Abstract:
In a semiconductor memory device capable of reading data regarding signature fuses through a normal read operation and a method of reading data regarding signature fuses in a semiconductor memory device through the normal read operation, a semiconductor memory device includes a memory cell array with a plurality of memory cells, an input buffer, and an output buffer. The input buffer writes signature fuse data related to signature fuses to the memory cells respectively when the semiconductor memory device enters a test mode. The output buffer reads the signature fuse data from the memory cells during a normal read operation of the semiconductor memory device. The signature fuse data comprises binary data that is determined based on whether the respective signature fuses are cut. Accordingly, the semiconductor memory device does not require connection of the output buffer to test-related circuits for outputting the signature fuse data while operating in a test mode. As a result, loads on the output buffer do not increase, and therefore, the speed of reading data from the output buffer is not adversely impacted during a normal read operation.

Description:
RELATED APPLICATIONS  
         [0001]    This application claims the priority of Korean Patent Application No. 2003-6366, filed on Jan. 30, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a semiconductor memory device, and more particularly, to a semiconductor memory device capable of reading data regarding signature fuses through a normal read operation and a method of reading data regarding signature fuses in the semiconductor device through the normal read operation.  
           [0004]    2. Description of the Related Art  
           [0005]    The fabrication history of a semiconductor memory device can include useful information about the device and generally includes information regarding the mask, a fuse, wafer fabrication, package assembly, test results, and so on. Sometimes, critical fabrication history information is lost during manufacturing when the semiconductor memory device undergoes a wafer fabrication process, a package assembly process, and a module assembly process. Therefore, to prevent the loss of information, critical fabrication history is recorded within the semiconductor memory device.  
           [0006]    Writing of the fabrication history can be performed on fuses included in the semiconductor memory device using a cutting process or a non-cutting process. Here, the fuses may be signature fuses that store data that specifies, for example, lot identification, the position of a die on a wafer, trimming fuses used to control AC timing and DC voltage/current, and redundancy fuses used to repair failed memory cells of the semiconductor memory device.  
           [0007]    The fabrication history may be read from the semiconductor memory device using one of the following methods: (1) decapping semiconductor chips which are assembled together through a packaging process, and determining a pattern of cut/not cut signature fuses with the naked eye; (2) measuring currents flowing through an input pin, such as an address pin, which is coupled to a circuit via the signature fuses and reading data from the signature fuses in an analog manner; and (3) reading data from signature fuses in a test mode in a digital manner. A circuit with signature fuses from which data is read in the analog manner described in method (2) above is disclosed in U.S. Pat. No. 4,480,199.  
           [0008]    [0008]FIG. 1 is a schematic block diagram illustrating the structure of a conventional semiconductor memory device  100  that is capable of reading data regarding the signature fuses in a test mode in a digital manner. Referring to FIG. 1, the semiconductor memory device  100  includes a fuse box selection circuit  110 , a plurality of fuse boxes  121 ,  122 , . . . ,  12   n , an output pipeline  130 , an output control circuit  140 , an output buffer  150 , and a data input/output (I/O) pin DQ  160 .  
           [0009]    The semiconductor memory device  100  is, for example, an apparatus that satisfies the RLC specifications of a Rambus DRAM that can operate above a predetermined frequency, i.e., a high frequency, or satisfies the Input/output buffer information specifications of a synchronous DRAM that can operate at a high frequency.  
           [0010]    The fuse box selection circuit  110  generates selection signals SELi (i is an integer from 1 to n) for respectively selecting the fuse boxes  121 ,  122 , . . . ,  12   n , in response to a command signal CMD that is in phase with a clock signal CLK and a combination of address signals ADDR. The command signal CMD and the combination of the address signals ADDR may enable the semiconductor memory device  100  to enter a direct access (DA) mode and a signature fuse read mode, which are test modes, or to exit the signature fuse read mode. The signature fuse read mode is an operation mode where data is output from the signature fuses.  
           [0011]    Each of the fuse boxes  121 ,  122 , . . . ,  12   n  includes a plurality of signature fuses and stores bit data regarding signature fuses, the data being at a logic high level (a level “1”) or a logic low level (a level “0”) depending on whether the signature fuses are cut or not. The fuse boxes  121 ,  122 , . . . ,  12   n  respectively output the stored bit data in response to the selection signals SELi. The output data may specify, for example, fabrication history information, related to the position of a semiconductor chip or a die on a wafer, a lot number, and a wafer number.  
           [0012]    The output control circuit  140  generates a control signal for controlling the output pipeline  130  in response to the command signal CMD synchronized with the clock signal CLK. The command signal CMD may instruct the semiconductor memory device  100  to enter the signature fuse read mode.  
           [0013]    The output pipeline  130  converts parallel data regarding the signature fuses, which is input from the fuse boxes  121 ,  121 , . . . ,  12   n , respectively, into serial data SIG_D in response to the control signal generated by the output control circuit  140 . Next, the output pipeline  130  sequentially outputs the serial data to be in phase with the clock signal CLK.  
           [0014]    The output buffer  150  selects one of the serial data SIG_D output from the output pipeline  130  and output data DOUT generated during a normal read operation, in response to a command signal CMD that is in phase with the clock signal CLK and a combination of address signals ADDR. Then, the output buffer  150  sends the selected data to the data  10  pin DQ  160 . The command signal CMD and the combination of the address signals ADDR may enable the semiconductor memory device  100  to enter the signature fuse read mode that is a test mode, or to perform the normal read operation. During the normal read operation, the data DOUT is read from memory cells (not shown) of the semiconductor memory device  100 .  
           [0015]    [0015]FIGS. 2A and 2B are illustrative timing diagrams of a read operation involving the reading of signature fuse data for the semiconductor memory device  100  of FIG. 1. More specifically, the timing diagrams shown in FIGS. 2A and 2B refer to a read operation for reading data regarding the signature fuses in a Rambus DRAM.  
           [0016]    Referring to FIGS. 2A and 2B, the read operation is performed in the following sequence of steps: direct access mode setting, signature fuse read mode setting, register framing, request packet inputting, address packet inputting, dummy packet outputting, and signature fuse data reading.  
           [0017]    During the direct access mode setting, a direct access (DA) mode, which is a test mode, is set in response to a command signal CMD, which is in phase with a clock signal SCK, and a combination of address signals ADDR input from a serial I/O pin SIOφ. Then, a direct access mode signal DAmode that indicates activation or inactivation of the DA mode, is activated to a high level.  
           [0018]    During the signature fuse read mode setting, the signature fuse read mode which is a test mode, is set in response to a command signal CMD which is in phase with a clock signal SCK and a combination of the address signals ADDR input from the serial I/O pin SIOφ. Then, a signature fuse read mode signal SIG_RD that indicates activation or inactivation of the signature fuse read mode, is activated to a high level.  
           [0019]    During register framing, a starting point of an input packet is determined. Next, during the request packet inputting step, a request packet is input via the serial I/O pin SIOφ, the request packet instructing a read operation of a control register in the Rambus DRAM. Next, during the address packet inputting section, an address packet, which is used to select one of the fuse boxes  121 ,  122 , . . . ,  12   n  of FIG. 1, is input via the serial I/O pin SIOφ. Thereafter, during the dummy packet outputting and the signature fuse data reading sections, a dummy packet is output via the serial I/O pin SIOφ, and then, signature fuse data SIG_D is read. FIG. 2B illustrates outputting of 16-bit signature fuse data F 1 , . . . , F 16  in synchronization with the clock signal SCK.  
           [0020]    However, since the conventional semiconductor memory device  100  reads the signature fuse data directly through a data read path, circuits, such as the fuse box selection circuit  110 , the fuse boxes  121 , . . . ,  12   n , the output control circuit  140 , and the output pipeline  130 , which output the signature fuse data are connected to the output buffer  150 , thereby increasing the load placed on the output buffer  150 . In view of the increased load on the output buffer due to the signature fuse test circuitry, the read speed of the output data DOUT output via the output buffer  150  may become slow during a normal read operation.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention provides a semiconductor memory device capable of preventing the reduction of the speed for reading output data during a normal read operation by writing the data regarding signature fuses stored in the semiconductor memory device to memory cells while operating in a test mode, and then reading the signature fuse data from the memory cells during a normal read operation. A method of reading the signature fuse data from the semiconductor memory device through a normal read operation is also provided.  
           [0022]    A semiconductor memory device according to the present invention, which is capable of reading the signature fuse data through the normal read operation, includes a memory cell array including a plurality of memory cells.  
           [0023]    According to an aspect of the present invention, there is provided a semiconductor memory device including a plurality of memory cells, the semiconductor memory device comprising: an input buffer that respectively writes signature fuse data related to the status of signature fuses to the memory cells when the semiconductor memory device enters a test mode; and an output buffer that reads the signature fuse data from the memory cells during a normal read operation of the semiconductor memory device, wherein the signature fuse data comprises binary data that is determined based on whether the respective signature fuses are cut.  
           [0024]    In one embodiment, the input buffer writes one of the signature fuse data and input data, which is generated during a normal write operation of the semiconductor memory device, to the respective memory cells in response to a command signal which is synchronized with a clock signal and a combination of address signals. The input data is initialization data at a low level “0” that is written to the memory cells prior to writing of the signature fuse data to the respective memory cells.  
           [0025]    In another embodiment, the semiconductor memory device further comprises a fuse box selection circuit that decodes the command signal which is synchronized with the clock signal and the combination of the address signals and generates a plurality of selection signals; and a plurality of fuse boxes including the signature fuses that output the signature fuse data in response to the selection signals, respectively.  
           [0026]    In another embodiment, the address signal for selecting the respective fuse boxes is the same as the address signal for selecting the respective memory cells to which the signature fuse data is written.  
           [0027]    In another embodiment, the command signal that is in phase with the clock signal and a combination of the address signals enable the semiconductor memory device to enter or exit a signature fuse read mode.  
           [0028]    In another embodiment, the address signal is one of an externally generated address signal and an internal address signal generated in an internal address generator of the semiconductor memory device.  
           [0029]    According to another aspect of the present invention, there is provided a method of reading data regarding signature fuses in a semiconductor memory device, the method comprising (a) entering a signature fuse read mode in response to a command signal that is in phase with the clock signal and a combination of address signals; (b) outputting signature fuse data related to signature fuses included in fuse boxes selected in response to the address signal; (c) writing the output signature fuse data to memory cells selected in response to the address signal that is the same as the address signal used in step (b); (d) exiting the signature fuse read mode in response to a command signal that is in phase with a clock signal and the combination of address signals; and (e) reading the signature fuse data from the memory cells through a normal read operation of the semiconductor memory device.  
           [0030]    In one embodiment, (a) includes initializing the memory cells by writing data at a low level “0” to the memory cells through a normal write operation.  
           [0031]    In another embodiment, (c) further comprises determining whether data related to a signature fuse to be written to a corresponding memory cell concerns a last signature fuse, and continuing to write data regarding the signature fuses to the corresponding memory cells until the signature fuse data of the last signature fuse is written to the memory cells.  
           [0032]    According to yet another aspect of the present invention, there is provided a method of reading data regarding signature fuses in a semiconductor memory device, the method comprising (a) entering a signature fuse read mode in response to a command signal that is in phase with a clock signal and a combination of internal address signals; (b) outputting signature fuse data regarding signature related to signature fuses that are included in respective fuse boxes selected in response to the internal address signal; (c) writing the output signature fuse data to memory cells selected in response to the internal address signal that is the same as the internal address signal used in (b); (d) exiting the signature fuse read mode in response to a command signal that is in phase with a clock signal and a combination of address signals; and (e) reading the written signature fuse data from the memory cells through a normal read operation of the semiconductor memory device.  
           [0033]    In one embodiment, (a) further comprises initializing the memory cells by writing data at a low level “0” to the memory cells through the normal write operation.  
           [0034]    In a semiconductor memory device capable of reading signature fuse data through a normal read operation and a method of reading data regarding signature fuses in the semiconductor memory device through the normal read operation, according to the present invention, the signature fuse data is written to memory cells of the semiconductor memory device in a test mode and is read through the normal read operation. Accordingly, in the test mode, there is no need for connecting test-related circuits for outputting data of the signature fuses to the output buffer from which the signature fuse data is output, thereby preventing an increase in the load applied to the output buffer. In this manner, the speed for reading the signature fuse data from the output buffer is not adversely affected by the signature fuse test circuitry during a normal read operation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    The above and other aspects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0036]    [0036]FIG. 1 is a schematic block diagram illustrating the structure of a conventional semiconductor memory device capable of reading data regarding signature fuses in a digital manner in a test mode;  
         [0037]    [0037]FIGS. 2A and 2B are timing diagrams of a read operation of reading data regarding signature fuses in the semiconductor memory device of FIG. 1;  
         [0038]    [0038]FIG. 3 is a block diagram illustrating the structure of a semiconductor memory device that reads data regarding signature fuses through a normal read operation, according to a preferred embodiment of the present invention;  
         [0039]    [0039]FIG. 4 is a detailed block diagram illustrating the structure of the fuse box selection circuit shown in FIG. 3;  
         [0040]    [0040]FIG. 5 is a detailed circuit diagram illustrating one of the fuse boxes shown in FIG. 3;  
         [0041]    [0041]FIG. 6 is a block diagram illustrating the structure of a semiconductor memory device that reads data regarding signature fuses through a normal read operation, according to another embodiment of the present invention;  
         [0042]    [0042]FIG. 7 is a flowchart illustrating a method of reading data regarding signature fuses in a semiconductor memory device through a normal read operation, according to a preferred embodiment of the present invention; and  
         [0043]    [0043]FIG. 8 is a flowchart illustrating a method of reading data regarding signature fuses in a semiconductor memory device through a normal read operation, according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference the accompanying drawings. The same reference numerals represent the same elements throughout the drawings.  
         [0045]    [0045]FIG. 3 is a block diagram illustrating the structure of a semiconductor memory device  200  capable of reading data stored in signature fuses through a normal read operation, according to a preferred embodiment of the present invention. Referring to FIG. 3, the semiconductor memory device  200  includes a fuse box selection circuit  210 , a plurality of fuse boxes  221 ,  222 , . . . ,  22   n , an input buffer  230 ; a column decoder  240 , a bit-line sense amplifier  250 , a memory cell array  260 , a row decoder  270 , an output buffer  280 , and a data input/output (I/O) pin DQ  290 .  
         [0046]    The fuse box selection circuit  210  generates selection signals CHOi (i is an integer from 1 to n) for selecting the respective fuse boxes  221 ,  222 , . . . ,  22   n , in response to a command signal CMD that is in phase with a clock signal CLK and a combination of address signals ADDR. The command signal CMD and the combination of the address signals ADDR may enable the semiconductor memory device  200  to enter a direct access (DA) mode and a signature fuse read mode, which are test modes, or to end the signature fuse read mode. The signature fuse read mode is an operation mode where data SIG_DAT from the signature fuses is output. The semiconductor memory device  200  may directly enter the signature fuse read mode without entering the DA mode.  
         [0047]    Each of the fuse boxes  221 ,  222 , . . . ,  22   n  includes signature fuses and stores several-bit data regarding the signature fuses, the data being at a logic high level (level “1”) or a logic low level (level “0”) depending on whether the signature fuses are cut or not. The respective fuse boxes  221 ,  222 , . . . ,  22   n  output data SIG_DAT as the stored data in response to the selection signals CHOi. The output data SIG_DAT may, for example, contain historical information related to the manufacture of the device, including for example, specifying the position of a semiconductor chip or a die on a wafer, a lot number, and a wafer number.  
         [0048]    The input buffer  230  selects one of the data SIG_DAT output from the respective fuse boxes  221 ,  222 , . . . ,  22   n  and input data DIN generated during a normal write operation, and sends the selected data to the bit-line sense amplifier  250 , in response to a command signal CMD that is in phase with a clock signal CLK and a combination of address signals ADDR. The command signal CMD and the combination of the address signals ADDR enable the semiconductor memory device  200  to enter a signature fuse read mode that is a test mode or to perform a normal write operation. During the normal write operation, the input data DIN is written to memory cells of the memory cell array  260 .  
         [0049]    The memory cell array  260  includes a plurality of memory cells. The row decoder  270  decodes a row address signal R_ADDR among the address signals ADDR which are used to select the signature fuses from the fuse boxes  221 ,  222 , . . . ,  22   n , and selects the memory cells connected with one another via a word line (not shown). The column decoder  240  decodes a column address signal C_ADDR among the address signals ADDR which are used to select the signature fuses from the fuse boxes  221 ,  222 , . . . ,  22   n , and generates a column selection line activation signal CSL. Here, the column selection line activation signal CSL activates column selection lines (not shown) connected with one another through the bit-line sense amplifier  250  that amplifies bit lines (not shown) connected to the respective memory cells. Address signals used in the fuse box selection circuit  210 , the input buffer  230 , the column decoder  240 , and the row decoder  270  are applied from a source external to the semiconductor memory device  200 .  
         [0050]    A method of writing the signature fuse data SIG_DAT to the memory cells of the memory cell array  260  and reading it back from the memory cells will now be described. First, during the normal write operation, the input data DIN at a level “0” is input at the data I/O pin DQ  290 , sequentially passes through the input buffer  230 , and the bit-line sense amplifier  250 , and is finally written to the memory cells of the memory cell array  260 . Next, when the semiconductor memory device  200  enters the signature fuse read mode that is a test mode, the signature fuse data SIG_DAT output from the respective fuse boxes  221 ,  222 , . . . ,  22   n  passes through the input buffer  230  and the bit-line sense amplifier  250 , and is then written to the memory cells of the memory cell array  260 . Next, when the signature fuse read mode ends, the normal read operation begins and the signature fuse data SIG_DAT written to the memory cells of the memory cell array  260  is read and output to the data I/O pin DQ  290  via the output buffer  280 .  
         [0051]    Accordingly, the semiconductor memory device  200  according to a preferred embodiment of the present invention does not require connection of the output buffer  280  to test-data related circuits, such as the fuse box selection circuit  210  and the fuse boxes  221 ,  222 , . . . ,  22   n , which output the signature fuse data SIG_DAT. Thus, the load applied to the output buffer  280  does not increase, and thus, the speed of reading data from the output buffer  280  does not decrease during the normal read operation.  
         [0052]    [0052]FIG. 4 is a detailed block diagram illustrating the structure of the fuse box selection circuit  210  shown in FIG. 3. Referring to FIG. 4, the fuse box selection circuit  210  includes a plurality of unit selection circuits  211 ,  212 , . . .  21   n.    
         [0053]    The first unit selection circuit  211  includes a first decoder  2111 , a second decoder  2112 , and an AND circuit  2113 . The first decoder  2111  decodes a command signal CMD which is in phase with a clock signal CLK and a combination of address signals ADDR, and generates a signature fuse read mode signal SIG_RD that indicates activation or inactivation of the signature read mode. Although not shown in FIG. 4, the signal that indicates the activation or inactivation of the DA mode and can be set prior to entering the signature read mode may also be generated by a decoder in a similar manner. The second decoder  2112  decodes a command signal CMD which is in phase with a clock signal CLK and a combination of address signals ADDR, and generates a first fuse box selection signal SEL_BOX 1 . The AND circuit  2113  performs an AND operation on the signature fuse read mode signal SIG_RD and the first fuse box selection signal SEL_BOX 1  to produce a selection signal CHOI for selecting the fuse box  221  of FIG. 3.  
         [0054]    The structures of the unit selection circuits  212  through  21   n  are the same as that of the first unit selection circuit  211  and their descriptions will not be described here. As in the first unit selection circuit  211 , fuse box selection signals SEL_BOX 2  through SEL_BOXn are generated by decoders of the unit selection circuits  212  through  21   n , the decoders corresponding to the second decoder  2112 , and selection signals CHO 2  through CHOn are generated by the fuse box selection circuits  212  through  21   n , respectively. The selection signals CHO 2  through CHOn are used to select the fuse boxes  222  through  22   n , respectively.  
         [0055]    [0055]FIG. 5 is a detailed circuit diagram illustrating the structure of the fuse box  221  of FIG. 3. Referring to FIG. 5, the fuse box  221  includes a fuse circuit  2211  and an NMOS transistor MN 2 . The fuse circuit  2211  includes a signature fuse F 1 , a PMOS transistor MP, an NMOS transistor MN 1 , and inverters INV 1  and INV 2  that constitute a latch circuit. A power supply voltage VDD is applied to an end of the signature fuse F 1  and a ground voltage VSS is applied to a source of the NMOS transistor MN 1 .  
         [0056]    The fuse circuit  2211  stores data, which reaches a level “1” or a level “0” depending on whether the signature fuse F 1  is cut or not, in an output node NODE of the inverters INV 1  and INV 2 , in response to a pulse signal VCCHB. The pulse signal VCCHB is activated to a high level for a predetermined time when the power supply voltage VDD is supplied to the semiconductor memory device  200 . In detail, if the signature fuse F 1  is not cut, a data signal at the level “0” is stored in the output node NODE in response to the pulse signal VCCHB. If the signature fuse F 1  is cut, a data at signal at the level “1” is stored in the output node NODE in response to the pulse signal VCCHB.  
         [0057]    The NMOS transistor MN 2  outputs signature fuse data SIG_DAT stored in the output node NODE of the fuse circuit  2211  in response to a selection signal CHOi.  
         [0058]    The structures of the fuse boxes  222  through  22   n  are the same as that of the fuse box  221  and their descriptions will be omitted. FIG. 5 illustrates a fuse box  221  capable of storing 1-bit data, but the fuse box  221  may include a plurality of fuse circuits that store several-bit data.  
         [0059]    [0059]FIG. 6 is a block diagram illustrating the structure of a semiconductor memory device  300  that is capable of reading data regarding signature fuses through the normal read operation, according to another embodiment of the present invention. Referring to FIG. 6, the semiconductor memory device  300  includes a fuse box selection circuit  310 , a plurality of fuse boxes  321 ,  322 , . . . ,  32   n , an input buffer  330 , a column decoder  340 , a bit-line sense amplifier  350 , a memory cell array  360 , a row decoder  370 , an internal address generator  380 , an output buffer  385 , and data I/O pin DQ  390 .  
         [0060]    As compared to the semiconductor memory device  200  according to a preferred embodiment of the present invention, the semiconductor memory device  300  according to another embodiment further includes the internal address generator  380 . For this reason, an address signal used in the semiconductor memory device  300  is an internal address signal I_ADDR generated in the internal address generator  380 . In contrast, the address signal used in the semiconductor memory device  200  is an external address signal applied from an external source. The internal address signal generator  380  automatically generates a first address through a last address for selecting the fuse boxes  321 ,  322 , . . . ,  32   n , respectively, and may include a counter for this purpose.  
         [0061]    The fuse box selection circuit  310  generates a selection signal OPTi (i is an integer from 1 through n) for selecting the fuse boxes  321 ,  322 , . . . ,  32   n , respectively, in response to a command signal CMD which is in phase with a clock signal CLK and a combination of the internal address signals I_ADDR. The command signal CMD and the combination of the internal address signals I_ADDR may enable the semiconductor memory device  300  to enter the DA mode and a signature fuse read mode which are test modes or to end the signature fuse read mode. Meanwhile, the semiconductor memory device  300  may alternatively directly enter the signature fuse read mode without entering the DA mode. The structure of the fuse box selection circuit  310  is the same as that of the fuse box selection circuit  210  shown in FIG. 4.  
         [0062]    Each of the fuse boxes  321 ,  322 , . . . ,  32   n  includes a signature fuse and stores data that reaches a level “1” or a level “0” depending on whether the signature fuse is cut or not. The respective fuse boxes  321 ,  322 , . . . ,  32   n  output the stored data as data SIG_DAT regarding the signature fuses in response to a selection signal OPTi. The data SIG_DAT may specify the position of a semiconductor chip or a die on a wafer, a lot number, and a wafer number. The fuse boxes  321 ,  322 , . . . ,  32   n  have the same constructions as the fuse box  221  of FIG. 5.  
         [0063]    The input buffer  330  selects one of the signature fuse data SIG_DAT, which is output from the fuse boxes  321 ,  322 , . . . ,  32   n , respectively, and input data DIN generated during the normal write operation, in response to a command signal CMD which is in phase with a clock signal CLK and a combination of internal address signals I_ADDR. The selected data is transmitted to the bit-line sense amplifier  350 . The command signal CMD and the combination of the internal address signals I_ADDR enable the semiconductor memory device  300  to enter the signature fuse read mode that is a test mode or to perform the normal write operation. During the normal write operation, the input data DIN is written to memory cells of the memory cell array  360 .  
         [0064]    The memory cell array  360  includes a plurality of memory cells. The row decoder  370  decodes a row address signal I_R_ADDR among the internal address signals I_ADDR which are used to select signature fuses of the fuse boxes  321 ,  322 , . . . ,  32   n , and selects the memory cells connected to one another via a word line (not shown). The column decoder  340  decodes a column address signal I_C_ADDR among the internal address signals I_ADDR which are used to select signature fuses of the fuse boxes  321 ,  322 , . . . ,  32   n , and generates a column selection line activation signal CSL. Here, the column selection line activation signal CSL activates column selection lines (not shown) connected with one another via the bit-line sense amplifier  350  that amplifies bit lines (not shown) connected to the respective memory cells.  
         [0065]    A method of writing the signature fuse data SIG_DAT to the memory cells of the memory cell array  360  and reading it back from the memory cells will now be described. First, when the normal write operation begins, the input data DIN at the level “0” is input at the data I/O pin DQ  390 , passes through the input buffer  330 , and the bit-line sense amplifier  350 , and is then written to the memory cells of the memory cell array  360 . Next, the semiconductor memory device  300  enters the signature fuse read mode that is a test mode, and the signature fuse data SIG_DAT which is output from the fuse boxes  321 ,  322 , . . . ,  32   n , respectively, passes through the input buffer  330  and the bit-line sense amplifier  350  and is written to the memory cells of the memory cell array  360 . After exiting of the signature fuse read mode, during the normal read operation, the signature fuse data SIG_DAT written to the memory cells of the memory cell array  360  is read and output to the data I/O pin DQ  390  via the output buffer  385 .  
         [0066]    Accordingly, the semiconductor memory device  300  according to another embodiment of the present invention does not require connection of the output buffer  385  to the internal address generator  380  or test-data related circuits, such as the fuse box selection circuit  310  and the fuse boxes  321 ,  322 , . . . ,  32   n , which output the signature fuse data SIG_DAT. Thus, the load applied to the output buffer  385  does not increase, and therefore, the speed of reading data from the output buffer  385  does not decrease during the normal read operation.  
         [0067]    [0067]FIG. 7 is a flowchart illustrating a method of reading data stored in signature fuses in a semiconductor memory device during a normal read operation, according to a preferred embodiment of the present invention.  
         [0068]    Referring to FIG. 7, in step  100 , data at a level “0” is written to memory cells during the normal write operation, thereby initializing data of the memory cells.  
         [0069]    In step  105 , the semiconductor memory device enters the signature fuse read mode, which is a test mode, in response to a command signal which is synchronized with a clock signal and a combination of address signals. Alternatively, the semiconductor memory device may enter the direct access (DA) mode before entering the signature fuse read mode. Here, the address signal is applied from a source that is external to the semiconductor memory device.  
         [0070]    In step  110 , data regarding signature fuses is output from fuse boxes selected in response to the address signal. The signature fuse data is at a level “1” or a level “0”, depending on whether the signature fuses are cut or not.  
         [0071]    In step  115 , the signature fuse data is written to memory cells selected in response to the address signal that is the same as the address signal used to select the fuse boxes.  
         [0072]    In step  120 , it is determined whether signature fuse data to be written to a selected memory cell is data regarding a last signature fuse.  
         [0073]    If it is determined in step  120  that the signature fuse data concerns the last signature fuse, the method proceeds to step  125 . In step  125 , the last signature data is written to the memory cell selected in response to the address signal that is the same as that used to select the fuse boxes.  
         [0074]    However, if is it determined in step  120  that the signature fuse data does not concern the last signature, the method returns to step  115  and the signature fuse data is written to the memory cell selected in response to the address signal that is the same as that used to selected the fuse box.  
         [0075]    In step  130 , the signature fuse read mode ends in response to a command signal that is synchronized with a clock signal and a combination of address signals.  
         [0076]    In step  135 , the signature fuse data written to the memory cells is read from the memory cells through the normal read operation.  
         [0077]    [0077]FIG. 8 is a flowchart illustrating a method of reading data related to signature fuses in a semiconductor memory device through a normal read operation, according to a preferred embodiment of the present invention.  
         [0078]    Referring to FIG. 8, in step  200 , data at a level “0” is written to memory cells through the normal write operation, thereby initializing data originally stored in the memory cells.  
         [0079]    In step  205 , the semiconductor memory device enters the signature fuse read mode that is a test mode in response to a command signal that is in phase with a clock signal and a combination of address signals. Alternatively, the semiconductor memory device may enter the DA mode before entering the signature fuse read mode. Here, the address signal is an internal address signal that is automatically generated in an internal address generator of the semiconductor memory device.  
         [0080]    In step  210 , data regarding the signature fuses is output from fuse boxes selected in response to the internal address signal. Each signature fuse data is at a level “0” or a level “1” depending on whether the signature fuse is cut or not.  
         [0081]    In step  215 , the data regarding the signature fuses are continuously written to memory cells selected in response to the internal address signal that is the same as that used to select the fuse boxes.  
         [0082]    In step  220 , the semiconductor memory device exits the signature fuse read mode in response to a command signal that is in phase with a clock signal and a combination of internal address signals.  
         [0083]    In step  225 , the signature fuse data written to the memory cells is read from the memory cells through the normal read operation.  
         [0084]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.