Semiconductor memory device having redundant memory cells and circuit therefor
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device having a digit line redundancy circuit. 
     2. Description of the Related Art 
     In a conventional semiconductor memory there may be found a digit line redundancy circuit having a digit line substitution address programming circuit. In such a semiconductor memory, redundant digit lines are used instead of the digit lines corresponding to input column addresses when the address programming circuit generates a redundancy signal. The digit line substitution address programming circuit is programmed by melting a fuse corresponding to the address of a defective digit line, discovered by electrical, laser irradiation or other types of inspection. 
     The operation of a conventional semiconductor memory device with a digit line redundancy circuit is as follows. First, the digit line substitution address programming circuit generates a redundancy signal when an input column address corresponds to the address of a defective digit line. In response to this redundancy signal, an enable signal for a column decoder goes to an inactive state, and a redundant digit line selection signal is activated. Since the enable signal for the column decoder is in an inactive state at this time, digit line selection signals all go to the inactive state, so that the input/output (IO) lines and the digit lines corresponding to the input column address are not connected. Accordingly, only the redundant digit lines are connected to the IO lines for a data write/read operation. 
     However, in the conventional digit line redundancy circuit mode, where selection is made between connecting the digit lines or the redundant digit lines to the same IO lines, there is a possibility of generating a multiple selected state in which the digit lines and the redundant digit lines are connected simultaneously to identical IO lines. That is, a redundant digit line selection signal is activated in response to the redundancy signal generated by the digit line substitution address programming circuit, but at the same time the enable signal for the column decoder starts to go to the inactive state, namely, the generation of the digit line selection signal of the column decoder begins to be inhibited. Accordingly, since the time of activation of the redundant digit line selection signal is simultaneous with the time of inactivation of the enable signal for the column decoder, there is a possibility of an instantaneous occurrence, at this time, of a multiple selected state in which the digit line selection signal and the redundant digit line selection signal are simultaneously in the active state. When the multiple selected state occurs, data on the IO lines become erroneous, and therefore the memory malfunctions. 
     Further, let us consider what happens when the selected state of the redundant digit lines goes to the selected state of the normal bit lines. The circuit which judges that the normal digit lines are selected with a change in column addresses is also the digit line substitution address programming circuit. It is this circuit which changes the redundancy signal from the active state to the inactive state in response to the input column addresses. In response to this change, the enable signal for the column decoder goes from the inactive state to the active state, causing the column decoder to operate. In other words, in the operation of returning to the normal digit lines, the column decoder is operated only after the change in the redundancy signal so that there is a problem that the activation and the operation of the column decoder are delayed. 
     BRIEF SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
     It is therefore an object of the invention to provide a semiconductor memory device which can prevent the occurrence of multiple, simultaneous selections of the digit lines and the redundant digit lines on the same IO lines. 
     It is another object of this invention to provide a semiconductor memory device which can realize high speed operation. 
     2. Summary of the Invention 
     The semiconductor memory device according to this invention comprises a decoder circuit which generates a digit line selection signal for selecting predetermined digit lines in response to input addresses, a discrimination circuit which discriminates whether redundant digit lines are to be selected based on the input addresses, a circuit for generating a redundant digit line selection signal which selects redundant digit lines in response to the discrimination signal of the discrimination circuit, first IO lines connected respectively to a digit line group via a switch whose activation is controlled in response to the digit line selection signal, second IO lines connected to the redundant digit lines via a switch whose activation is controlled in response to the redundant digit line selection signal, and selective amplifier means which selectively amplifies the second IO lines in response to the discrimination signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram for the semiconductor memory device which is a first embodiment of the invention; 
     FIG. 2 is another circuit diagram for the semiconductor memory device of the first embodiment of the invention; 
     FIG. 3 is a waveform diagram for describing the operation of the semiconductor memory device shown in FIG. 1 and FIG. 2; 
     FIG. 4 is another waveform diagram for describing the operation of the semiconductor memory device shown in FIG. 1 and FIG. 2; 
     FIG. 5 is a circuit diagram for the semiconductor memory device which is a second embodiment of the invention; and 
     FIG. 6 is another circuit diagram for the semiconductor memory device of the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 and FIG. 2 showing a first embodiment of this invention, the semiconductor memory device of this embodiment has a configuration in which the digit lines are subdivided into two groups, and is operated in its normal mode by bringing one of the two column decoders (14 and 14&#39;) to an active state in response to input addresses. 
     In addition, as a countermeasure to the increase in testing time accompanying a high capacity memory, the memory has a parallel test mode which reads simultaneously data of a plurality of cells on the same word lines. In the parallel test mode, data in two memory cells connected to the same word lines in the same cell array are output simultaneously to two pairs of IO lines, IO,IOb and IO&#39;,IOb&#39; (see FIG. 2). 
     A digit line substitution address programming circuit 11 is programmed by melting the fuse of the column address corresponding to a digit line found defective by inspection. This programming circuit 11 receives column addresses Y0 to Yk, and brings φR and φRS to the active state when it detects that an input address is for a redundant digit line. 
     The column addresses Y0 to Yk are input to a column address predecoder circuit 12 and column decoder enable signal generating circuit 10 and 10&#39;. The circuit 12 predecodes the column addresses, and outputs predecoded signals Y01 to Yi-li to column decoder circuits 14 and 14&#39;. An address signal Aj and a parallel test signal φT are input to an address buffer circuit 15 that detects which one of the two groups of the digit line groups is selected. When φT is in the inactive state, φYjT is brought to the active state if Aj is a positive logic whereas φYjN is brought to the active state if Aj is a negative logic. On the other hand, when φT is in the active state, φYjT and φYjN are invariably in the active state. The circuit 10 receives the column address Yk, an enable inhibit signal φRD and φYjN, and generates column decoder enable signals φE and φEB (the two signals are in complementary relation) in response to the column address Yk when φYjN is in the active state and φRD is in the inactive state. The circuit 10&#39; receives the column address Yk, the enable inhibit signal φRD and φYjT, and generates column decoder enable signals φE&#39; and φEB&#39; under similar conditions. 
     An inhibit circuit 16 receives φR and φT, and inhibits activation of the column decode enable signals φE, φEB or φE&#39;, φEB&#39; when φT is in the active state. Accordingly, at the time of normal operation other than during the parallel test mode, activation of the column decoder enable signal will not be inhibited even if the programming circuit 11 detects that the input column addresses are for redundant digit lines. 
     The column decoder 14 has the predecoded signals Y01 to Yi-li and the enable signals φE and φEB as inputs, and brings either one of digit line selection signals φSW1 to φSWx to the active state in response to the predecoded signals when the column decoder enable signals are in the active state. The column decoder 14&#39; has a similar configuration. 
     Redundant digit line selection signal generating circuits 13 and 13&#39; receive φRS generated by the programming circuit 11 and φYjT or φYjN, and genearte selection signals φRSW and φRSW&#39;, respectively, when both φRS and φYjN are in the active state. 
     Next, referring to FIG. 2, the configuration of the memory cell array and the IO lines will be described. The digit line group is subdivided into two groups (D, Db and D&#39;, Db&#39;), and there exist redundant digit line pairs RD, RDb and RD&#39;, RDb&#39; that correspond to the groups, respectively. The digit lines D and Db forming a pair (in reality there are a plurality of such pairs) are in complementary relationships, and a sense amplifier SA for amplifying the potentials of the two digit lines is connected to the pair. The outputs of the sense amplifier SA are connected to a first IO line pair, IO and IOb, via a switching circuit SW whose switching is controlled by a digit line selection signal φSW1. The redundant digit line pair, RD and RDb, corresponding to the digit line pair, D and Db, is connected to the sense amplifier RSA, and the output of the same amplifier RSA are connected to a second IO line pair, IO&#39; and IOb&#39;, via a switching circuit RSW whose switching is controlled by the redundant digit line selection signal φRSW. The configuration of the other digit line pair, D&#39; and Db&#39;, and the corresponding redundant digit line pair, RD&#39; and RDb&#39;, is similar to the above. What is important here is the fact that the IO line pair connected to the digit line pair and the IO line pair connected to the redundant digit line pair are different IO line pairs. 
     The first IO line pair, IO and IOb, and the second IO line pair, IO&#39; and IOb&#39;, are connected to IOSW, RIOSW, IOSW&#39; and RIOSW&#39; that constitution an IO line pair switching circuit, and the switching of the IO line pair switching circuit is controlled by the signal φR. When φR is in the inactive state, namely, when the normal digit lines are selected, IOSW and IOSW&#39; go to the on-state, whereas when φR is in the active state, namely, when the redundant digit lines are selected, RIOSW and RIOSW&#39; go to the on-state. The outputs of the IO line pair switching circuit are connected to data amplifier circuits 41 and 42, and write circuits 43 and 44. The data amplifier 41 and write circuit 43 operate when φYjN is in the active state, and the data amplifier circuit 42 and the write circuit 44 operate when φYjT is in the active state. 
     Next, referring also to FIG. 3 which is a timing diagram when redundant digit lines are selected and FIG. 4, the operation of this embodiment will be described. First, the normal operation mode will be described. At the time of the normal operation, the parallel test signal φT stays in the inactive state all the time and the inhibit circuit 16 always outputs φRD at the low level, so that activation of the enable signals φE, φEB or φE&#39;, φEB&#39; will not be inhibited. In other words, even when the redundant digit lines are used, activation of the column decoder enable signals will not be inhibited. 
     Further, since φT is in the inactive state, the address buffer circuit 15 brings either one of φYjT and φYjN to the active state in response to the address signal Aj. In this embodiment, the case of φYjN being in the active state will be described as an example. The circuit 10 will go to the active state in response to φYjN in the active state. Accordingly, the enable signals φE and φEB go to the active state, and as a result, the column decoder circuit 14 goes to the active state. 
     When the input column addresses Y0 to Yk are for the normal digit lines, the circuit 11 keeps φR and φRS at the low level. Consequently, the circuit 13 is not activated, and the selection signal φRSW also maintains the low level. The column decoder circuit 14 brings either one of the selection signals φSW1 to φSWx (for example φSW1) to the active state. As φSW1 goes to the active state, the switching circuit SW (see FIG. 2) goes to the on-state, and the data on the digit line pair, D and Db, are amplified by the sense amplifier SA to be output to the IO line pair, IO and IOb. Since φR is at the low level at this time, IOSW and IOSW&#39; of the IO line pair switching circuit go to the on-state, and the first IO line pair, IO and IOb, is connected by φYjN to the data amplifier circuit 41 and the write circuit 43 that are in the active state. 
     On the other hand, when the input column address Y0 to Yk are for redundant digit lines (see the time t1 in FIG. 3), the circuit 11 brings φR and φRS to the high level. Accordingly, the circuit 13 is activated, and the selection signal φRSW is brought to the high level. Now, when φT is in the inactive state, the inhibit circuit 16 will not inhibit the activation of the column decoder enable signal, as mentioned above. Accordingly, the column decoder circuit 14 brings either one of the digit line selection signals φSW1 to φSWx (for example, φSW2) to the active state in response to the changed predecoded signals. 
     As the selection signal φRSW goes to the active state, the switching circuit RSW (see FIG. 2) goes to the on-state, data on the digit line pair, RD and RDb, are amplified by the sense amplifier RSA, and are output to the second line pair, IO&#39; and IOb&#39;. Since φR is at the high level at this time, RIOSW&#39; of the IO line pair switching circuit goes to the on-state and the second IO line pair, IO&#39; and IOb&#39;, is in the active state in response to φYjN being in the active state. 
     On the other hand, the first IO line pair, IO and IOb, receives data on a digit line pair (not shown) corresponding to the digit line selection signal φSW2, and is connected to the data amplifier circuit 42 and the write circuit 44 via RIOSW that is in the on-state, of the IO line pair switching circuit. Since, however, φYjT is in the inactive state, the data amplifier circuit 42 and the write circuit 44 will not operate. 
     As is clear from the above operations, since the IO line pair that is connected to the digit line pair is different from the IO line pair that is connected to the redundant digit line pair, in this embodiment, even when the redundant digit line pair is substituted for the digit line pair, it is possible to perform the redundancy operation by the mere substitution of these IO line pairs. Accordingly, there is absolutely no possibility of having multiple selections on the same IO line pair as is the case with the conventional device; and there is no need for the operation, at the time of switching from the redundant digit lines to the normal digit lines, of releasing the inhibition on the activation of the column decoder enable signal and activating the column decoder as is required in the conventional case, enabling thereby to increase the operating speed. In the present embodiment, the access speed to the column address was increased by 10 to 20%. Moreover, since it is possible to use as the second pair of IO lines the IO line pair which has been used conventionally in the parallel test that will be described next, this invention becomes applicable without increasing the occupation area of the device. 
     Next, referring to FIG. 4, the operation at the time of parallel test will be described. Since the parallel test signal φT is in the active state at the time of parallel test, and since the inhibit circuit 16 brings φRD to the active state when φR is in the active state, the circuit 16 inhibits the column decoder enable signal generating circuits 10 and 10&#39; to activate the column decoder enable signals φE, φEB or φE&#39;, φEB&#39;. In addition, since φT is in the active state, the address buffer circuit 15 always brings both φYjT and φYjN to the active state. In other words, at the parallel test both the digit line selection signal and the redundant digit line selection signal are generated simultaneously. 
     When the column addresses Y0 to Yk are for the normal digit lines, the circuit 11 maintains φR and φRS at the low level. Accordingly, since the inhibit circuit 16 brings φRD to the inactive state, it will not inhibit the circuits 10 and 10&#39; to activate the column decoder enable signals φE, φEB or φE&#39;, φEB&#39;. 
     Moreover, the circuits 13 and 13&#39; are not activated, and the selection signals φRSW and φRSW&#39; will also be maintained at the low level. The column decoder circuits 14 and 14&#39; activate one each out of the digit line selection signals φSW1 to φSWx and φSW1&#39; to φSWx&#39; (for example, φSW1 and φSW1&#39;) in response to the predecorded signals. As both φSW1 and φSW1&#39; go to the active state, the switching circuits SW and SW&#39; (see FIG. 2) are brought to the on-state, and the data on the digit line pair, D and Db, is output to the first IO line pair, IO and IOb, and at the same time the data on the digit line pair D&#39; and Db&#39;,is output to the second IO line pair IO&#39; and IOb&#39;. Since both OYjT and OYjN are in the active state, the amplifier circuits 41 and 42, and the write circuits 43 and 44 all go to the active state. Further, since φR is at the low level, IOSW and IOSW&#39; of the IO line pair switching circuit go to the on-state and the first IO line pair, IO and IOb, is connected to the data amplifier circuit 41 and the write circuit 43, and the second IO line pair, IO&#39; and IOb&#39;, is connected to the data amplifier 42 and the write circuit 44. With the above operation, a simultaneous read/write operation of two data becomes possible. 
     On the other hand, when the input column addresses Y0 to Yk are for redundant digit lines (see the time t1 in FIG. 4), the circuit 11 brings φR and φRS to the high level. Accordingly, the circuit 13 and 13&#39; are activated, and the selection signals φRSW and φRSW&#39; go to the high level. In addition, since the inhibit circuit 16 activates φRD, the circuit 10 and 10&#39; are inhibited to activate the column decoder enable signals φE, φEB and φE&#39;, φEB&#39;. Accordingly, the column decoder circuits 14 and 14&#39; will not generate activated digit line selection signals. 
     As the selection signals φRSW and φRSW&#39; go to the active state, both switching circuits RSW and RSW&#39;(see FIG. 2) go to the on-state, the data on the digit line pair, RD and RDb, is output to the second IO line pair, IO&#39; and IOb&#39;, and the data on the digit line pair, RD&#39; and RDb&#39;, is output to the first IO line pair, IO and IOb. Since OR is at the high level at this time, RIOSW and RIOSW&#39; of the IO line pair switching circuit go to the on-state and the second IO line pair, IO&#39; and IOb&#39;, is connected to the data amplifier circuit 41 and the write circuit 43 that are in the active state, and the first IO line pair, IO and IOb, is connected to the data amplifier circuit 41 and the write circuit 43 that are in the active state. 
     Next, referring to FIG. 5, the second embodiment of this invention is the same as the first embodiment except that IOSW, RIOSW, IOSW&#39; and RIOSW&#39; that constitute the IO line pair switching circuits in the first embodiment are absent, and that signals for controlling the data amplifier circuit 41 and 42 and the write circuit 43 and 44 are φYjT&#39; and φYjN&#39;. Therefore, its detailed description will be omitted. 
     The generating circuit of φYjT&#39; and φYjN&#39; is shown in FIG. 6. In the following, in which the group of digit lines D and Db of the digit line group is selected, the case of φYjN being at the high level, will be described as an example. 
     When the normal digit lines are in use, φYjT&#39; and φYjN&#39; are identical to φYjT and φYjN&#39; since φR is at the low level. As φYjN&#39; goes to the active level, the first IO line pair, IO and IOb, connected to the digit lines D and Db is connected to the data amplifier circuit 41 and the write circuit 43 that are in the active state. 
     On the other hand, when the redundant digit lines are in use, φYjT&#39; and φYjN&#39; are identical to φYjN and φYjT since φR is at the high level. In other words, the logical values of φYjT and φYjN are output inverted. Accordingly, φYjT&#39; goes to the active state, and the second IO line pair, IO&#39; and IOb&#39;, connected to the redundant digit line pair, RD and RDb, is connected to the data amplifier circuit 42 and the write circuit 44 that are in the active state. 
     As is clear from the above description, this embodiment makes it possible to execute the selection of the two IO line pairs identical to that of the first embodiment without the use of the IO line pair switching circuit of the first embodiment by changing the activity of the group of the data amplifier circuit and the write circuit (that is, whether 41 and 43 are to be activated or 42 and 44 are to be activated). Accordingly, this invention has the effect of eliminating the need for the switching.