Abstract:
Provided is a decoding circuit for a memory device which is improved in an operation of chip so as to enable the operation to be predictable by making a decoded result corresponding to an undefined code get a specific value. The decoding circuit for a memory device generates address signals by control signals set with a mode, and comprises a first logical circuit for decoding and outputting a result value defined by logically-combining the address signals corresponding to a first group and a second logical circuit for performing a decoding operation to have address signals with a specific value included in the defined result value by logically-combining address signals corresponding to a second group, by dividing the address signals into the first group corresponding to at least one defined result value and the second group corresponding to an undefined result value.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a decoding circuit, and more particularly to, a decoding circuit for a memory device, which is improved in an operation of chip so as to enable the operation to be predictable by making a decoded result corresponding to an undefined code become a specific value. 
   2. Description of the Related Art 
   Generally, a semiconductor memory device  10  (referring to  FIG. 1D ) such as SRAM, DRAM, and a Flash memory is comprised of a decoding circuit  12  which defines a control signal -for instance, CAS Latency (namely, CL) or Write recovery time (namely, tWR)- for setting an internal operation of the memory device  10  by receiving an address signal. For reference, CL denotes the minimum number of clocks from inputting a column address to outputting data and tWR denotes time from writing data in a cell to precharging the data. 
   For instance, the decoding circuit  12  as shown in  FIG. 1D  applied to CL combines an address signal applied through the fourth (A 4 ), fifth (A 5 ), and sixth (A 6 ) address pins among address signals, and outputs signal corresponding to one of CL  2 ,  3 ,  4 ,  5 , and  6 . Furthermore, the decoding circuit  12  as shown in  FIG. 1D  applied to tWR combines an address signal applied through the ninth (A 9 ), tenth (A 10 ), and eleventh (A 11 ) address pins among address signals, and outputs a signal corresponding to  2 ,  3 ,  4 ,  5 , and  6 . 
   Hereinafter, it will be described about the conventional decoding circuit with reference to  FIGS. 1   a ,  1   b , and  1   c , and  FIGS. 2   a ,  2   b , and  2   c.    
     FIGS. 1   a ,  1   b , and  1   c  are circuit diagrams illustrating a decoding circuit for a memory device  10  applied to the conventional CAS latency CL.  FIG. 1   a  shows a definition (truth) table for CL, (wherein A 4 . A 5 . and A 6  correspond to the respective fourth, fifth and sixth address pins, wherein the “Reserved” items indicated in  FIG. 1   a  correspond to the unspecified address signals, and “ 2 ”, “ 3 ”, “ 4 ”, “ 5 ”, and “ 6 ” items Indicated in  FIG. 1   a  correspond to the specified address signals),  FIG. 1   b  shows a circuit for generating an address signal used in a semiconductor memory device by a control signal (Mode REGister set: MREG) (wherein A 4 . A 5 . and A 6  correspond to the respective fourth, fifth and sixth address pins, and /A 4 , /A 5 , and /A 6  correspond to the respective fourth, fifth and sixth complementary address pins), and  FIG. 1   c  shows a decoding circuit for outputting CL (wherein A 4 , A 5 , and A 6  correspond to the respective fourth, fifth and sixth address pins, and /A 4 . /A 5 , and /A 6  correspond to the respective fourth, fifth and sixth complementary address pins) with a specific value by combining with an address signal generated from the circuit in  FIG. 1   b.    
   For Example, if the specified address signals at the address pins A 6 , A 5 , A 4  generated by control signals (MREG 6 , MREG 5 , MREG 4  are ( 1 ,  0 ,  0 ) in sequence, then the value of the outputted control signal is a predetermined value corresponding to 4 clock periods of the CL as shown outputted. In  FIG. 1   a  and in  FIG. 1   c  as indicated by a “4”. 
   On the other hand, if the unspecified address signals at the address pins A 6 , A 5 , A 4  generated by control signals MREG 6 , MREG 5 , MREG 4  in  FIG. 1   b  are ( 0 ,  0 , 0 ), ( 0 ,  0 ,  1 ), or( 1 ,  1 ,  1 ), CL outputted from  FIG. 1   c  is set in an undefined state (i.e., a reserved state) as shown in  FIG. 1   a.    
   Therefore, if the address signal corresponding to the undefined CL is inputted, it can&#39;t be predicted for an output from the conventional decoding circuit. In response to this, it can&#39;t be checked for an operational condition of a memory device as well. Especially, a peak current may flow in the memory device. 
   Meanwhile,  FIGS. 2   a ,  2   b , and  2   c  are circuit diagrams illustrating a decoding circuit applied to the conventional tWR.  FIG. 2   a  shows a definition table for tWR (wherein A 9 , A 10 , and A 11  correspond to the respective ninth, tenth and eleventh address pins and /A 9 , /A 10 , and /A 11  correspond to the respective ninth, tenth and eleventh complementary address pins, wherein the “Reserved” item in  FIG. 2   a  correspond to the unspecified signals, and “ 2 ”, “ 3 ”, “ 4 ”, “ 5 ”, and “ 6 ” items in  FIG. 2   a  correspond to the specified signals),  FIG. 2   b  shows a circuit for generating an address signal (wherein A 9 , A 10 , and A 11  correspond to the respective ninth, tenth and eleventh address pins and /A 9 , /A 10 , and /A 11  correspond to the respective ninth, tenth and eleventh complementary address pins), used in a semiconductor memory device  10  by a control signal (Mode REGister set: MREG), and  FIG. 2   c  shows a decoding circuit (wherein A 9 , A 10 , and A 11  correspond to the respective ninth, tenth and eleventh address pins and /A 9 , /A 10 , and /A 11  correspond to the respective ninth, tenth and eleventh complementary address pins) for outputting tWR with a specific value by combining the specified or unspecified address signal generated from the circuit in  FIG. 2   b.    
   That is, if the specified address signals at address pins A 11 , A 10 , A 9  generated by control signals MREG 11 , MREG 10 , MREG 9  are ( 1 ,  0 ,  0 ) in sequence, then the value of the outputted control signal is a predetermined value corresponding to 5 clock periods for tWR as shown outputted in  FIG. 2   a  and  FIG. 2   c  as a “5”. 
   On the other hand, if the unspecified address signals delivered to the address pins A 11 , A 10 , A 9  generated by control signals MREG 11 , MREG 10 , MREG 9  are ( 0 ,  0 ,  0 ), ( 0 ,  0 ,  1 ) or ( 1 ,  1 ,  1 ), then the value of the outputted control signal is a reserved value directing the tWR in an undefined state (i.e., a reserved state) as shown in  FIG. 2   a.    
   Accordingly, if the address signal corresponding to the undefined tWR is inputted, it can&#39;t be predicted for an output from the conventional decoding circuit. In response to this, it can&#39;t be checked for an operational state of a memory device as well. Especially, a peak current may flow in the memory device. 
   As aforementioned, in case that an address signal which is not defined for CL or tWR is generated, the conventional decoding circuit for a memory device causes a mis-operation in a memory device. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and an object of the present invention is to stabilize an operation of a semiconductor device by making a decoded result for an undefined value get a specific value, in a decoding operation by using an address signal. 
   Another object of the present invention is to prevent a peak current from being flown in a semiconductor device as the decoded result for an undefined value, as aforementioned, has a specific value. 
   The other object of the present invention is to stabilize an operation of a semiconductor device and prevent a peak current from being flown in a semiconductor device by making a decoded result for an undefined value become a specific value, in a decoding operation for performing CL function and tWR function. 
   In order to achieve the above object, according to one aspect of the present invention, there is provided a decoding circuit for a memory device, receiving an address signal and outputting a control signal to control an internal operation of the memory device, the decoding circuit comprising: a logical circuit outputting a signal to determine CL, tWR, and so forth for controlling the internal operation of the memory device when an address signal defined by a spec is applied, and outputting a control signal with a specific value when an undefined address signal is applied. 
   Here, preferably, the control signal is corresponding to CAS latency CL or Write recovery time tWR. 
   Furthermore, preferably, if the undefined address signal is applied, the CAS latency CL or the Write recovery time tWR is set on a specific value which is used for selecting a standard operation of a memory device. 
   In addition, a decoding circuit for a memory device in accordance with the present invention, from which address signals are generated by a control signal set with a mode, the address signals being divided into a first group corresponding to at least a defined result value and a second group corresponding to a undefined result value, decoding the address signals in logical combinations, the decoding circuit comprising: a first logical circuit for decoding and outputting the defined result value by logically combining the address signals corresponding to the first group; and a second logical circuit for performing a decoding operation to make the address signals have a specific value included in the defined result value by logically combining the address signals corresponding to the second group. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects, and other features and advantages of the present invention will become more apparent after a reading of the following detailed description when taken in conjunction with the drawings, in which: 
       FIGS. 1   a ,  1   b , and  1   c  and  1   d  are circuit diagrams illustrating a decoding circuit for a memory device applied to CAS latency CL; 
       FIGS. 2   a ,  2   b , and  2   c  are circuit diagrams illustrating a decoding circuit for a memory device applied to the conventional Write recovery time tWR; 
       FIGS. 3   a  and  3   b  are circuit diagrams illustrating an embodiment of which a decoding circuit for a memory device in accordance with the present invention is applied to CL; and 
       FIGS. 4   a  and  4   b  are circuit diagrams illustrating an embodiment of which a decoding circuit for a memory device in accordance with the present invention is applied to tWR. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
   Hereinafter, it will be described about preferred embodiments of the present invention in detail with reference to the drawings. 
   A decoding circuit for a semiconductor device in accordance with the present invention is schemed to decode an output to have a specific value when an undefined address signal is inputted. 
   For the scheme, address signals applied to the decoding circuit for the semiconductor device are divided into a first group corresponding to a defined result value and a second group corresponding to an undefined result value. The decoding circuit is comprised of a first logical circuit for outputting a result value defined by logically-combining the address signals corresponding to the first group, and a second logical circuit for performing a decoding operation to make the address signals have a specific value included in the defined result value by logically-combining the address signals corresponding to the second group. 
   Here, the first logical circuit may be schemed by such as a circuit in  FIG. 1   c , and describes about preferred embodiments corresponding to the second logical circuit according to the present invention. 
     FIGS. 3   a  and  3   b  show embodiments of some of the internal operations of a decoding circuit for a semiconductor memory device  10  in accordance with the present invention, which has applied to CL (wherein A 4 , A 5 , and A 6  correspond to the respective fourth, fifth and sixth address pins).  FIGS. 4   a  and  4   b  show embodiments of a decoding circuit for a semiconductor memory device  10  in accordance with the present invention, which has applied to tWR (wherein A 9 , A 10 , and A 11  correspond to the respective ninth, tenth and eleventh address pins). 
   First, with reference to  FIGS. 3   a  and  3   b , it will be described about embodiments of the decoding circuit applied to CL. 
   If an undefined address signal is applied in case of applying to CL, the decoding circuit outputs CL set with a specific value. That is, with reference to  FIG. 1   a , if address signals A 6 , A 5 , A 4  are applied to ( 0 ,  0 ,  0 ), ( 0 ,  0 ,  1 ), or ( 1 ,  1 ,  1 ), the decoding circuit outputs CL with a specific value. 
   Here, the CL with a specific value represents one of values corresponding to the CL function. For instance, it is preferable to set the CL with a control signal corresponding to 5 clock periods, i.e., a “5” (tAA= 15  nsec=tCK*CL) to embody function of DDR667 model of semiconductor memories which have 3 nsec for a clock period. Note that the DDR667 model memory device  10  corresponds to a commercially available semiconductor memory devices  10  as illustrated In  FIG. 1   d . Therefore,  FIGS. 3   a  and  3   b , as embodiments, explain that the CL is set with a control signal to correspond to 5 clock periods (indicated as a “5”) if an undefined address signal is applied. That is, DDR667 model memory device  10  of semiconductor memories is designed to be operated in the state of CL=5 when an undefined address signal is applied. For reference, tAA is an address access time taken by outputting data after an address signal is applied, and tCK is a clock period. 
     FIG. 3   a  is an embodiment for a case of CL=5 corresponding to the undefined address signal, and  FIG. 3   b  is an equivalent circuit diagram with  FIG. 3   a . The decoding circuit can be designed with various schemes except those in  FIGS. 3   a  and  3   b.    
   The decoding circuit in  FIG. 3   a  is configured by a logical circuit to output CL=5 when an undefined address signal is applied, and the logical circuit is comprised of NOR gate, NAND gate, and inverter. 
   A first NOR gate receives and NOR-combines address signals A 6 , A 5 , and the NAND gate receives and NAND-combines address signals A 6 , A 4 . A first inverter inverts an output from the NAND gate, and a second NOR gate receives and NOR-combines an output from the first NOR gate and an output from the first inverter. A second inverter outputs an output from the second NOR gate by inverting it. 
   According to the combinations, if the address signals A 6 , A 5 , A 4  are ( 0 ,  0 ,  0 ), ( 0 ,  0 ,  1 ), ( 1 ,  0 ,  1 ) or ( 1 ,  1 ,  1 ), the CL is set with “5” in the embodiment according to the present invention. 
   Therefore, a semiconductor device to which applies an embodiment in accordance with the present invention is operated in a state to set CL with a specific value even when an undefined address signals is applied. 
   It is possible to set CL with various values except setting by CL=5 according to the semiconductor device in accordance with the present invention. 
     FIG. 3   b  is an equivalent circuit with  FIG. 3   a , and thus possible to change to various circuits with the same function to the circuit in  FIG. 3   a.    
   On the other hand, with reference to  FIGS. 4   a  and  4   b , it will be described embodiments of a decoding circuit applied to tWR. 
   If an undefined address signal is applied in case of applying to tWR, the circuit outputs tWR set with a specific value. That is, with reference to  FIG. 1   a , if address signals A 11 , A 10 , A 9  are applied to ( 0 ,  0 ,  0 ), ( 1 ,  1 ,  0 ), or ( 1 ,  1 ,  1 ), the decoding circuit outputs tWR with a specific value. 
   Here, the tWR with a specific value represents one of values corresponding to the tWR function. For instance, it is preferable to set the tWR with “5”(tAA= 15  nsec=tCK*CL) to embody function of DDR667 memory device  10  of semiconductor memories which have 3 nsec for a clock period. Note that the DDR667 memory device  10  corresponds a commercially available semiconductor memory device  10  as illustrated in  FIG. 1   d . Therefore,  FIGS. 4   a  and  4   b , as embodiments, explain that the tWR is set with “5” if an undefined address signal is applied. That is, in applying the embodiment, DDR667 memory device  10  of semiconductor memories is designed to be operated in the state of tWR=5 when an undefined address signal is applied. 
     FIG. 4   a  is an embodiment for a case of tWR=5 corresponding to the undefined address signal, and  FIG. 3   b  is an equivalent circuit diagram with  FIG. 3   a . The decoding circuit can be designed with various schemes except those in  FIGS. 4   a  and  4   b.    
   The decoding circuit in  FIG. 4   a  is configured by a logical circuit to output tWR=5 when an undefined address signal is applied, and the logical circuit is comprised of NOR gate, NAND gate, and inverter. 
   A first NOR gate receives and NOR-combines address signals A 9 , A 10 , and the NAND gate receives and NAND-combines address signals A 9 , A 11 . A first inverter inverts an output from the NAND gate, and a second NOR gate receives and NOR-combines an output from the first NOR gate and an output from the first inverter. A second inverter outputs an output from the second NOR gate by inverting it. 
   According to the combinations, if the address signals A 11 , A 10 , A 9  are ( 0 ,  0 ,  0 ), ( 1 ,  0 ,  0 ), ( 1 ,  1 ,  0 ), ( 1 ,  1 ,  1 ), the tWR is set with “5” in the embodiment according to the present invention. 
   Therefore, a semiconductor device to which applies the embodiment in accordance with the present invention is operated in a state to set tWR with a specific value even when an undefined address signals is applied. 
   It is possible to set tWR with various values except setting by tWR=5 according to the semiconductor device in accordance with the present invention. 
     FIG. 4   b  is an equivalent circuit with  FIG. 4   a , and thus possible to change to various circuits with the same function to the circuit in  FIG. 4   a.    
   As aforementioned, the present invention shows a decoding circuit about CL and tWR with reference to preferred embodiments. However, the present invention can be utilized variously in case of embodying a predetermined decoding circuit by using an address signal without being limited aforementioned cases. 
   Furthermore, if an undefined address signal is applied, the present invention has suggested embodiments set by CL=5 and tWR=5, but CL and tWR can be set with various cases by necessity. 
   As apparent from the above description, the present invention provides a decoding circuit for a memory device. As a result, in performing a decoding operation using an address signal, the present invention lets the decoded result for an undefined value have a specific value, which leads the semiconductor device to be stabilized by performing function corresponding to the specific value. Especially, it is advantageous to prevent an over current flow and stabilize an operation by applying to functions of CL and tWR. 
   In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.