chip selection signal. The present application contains subject matter related to the Korean patent application No. KR 2005-0058496, filed in the Korean Patent Office on Jun. 30, 2005, the entire contents of which being incorporated herein by reference.  
         [0065]     While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.