Patent Publication Number: US-6037799-A

Title: Circuit and method for selecting a signal

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/582,424, entitled REDUNDANCY ARCHITECTURE, filed Dec. 29, 1995, now U.S. Pat. No. 5,612,918. 
     This application is related to U.S. patent application Ser. No. 08/710,357, entitled INTEGRATED CIRCUIT THAT SUPPORTS AND METHOD FOR WAFER-LEVEL TESTING, filed Sep. 17, 1996, now U.S. Pat. No. 5,808,947; U.S. patent application Ser. No. 08/710,356, entitled INTEGRATED-CIRCUIT DIE SUITABLE FOR WAFER-LEVEL TESTING AND METHOD FOR FORMING THE SAME, filed Sep. 17, 1996, now U.S. Pat. No. 5,861,660; U.S. patent application Ser. No. 08/758,582, entitled CIRCUIT AND METHOD FOR REPLACING A DEFECTIVE MEMORY CELL WITH A REDUNDANT MEMORY CELL, filed Nov. 27, 1996, now U.S. Pat. No. 5,771,195; and U.S. patent application Ser. No. 08/758,586, entitled MEMORY HAVING AND METHOD FOR TESTING REDUNDANT MEMORY CELLS filed Nov. 27, 1996, now U.S. Pat. No. 5,841,709 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to integrated circuits, and more specifically to a circuit and method for selecting one out of a plurality of signals. 
     BACKGROUND OF THE INVENTION 
     Many of today&#39;s integrated memory circuits include redundant memory cells that are available to replace malfunctioning or defective matrix memory cells. Typically, the entire matrix row or column to which a defective memory cell belongs is identified as defective and is respectively replaced with a redundant row or column. Often, one defective matrix row or column alone can cause a memory circuit to memory circuits, engineers can repair otherwise unusable memory circuits and increase the overall manufacturing yield of operational memory circuits. 
     Typically, engineers test a memory circuit shortly after it is manufactured to find and identify rows and columns that contain defective memory cells. For example, after a test station identifies a defective column, it maps a redundant column to the address of the defective column. During normal operation of the memory circuit when an external circuit, such as a processor, writes data to the address of a memory cell in the defective column via a data bus, a redundancy circuit within the memory circuit isolates the defective column from the data bus and diverts the data to a corresponding memory cell in the redundant column. The redundancy circuit within the memory circuit performs this isolation and diversion in a manner that is transparent to the external circuit. 
     Often, select circuits such as programmable multiplexers are used to enable a selected redundant memory row or column to replace a defective matrix row or column, respectively. For example, FIG. 1 is a schematic diagram of a known multiplexer circuit 1, which, when in an unprogrammed state, disables a corresponding redundant row or column, and which, when in a programmed state, enables the redundant row or column in response to the desired one of n select signals S 0  -S n-1 . For example, if the corresponding redundant row or column is to be enabled whenever S 1  is active logic 1, all the selectively conductive elements F 0  -F n-1 , such as fuse elements, are made nonconductive except the element F 1 , which is made conductive. When S 1  is active logic 1, the transistor T 1  is active, and thus pulls the node Na to an active logic 0. A pair 2 of serially coupled inverters, which together act as a noninverting buffer, provides the active logic 0 at the output of the multiplexer 1 to enable the redundant row or column. When S 1  is inactive logic 0, the transistor T 1  is inactive and the transistor T p  pulls up the node Na to an inactive logic 1, which the multiplexer 1 provides to disable the redundant row or column. 
     One problem with many such select circuits is that they occupy a relatively large area of the memory device. Because the memory device typically includes many of these select circuits, the area that they occupy may significantly increase the production costs of the memory device. 
     Further background on memories, redundant memory cells, and redundancy circuits can be found in: Prince, Betty, Semiconductor Memories, A Handbook of Design, Manufacture, and Applications, 2nd Edition, John Wiley and Sons, (1991); Hardee et al., &#34;A Fault-Tolerant 30 ns/375 mW 16K X 1 NMOS Static RAM,&#34; Journal of Solid State Circuits SC-16(5):435-43 (IEEE, 1981); Childs et al., &#34;An 18 ns 4K X 4 CMOS SRAM,&#34; Journal of Solid State Circuits SC-19(5):545-51 (IEEE, 1984); and ISSCC Proceedings From 1975 to the present, all of which are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a multiplexing circuit includes a reference terminal, a plurality of multiplexing input terminals, and a buffer having an input terminal and an output terminal. The multiplexing circuit also includes a plurality of first elements that each have a programmable conductivity and that are each serially coupled between a corresponding one of the multiplexing input terminals and the input terminal of the buffer. 
     An advantage provided by one aspect of the invention is a multiplexer circuit that occupies a significantly smaller area than known multiplexer circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a known multiplexer circuit. 
     FIG. 2 is a diagram of an integrated memory circuit according to the present invention. 
     FIG. 3 is a schematic block diagram of one embodiment of the memory circuit of FIG. 2. 
     FIG. 4 is a diagram of a memory block of the memory circuit of FIG. 2. 
     FIG. 5a is a schematic diagram of one embodiment of a redundant-column decoder of the redundant column select circuitry of FIG. 3. 
     FIG. 5b is a schematic diagram of a redundant-address-signal generator of FIG. 5a. 
     FIG. 6 is a schematic diagram of a first embodiment of a memory-column select circuit of the matrix-column select circuit of FIG. 3. 
     FIG. 7 is a schematic diagram of a first embodiment of a redundant-column select circuit of the redundant-column decode and select circuit of FIG. 3. 
     FIG. 8 is a schematic diagram of a second embodiment of a memory-column select circuit of the matrix-column select circuit of FIG. 3. 
     FIG. 9 is a schematic diagram of a second embodiment of a redundant-column select circuit of the redundant-column decode and select circuit of FIG. 3. 
     FIG. 10 is a schematic diagram of one embodiment of a portion of the wafer test-mode circuitry of FIG. 3. 
     FIG. 11 is a schematic diagram of one embodiment of a first portion of the control circuit of FIG. 3. 
     FIG. 12 is a schematic diagram of one embodiment of a test-mode logic circuit that forms a second portion of the control circuit of FIG. 3. 
     FIG. 13 is a schematic diagram of one embodiment of the redundant-row decode and select circuit of FIG. 3. 
     FIG. 14 is a schematic diagram of a multiplexer circuit according to the present invention. 
     FIG. 15 is a schematic block diagram of a computer system that incorporates the memory circuit of FIGS. 2 and 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a block diagram of a memory device or circuit 10 according to the present invention. In one aspect of the invention, the memory device 10 is a 32K×32-bit Burst Static Random Access Memory (Burst SRAM). 
     The memory 10 has its matrix memory cells arranged in an even number of memory blocks B 0  -B 31 , although other embodiments of the memory 10 may have more, fewer, or an odd number of such memory blocks. The memory cells in each block B 0  -B 31 , are arranged in rows and columns. A row refers to a group of memory cells that is coupled to a common word line, and a column refers to a group of memory cells that is coupled to a common bit line, or in the case of an SRAM, to a common pair of complementary bit lines. 
     The blocks B 0  -B 31 , are divided into four quadrants Q 0  -Q 3  of 8 blocks each. That is, quadrant Q 0  includes blocks B 0  -B 7 , quadrant Q 1  includes blocks B 8  -B 15 , quadrant Q 2  includes blocks B 16  -B 23 , and quadrant Q 4  includes blocks B 24  -B 31 . Each quadrant Q 0  -Q 3  provides 8 of the 32 data bits D 0  -D 31  that the memory 10 provides. During a read or write cycle, one block from each quadrant is accessed and provides the 8 bits of data for that quadrant. For example, during a read or write cycle, blocks B 0  from quadrant Q 0 , B 8  from quadrant Q 1 , B 16  from quadrant Q 2 , and B 24  of quadrant Q 3 , are simultaneously activated together to provide D 0  -D 31 . A master word-line decoder 12 is located along one center axis of the memory device 10. Master word lines MWL 0  -MWL 3  run through each quadrant Q 0  -Q 3 , respectively. Local-word-line decoders LWD 0  -LWD 15  are respectively located between each pair of blocks B 0  -B 31 . 
     The memory 10 also includes 16 Block Input/Output circuits (BLKIO 0-15 ) that are each associated with a corresponding pair of blocks B 0  -B 31 . The BLKIO circuits couple the memory cells within the blocks B 0  -B 31  to corresponding ones of the 32 external data input/output pins or terminals DQ 0  -DQ 31 . The memory 10 also includes other external terminals for receiving address signals, control signals, and power signals from external circuitry (not shown). A memory device that is similar to the memory 10 is discussed in U.S. patent application Ser. No. 08/587,708, entitled DEVICE AND METHOD FOR DRIVING A CONDUCTIVE PATH WITH A SIGNAL, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/588,762, entitled DATA-INPUT DEVICE FOR GENERATING TEST SIGNALS ON BIT AND BIT-COMPLEMENT LINES, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/589,141, entitled WRITE DRIVER HAVING A TEST FUNCTION, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/589,140, entitled MEMORY-ROW SELECTOR HAVING A TEST FUNCTION, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/588,740, entitled DEVICE AND METHOD FOR ISOLATING BIT LINES FROM A DATA LINE, filed Jan. 19, 1996; and U.S. patent application Ser. No. 08/589,024, entitled LOW-POWER READ CIRCUIT AND METHOD FOR CONTROLLING A SENSE AMPLIFIER, filed Jan. 19, 1996, which are incorporated by reference herein. 
     FIG. 3 is a schematic block diagram of the memory 10 of FIG. 2. An address decoder 14 receives an address signal from external circuitry (not shown), 5 decodes the address signal, and provides corresponding column-select and row-select signals to a row-and-column select circuit 16. A matrix-row select circuit 18 receives the row-select signals from the address decoder 14, and activates the word lines of the addressed matrix-memory rows in the matrix-memory array 20, which composes a portion of the memory array 21. Likewise, the matrix-column select circuit 22 receives the column-select signals from the address decoder 14, and couples the bit lines of the addressed matrix-memory columns in the matrix array 20 to the read/write circuit 24. 
     If, during initial testing of the memory 10, a defective matrix row of memory cells in the array 20 is found, it is replaced with one of the redundant memory rows 26 of redundant memory cells. That is, the redundant row is mapped to the address of the defective matrix row. When the defective matrix row is addressed, the redundant-row decode and select circuit 28 decodes the row-select signal from the address decoder 14 and activates the redundant word line associated with the mapped redundant row. Likewise, if during initial testing of the memory 10, a defective matrix column of memory cells in the array 20 is found, it is replaced with one of the redundant memory columns 32. That is, the redundant column is mapped to the address of the defective matrix column. When the defective matrix column is addressed, redundant-column decode and select circuit 30 decodes the column-select signal from the address decoder 14 and couples the read/write circuitry 24 to the complementary bit lines of the mapped redundant column. As discussed below in conjunction with FIGS. 6-8, the defective rows and columns of memory cells in the matrix array 20 are isolated from the read/write circuitry 24. Thus, the circuits 28 and 30 respectively allow mapping of the redundant rows 26 and the redundant columns 32 to the addresses of defective matrix rows and columns in the matrix array 20 in a manner that is transparent to the external circuitry that provides the address signal to the address decoder 14. 
     Still referring to FIG. 3, during a write cycle, the read/write circuit 24 couples data from the data bus to an addressed memory cell in the memory array 21 via input/output (I/O) buffers 34. During a read cycle, the read/write circuit 24 couples data from an addressed memory cell to the data bus via the I/O buffers 34. The read/write circuit 24 includes one or more write drivers for writing data to an addressed memory cell, and includes one or more sense amplifiers for reading data from an addressed memory cell. The I/O buffers 34 include input buffers for providing data from the data bus to the read/write circuit 24 during a write cycle, and include output buffers for providing data from the read/write circuit 24 to the data bus during a read cycle. 
     A control circuit 36 receives control signals from a control bus, and in response thereto, controls the operation of the address decoder 14, the row-and-column select circuit 16, the memory array 21, the read/write circuit 24, and the I/O buffers 34. 
     A wafer test-mode circuit 38 receives wafer test signals from a wafer test bus, and allows the memory 10 to operate in one or more test modes while the die (not shown) that includes the memory 10 is part of the wafer (not shown) on which it was formed, i.e., before the die is scribed from the wafer. 
     FIG. 4 is a block diagram of the memory block B 0  of the memory 10 of FIG. 2 Although the block B 0  is shown and discussed, it is understood that the remaining blocks B 1  -B 31  are constructed and operate in a similar fashion. In the described embodiment of the invention, the block B 0  has 8 matrix-column groups 40a-40h, which each include 16 matrix memory columns. Thus, the block B 0  includes a total of 128 matrix memory columns. 
     Each matrix-column group 40a-40h has associated therewith a respective input/output (I/O) circuit I/O 0  -I/O 7 . The associated I/O circuit selects the addressed one of the 16 columns in a group 40, and during a read cycle, respectively couples the bit-true and bit-complement lines of the selected matrix column to the associated one of the read-true (RBT 0  -RBT 7 ) lines and the associated one of the read-complement (RBC 0  -RBC 7 ) lines. Likewise, during a write cycle, the I/O circuit respectively couples the bit-true and bit-complement lines of the selected matrix column to the associated one of the write-true (WBT 0  -WBT 7 ) lines and the associated one of the write-complement (WBC 0  -WBC 7 ) lines. 
     The block B 0  also includes a redundant-column group 42, which includes one or more redundant memory columns that are available to replace defective columns in any of the matrix-column groups 40a-40h. The number of redundant columns in the redundant-column group 42 is based on a balancing between the anticipated number of matrix columns within the groups 40a-40h that will have defective memory cells and the amount and cost of the additional area required for each redundant column. In the described embodiment, the redundant-column group 42 includes two redundant columns. Because each block B 0  -B 31 , has its own redundant-column group 42, the redundant-column decode and select circuit 30 (FIG. 3) need not be centrally located, and can be distributed among each of the blocks B 0  -B 31 . Such local distribution often reduces the complexity of and the area required by the routing of interconnections within the circuit 30. Furthermore, the redundant columns of the block B 0  can use the same local-word-line driver LWD 0  as the matrix memory columns, thus reducing the programming overhead. 
     Redundant input/output select circuits RI/O 0  -RI/0 7  are coupled between each of the redundant columns in the redundant-column group 42 and associated ones of the read-bit true lines RBT 0  -RBT 7 , the read-bit complement lines RBC 0  -RBC 7 , the write-bit true lines WBT 0  -WBT 7 , and the write-bit complement lines WBC 0  -WBC 7 . For example, the block RI/O 0  is coupled between each of the redundant columns and RBT 0 , RBC 0 , WBT 0 , and WBC 0 . 
     A redundant-column decode circuit RD 0  receives a matrix-column select signal from the address decoder 14 (FIG. 3), and, when the corresponding matrix column in one of the groups 40a-40h is defective, activates a selected one of the redundant columns in the group 42 to replace the defective matrix column. 
     In operation during a normal read or write cycle, the local word line decoder LWD 0  activates the addressed row of matrix memory cells in the block B 0 . The blocks I/O 0  -I/O 7  each select the addressed one of the columns in a corresponding group 40a-40h (for a total of 8 selected columns, one from each group 40a-40h), and couple the bit-true and bit-complement lines of the selected column to the corresponding lines RBT and RBC, or WPT and WBC, depending upon whether the cycle is a read or a write. Thus, for example, during a read cycle, the circuit I/O 0  couples the bit-true line of a selected column in the group 40a to RBT 0  and couples the bit-complement line of the same column to RBC 0 . During a write cycle, the circuit I/0 0  respectively couples the bit-true and bit-complement lines of the selected column in the group 40a to WBT 0  and WBC 0 . As discussed below in conjunction with FIG. 6, in one embodiment of the invention, during both a read cycle and a write cycle, the circuit I/O 0  couples the bit-true line of the selected column in the group 40a to both RBT 0  and WBT 0 , and couples the bit-complement line to both RBC 0  and WBC 0 . 
     If a matrix column in one of the matrix-column groups 40a-40h is found to be defective, one of the redundant columns in the redundant-column group 42 is mapped to replace the defective matrix column. For example, suppose that a defective column is found in the group 40a. When this defective column is addressed, the circuit RD 0  generates a redundant-column select signal. In response to the redundant-column select signal, the circuit RI/O 0  respectively couples the bit-true and bit-complement lines of the mapped redundant column to RBT 0  and RBT 0  during a read cycle, and respectively couples the bit-true and bit-complement lines of the mapped redundant column to WBT 0  and WBC 0  during a write cycle. The circuit I/O 0  isolates the defective matrix column in the group 40a from RBT 0 , RBC 0 , WBT 0 , and WBC 0  such that all data transactions are routed to the mapped redundant column instead of the defective memory column. 
     FIG. 5a is a schematic diagram of one embodiment of the redundant decoder RD 0  of FIG. 4. In the described embodiment of the memory 10, there are 16 placements of redundant decoders RD 0  -RD 15 , one for every pair of blocks B 0  -B 31 . The decoder RD 0  is associated with blocks B 0  and B 1 , and the remaining decoders RD 1  -RD 15  are similar in structure and operation to the decoder RD 0 . Furthermore, because each pair of blocks has a total of 4 redundant columns, each redundant decoder includes 4 redundant-address-signal generators RSC 0  -RSC 3 , one for each redundant column. Thus, with respect to RD 0 , the address-signal generators RSC 0  and RSC 0  are available to generate redundant-address signals for the 2 redundant columns in the block B 0 , and the generators RSC 2  and RSC 3  are available to generate redundant-address signals for the 2 redundant columns in the block B 1 . 
     FIG. 5b is a schematic diagram of the signal generator RSC 0  of FIG. 5a. The generator RSC 0  includes an enable circuit 44, which enables the generator RSC 0  when the associated redundant column, redundant column 0, has been mapped to the address of a defective matrix column, and disables the generator RSC 0  when the redundant column 0 has not been so mapped. A control circuit 46 receives the matrix-column select signals COL &lt;0:15&gt; from the address decoder 14 (FIG. 3), and generates an active logic 1 at a node N2 when the matrix-column select signal COL &lt;0:15&gt; that corresponds to the defective column to which the redundant column 0 has been mapped has an active logic 1 signal level. There are a total of 16 placements of the circuit 46 within the generator RSC 0 , one for each matrix-column select signal COL &lt;0:15&gt;. An output circuit 50 receives the signal at node N2 and generates on the output line 52 the active-low, redundant-column-select signal RCOL0. 
     During a read or a write cycle, when the redundant column 0 has not been mapped to replace a defective matrix column, a selectively conductive element 54 of the enable circuit 44 is made conductive. The element 54 and similar elements discussed below may be a laser fuse, an electrical fuse, a programmable memory cell, or other element that has selectable conductivity. The conductive element 54 couples the supply voltage Vcc, which is equivalent to logic 1, to the input of a latch circuit 56, which includes an inverter 58 and an NMOS transistor 60. The latch 56 generates logic 0 at the output of the enable circuit 44. A NOR gate 64 of the control circuit 46 receives the logic 0 from the latch 56 at one input and a signal CRS, which is an inactive logic 0, at its other input terminal, and thus generates logic 1 at its output terminal. The logic 0 from the latch 56 and the logic 1 from the NOR gate 64 deactivate pass gates 65, which are each formed from an NMOS transistor that is coupled in parallel to a PMOS transistor. The inactive pass gates 65 thus prevent all of the signals COL &lt;0:15&gt; from propagating to the node N2. Furthermore, the logic 1 generated by the NOR gate 64 activates an NMOS transistor 66, which drives the node N2 to logic 0. The output circuit receives at one input terminal the logic 0 at the node N2 and receives a signal ISO, which is an active logic 1 to select the block B 0  at the other input terminal. Thus, the output circuit 50 generates an inactive logic 1 for the redundant-column select signal RCOL0 on the line 52. 
     During a read or a write cycle, when the redundant column 0 has been mapped to replace a defective matrix column, the element 54 is made nonconductive. During power-up of the memory 10, a power on reset (POR) signal is logic 1 for a predetermined period of time, and during this period, activates a transistor 62 of the enable circuit 44. The active transistor 62 couples the input of the latch circuit 56 to ground, which is equivalent to logic 0. The latch circuit 56 maintains a logic 1 at its output even after POR returns to logic 0. The NOR gate 64 receives the logic 1 from the latch 56 and the signal CRS, which is an inactive logic 0, at its other input, and thus generates logic 0 at its output. The logic 1 from the latch 56 and the logic 0 from the NOR gate 64 activate the pass gates 65. All selectively conductive elements 69 are made nonconductive except the element 69 that corresponds to the defective column. Thus, when the one of the signals COL &lt;0:15&gt; that corresponds to the defective matrix column goes active logic 1, the logic 1 propagates through the corresponding pass gate 65 and conductive element 69 to the node N2. Because the logic 0 from the NOR gate 64 deactivates the transistor 66, the output circuit converts the logic 1 at the node N2 and the logic 1 ISO signal to an active logic 0 for RCOL0. The active logic 0 for RCOL0 selects the redundant column 0. 
     During a first test mode where all of the matrix-memory columns are tested simultaneously, and all of the redundant-memory columns are tested simultaneously whether or not they are mapped to replace defective memory columns, the signal CRS is active logic 1. If the redundant column 0 is not mapped to replace a defective matrix column, the element 54 is conductive, the output of the latch 56 is logic 0, and the output of the NOR gate 64 is logic 0. Thus, the PMOS transistors of the pass gates 65 are active, the elements 69 are conductive, and the signals COL &lt;0:15&gt;, which are all active logic 1 to simultaneously select all matrix columns, are coupled to the node N2. Furthermore, ISO is active logic 1, and the output circuit 50 generates RCOL0 equal to logic 0, which thus selects the redundant column 0. If the redundant column 0 is mapped to replace a defective matrix column, the element 54 is nonconductive, the output of the latch 56 is logic 1, and the output of the NOR gate 64 is logic 0. Thus, the PMOS transistors of the pass gates 65 are active, only the element 69 that corresponds to the defective column is conductive, and the corresponding one of the signals COL &lt;0:15&gt;, which are all active logic 1 to simultaneously select all matrix columns, is coupled to the node N2. Furthermore, the transistor 66 is inactive, ISO is active logic 1, and the output circuit 50 generates RCOL0 equal to logic 0, which thus selects the redundant column 0. 
     During a second test mode where all of the nondefective matrix columns are tested simultaneously, but only the redundant columns that are mapped to replace defective matrix columns are tested along with the nondefective matrix columns, CRS is inactive logic 0. Thus, the operation of the circuit RSC 0  is the same as discussed above during normal operation of the memory 10. That is, if the redundant column 0 is mapped to replace a defective matrix column, the circuit RSC 0  selects it during the second test mode, and if the redundant column 0 is not so mapped, RSC 0  does not select it. 
     In both the first and second test modes, the memory cells in the selected matrix and redundant columns are often stressed with an elevated DC voltage, for example 7-9 V. This elevated voltage is typically applied to one of the bit-true and bit-complement lines of a column, while 0 V is applied to the other line. Then the voltages are reversed. If the memory cells do not fail when subjected to this elevated voltage, then one can be pretty sure that they will not fail when subjected to normal operating voltages, for example 5 V. 
     An advantage provided by the second test mode is that it can be performed after defective matrix columns have been replaced with redundant columns, and only the redundant columns that are mapped will be tested. Thus, because it will not affect the operation of the memory 10, a defective, unmapped redundant column will not cause the second test mode to identify the memory 10 as defective. Therefore, the second test mode prevents the possible discarding of a functional memory 10 because an unmapped redundant column is defective. 
     FIG. 6 is a schematic diagram of a first embodiment of a section 23 of the matrix column select circuit 22 of FIG. 3. There is one placement of the section 23 for each column within a matrix-column group 40a-40h of FIG. 4. Although only the one placement for the matrix column 0 in group 40a is shown, it is understood that each placement is structured and operates in a similar fashion. The section 23 includes matrix pass gates or switches 82 and 84, which are respectively coupled between the bit-true and bit-complement lines of the matrix column 0 and the read lines RBT 0  and RBC 0 , and which each include a control terminal that is coupled to the active-low column select signal COL0. In the illustrated embodiment of the invention, the switches 82 and 84 are PMOS transistors. The section 23 also includes matrix pass gates or switches 86 and 88, which are respectively coupled between the bit-true and bit-complement lines of the matrix column 0 and the write lines WBT 0  and WBC 0 , and which each include a control terminal that is coupled to COL0 via an inverter 90. In the illustrated embodiment, the switches 86 and 88 are NMOS transistors. Selectively conductive isolation elements 92 and 94, when nonconductive, respectively isolate the bit-true line from the switches 82 and 86 and the bit-complement line from the switches 84 and 88. 
     In operation during a read or write cycle when the matrix column 0 is not selected, but is functional, i.e., is not defective and thus has not been replaced with a redundant column, COL0 is inactive logic 1, which inactivates the switches 82, 84, 86, and 88. The inactive switches 82, 84, 86, and 88 uncouple, i.e., isolate, the bit-true and bit-complement lines of the matrix column 0 from the lines RBT 0  and WBT 0 , and RBC 0  and WBC 0  respectively, and thus prevent any reading of data from or writing of data to the memory cells in the matrix column 0. When the column 0 is selected, COL0 is active logic 0, which activates the switches 82, 84, 86, and 88. The active switches 82, 84, 86, and 88 couple the bit-true and bit-complement lines of the matrix column 0 to the lines RBT 0  and WBT 0 , and RBC 0  and WBC 0 , respectively, and thus allow the reading of data from or writing of data to the memory cells in the matrix column 0. 
     If the matrix column 0 is found to be defective during testing of the memory 10, a redundant memory column is mapped to the address of the defective matrix column 0, and the isolation elements 92 and 94 are made nonconductive to isolate the bit-true and bit-complements lines of the defective matrix column 0 from the lines RBT 0  and WBT 0 , and RBT 0  and WBC 0 , respectively. Thus, even when COL0 is active logic 0 and the switches 82, 84, 86, and 88 are active, the defective matrix column 0 is isolated from the read and write lines and does not adversely affect the operation of the memory 10. 
     FIG. 7 is a schematic diagram of two redundant-column select circuits 96a and 96b of the RI/O 0  circuit of FIG. 4 according to a first embodiment of the invention, it being understood that the redundant-column select circuits of RI/O 1  -RI/O 7  are structured and operate in a similar manner. When selected, the circuit 96a couples the redundant column 0 from the redundant-column group 42 (FIG. 4) to the read and write lines RBT 0 , WBT 0 , RBC 0 , and WBC 0 , and the circuit 96b couples the redundant column 1 from the redundant-column group 42 to the same read and write lines. For clarity, only the circuit 96a is described in detail, it being understood that the circuit 96b is structured and operates in a similar manner. 
     The redundant-column select circuit 96a includes a first pair of redundant pass gates or switches 98 and 100, which are respectively coupled between the bit-true and bit-complement lines of the redundant column 0 and RBT 0  and RBC 0 , and which each have a control terminal coupled to RCOL0. A second pair of redundant switches 102 and 104 are respectively coupled between the bit-true and bit-complement lines of the redundant column 0 and WBT 0  and WBC 0 , and each have a control terminal coupled to RCOL0 via an inverter 106. In the illustrated embodiment of the invention, the switches 98 and 100 are PMOS transistors, and the switches 102 and 104 are NMOS transistors. Selectively conductive elements 116 and 118 are respectively coupled between RBT 0  and RBC 0  and the switches 98 and 100, and selectively conductive elements 120 and 122 are respectively coupled between the bit-true line of the redundant column 0 and the switches 100 and 104 and the bit-complement line and the switches 98 and 102. 
     An equilibrate and precharge circuit 108 includes PMOS transistors 110, 112, and 114, which precharge and equilibrate the bit-true and bit-complement lines of the redundant column 0 in a conventional manner. During operation between each read and write cycle, the equilibrate and precharge circuit 108 receives an active-low equilibrate/precharge signal, and, in response thereto, couples the bit-true and bit-complement lines of the redundant column 0 to both Vcc and one another such that these complementary bit lines carry the same voltage level at the beginning of each read and write cycle. 
     In operation when the redundant column 0 has not been mapped to the address of a defective matrix column, as described above in conjunction with FIG. 5b, RCOL0 is inactive logic 1, which inactivates the switches 98, 100, 102, and 104. These inactive switches uncouple the redundant column 0 from the read and write lines RBT 0 , RBC 0 , WBT 0 , and WBC 0 . 
     If the redundant column 0 is mapped to replace a matrix column that is coupled to read and write lines other than RBT 0 , RBC 0 , WBT 0 , and WBC 0 , the elements 116, 118, 120 and 122 are made nonconductive such that the redundant column 0 is isolated from RBT 0 , RBC 0 , WBT 0 , and WBC 0  regardless of the logic level of RCOL0. 
     If the redundant column 0 is mapped to replace the matrix column 0 of the matrix-column group 40a (FIG. 4), the elements 116, 118, 120 and 122 are made conductive, and the corresponding elements in the other redundant-column select circuits of RI/O 1  -RI/O 7  are made nonconductive such that the redundant column 0 is isolated from RBT 1  -RBT 7 , RBC 1  -RBC 7 , WBT 1  -WBT 7 , and WBC 1  -WBC 7 . As discussed above in conjunction with FIG. 5b, in operation when the defective matrix column 0 is addressed, RCOL0 is active logic 0, which activates the switches 98, 100, 102, 104. These active switches couple the bit-true line of the redundant column 0 to RBT 0  and WBT 0 , and couple the bit-complement line to RBC 0  and WBC 0  such that data is read from and written to the redundant column 0 instead of the defective matrix column 0. 
     FIG. 8 is a schematic diagram of a second embodiment of a section of the matrix-column select circuit 22 of FIG. 3. There is one placement of the section 128 for each column within a matrix-column group 40a-40h (FIG. 4). Although only the placement of the section 128 for the matrix column 0 in group 40a is shown, it is understood that each placement of the section 128 is structured and operates in a similar fashion. 
     The section 128 includes a conventional precharge and equilibrate circuit 134, which precharges and equilibrates the bit-true and bit-complement lines of the matrix column 0 between read and write cycles. The section 128 also includes a matrix switch circuit 130, which includes matrix switches 136 and 138. The switches 136 and 138 are respectively coupled between the bit-true and bit-complement lines of the matrix column 0 and the read lines RBT 0  and RBC 0 . Each of the switches 136 and 138 include a control terminal that is coupled to a first output 141 of a matrix-switch control circuit 132. The matrix switch circuit 130 also includes matrix switches 140 and 142, which are respectively coupled between the bit-true and bit-complement lines of the matrix column 0 and the write lines WBT 0  and WBC 0 . Each of the switches 140 and 142 include a control terminal that is coupled to a second output 143 of the matrix-switch control circuit 132. In the illustrated embodiment of the invention, the switches 136 and 138 are PMOS transistors, and the switches 140 and 142 are NMOS transistors. 
     The matrix-switch control circuit 132 includes a first inverter that is formed from a PMOS transistor 144 and an NMOS transistor 146. The first inverter receives active-high COL0 at its input, and provides the first output 141 of the circuit 132. A second inverter 148 inverts the signal at the first output 141 and provides the second output 143 of the circuit 132. The inverter 148 and a PMOS transistor 150 are coupled to form a latch circuit 151. The control circuit 132 also receives the block-enable signal ISO, which as described above in conjunction with FIG. 5b, is active logic 1 when the block B 0  (FIG. 2) is selected. ISO is coupled to the gates of an optional enable NMOS transistor 152 and an optional reset PMOS transistor 154. A selectively conductive disable element 156 is coupled between the transistor 146 and the supply voltage Vss. 
     In operation between read and write cycles, the precharge and equilibrate circuit 134 operates in a manner similar to that of the precharge and equilibrate circuit 108 of FIG. 7 to precharge and equilibrate the bit-true and bit-complement lines of the matrix column 0. 
     During a read or write cycle involving the matrix column 0 when it is not defective, ISO is active logic 1, which activates the transistor 152 and deactivates the transistor 154. COL0 is active logic 1, which activates the transistor 146 and deactivates the transistor 144. The active transistor 146 drives the gates of the transistors 136 and 138 to logic 0, which activates these transistors. The active transistors 136 and 138 respectively couple RBT 0  and RBC 0  to the bit-true and bit-complement lines of the matrix column 0. Likewise, the inverter 148 inverts the logic 0 at the gates of the transistors 136 and 138 to logic 1, which activates the transistors 140 and 142. The active transistors 140 and 142 respectively couple WBT 0  and WBC 0  to the bit-true and bit-complement lines of the matrix column 0. 
     During a read or write cycle involving a matrix column other than the matrix column 0 of the block B 0 , either or both ISO or COL0 is inactive logic 0. When ISO is logic 0 to indicate that the block B 0  is not selected, the transistor 152 is inactive and the transistor 154 is active. The active transistor 154 couples Vcc, which is equivalent to logic 1, to the gates of the transistors 136 and 138, and thus deactivates them. The inverter 148 provides logic 0 to the gates of transistors 140 and 142, and thus deactivates them. Likewise, when COL0 is logic 0 to indicate that the matrix column 0 of any block B 0  -B 31  is not selected, the transistor 146 is inactive and the transistor 144 is active. The active transistor 144 couples Vcc to the gates of the transistors 136 and 138 and thus deactivates them. The inverter 148 provides logic 0 to the gates of transistors 140 and 142 and thus deactivates them. Thus, when either the block B 0  or the matrix column 0 is not selected, the section 128 isolates the matrix column 0 of the block B 0  from RBT 0 , RBC 0 , WBT 0 , and WBC 0 . 
     During a read or write cycle involving the matrix column 0 when it is defective, the element 156 is made nonconductive such that it does not couple the source of the transistor 146 to Vss. During an initialization routine when the memory 10 is powered up, ISO goes inactive logic 0 for a predetermined time and activates the transistor 154, which provides a logic 1 to the input of the inverter 148. The logic 0 at the output of the inverter 148 activates the transistor 150, which reinforces the logic 1 at the input of the inverter 148. The latch circuit 151 thus latches the logic 1 at the gates of the transistors 136 and 138 and the logic 0 at the gates of the transistors 140 and 142, and thus disables these transistors. Therefore, by making the element 156 nonconductive when the matrix column 0 is defective, the control circuit 132 disables the circuit 130 such that it isolates the defective matrix column 0 from the read and write lines. 
     An advantage of the circuits 132 and 130 is that selectably conductive elements such as fuses are not required between the bit-true and bit-complement lines of matrix column 0 and the circuit 130. Thus, the total number of such elements is reduced by approximately half. Furthermore, the cutting of elements such as laser fuses that are in adjacent bit lines is not required. This reduces the chance of error during the laser cutting of the elements, and is particularly advantageous as the size of memory devices in general decreases, and, therefore, as the pitch between the bit-true and bit-complement lines of each column decreases. Additionally, the series resistances of the bit lines are reduced when such fuses are omitted. This can significantly increase the speed at which the memory cells are accessed. Moreover, the use of such bit-line fuses may put limitations on the manufacturing process, because the sheet resistance of the layer in which the fuses are made must be relatively low. Conversely, the circuits 130 and 132 impose no such limitation. 
     FIG. 9 is a schematic diagram of a second embodiment of a redundant select circuit of the redundant-column decode and select circuit 30 of FIG. 3. Although for clarity only the placement of the redundant select circuit 158 of RI/O 0  (FIG. 4) that corresponds to the redundant column 0 of the redundant-column group 42 is described, it is understood that the remaining redundant select circuits 158 of RI/0 0  -RI/0 7  have a similar structure and operate in a similar manner. Furthermore, the combination of the section 128 (FIG. 8) of the matrix-column select circuit 22 (FIG. 3) and the redundant select circuit 158 may be referred to as a memory access circuit. The redundant select circuit 158 includes a precharge and equilibrate circuit 160, which is similar in structure and operation to the precharge and equilibrate circuit 134 of FIG. 8. A redundant switch circuit 162 selectively couples RBT 0 , RBC 0 , WBT 0 , and WBC 0  to the bit-true and bit-complement lines of the redundant column 0 of the block B 0  (FIG. 4). A redundant-switch control circuit 164 controls the switch circuit 162. 
     More specifically, the redundant switch circuit 162 includes switches 166 and 168, which are respectively coupled between the bit-true and bit-complement lines of the redundant column 0 and RBT 0  and RBC 0 , and includes switches 170 and 172, which are respectively coupled between the bit-true and bit-complement lines and WBT 0  and WBC 0 . In the illustrated embodiment, the switches 166 and 168 are PMOS transistors, and the switches 170 and 172 are NMOS transistors. The redundant-switch control circuit 164 includes a first selectively conductive element 174, which is coupled between RCOL0 (from the decoder RSC 0  of FIG. 5b) and the control terminals of the switches 166 and 168, and a second selectively conductive element 176, which is coupled between the active-low signal CRS and the control terminals of the switches 166 and 168. An inverter 178 is coupled between the elements 174 and 176 and the control terminals of the switches 170 and 172. 
     In operation during a read or write cycle when the redundant column 0 has not been mapped to replace any defective matrix columns, both elements 174 and 176 are made conductive. Furthermore, both RCOL0 and CRS are inactive logic 1 to disable the switch circuit 162, which thus isolates the redundant column 0 from RBT 0 , RBC 0 , WBT 0 , and WBC 0 . The elements 174 and 176 in the circuits 158 of RI/O 1  -RI/O 7  are also made conductive, such that the unmapped redundant column 0 is also isolated from RBT 1  -RBT 7 , RBC 1  -RBC 7 , WBT 1  -WBT 7 , and WBC 1  -WBC 7 . 
     During a read or write cycle when the redundant column 0 has been mapped to replace a defective matrix column in one of the matrix-column groups 40b-40h of the block B0, such that the redundant column 0 is to be coupled to a corresponding foursome of RBT 1  -RBT 7 , RBC 1  -RBC 7 , WBT 1  -WBT 7 , and WBC 1  -WBC 7 , the element 174 is made nonconductive and the element 176 is made conductive. Thus, whenever RCOL0 goes active logic 0, CRS remains inactive logic 1 to disable the circuit 162 such that the redundant column 0 is isolated from RBT 0 , RBC 0 , WBT 0 , and WBC 0 . 
     During a read or write cycle when the redundant column 0 is mapped to replace the matrix column 0 of the matrix-column group 40a, the element 174 is made conductive, and the element 176 is made nonconductive. Thus, when the defective matrix column 0 is addressed, the decoder circuit RSC 0  of FIG. 5b drives RCOL0 active logic 0, which activates the switches 166, 168, 170, and 172. These active switches couple the bit-true line of the redundant column 0 to RBT 0  and WBT 0 , and couple the bit-complement line of the redundant column 0 to RBC 0  and WBC 0 . In the circuits 158 of RI/O 1  -RI/O 7 , the elements 174 are made nonconductive and the elements 176 are made conductive such that the redundant column 0 is isolated from RBT 1  -RBT 7 , RBC 1  -RBC 7 , WBT 1  -WBT 7 , and WBC 1  -WBC 7 . 
     As discussed above in conjunction with FIG. 5b, during the first test mode that simultaneously tests all the matrix columns and all of the redundant columns before any redundant columns have been mapped to replace defective matrix columns, the elements 174 and 176 are conductive in all of the redundant select circuits 158 of RI/O 0  -RI/O 7 . Furthermore, RCOL0 is active logic 0 such that the redundant column 0 is coupled to all of the lines RBT 0  -RBT 7 , RBC 0  -RBC 7 , WBT 0  -WBT 7 , and WBC 0  -WBC 7 . In order that there is no signal conflict at the input of the inverter 178, CRS is also active logic 0. Thus, an advantage of the control circuit 164 is that by ensuring that there is no conflict between RCOL0 and CRS when the both of the elements 174 and 176 are conductive, the switching transistors of conventional control circuits may be omitted. Furthermore, the circuit 164 also allows one to omit the selectively conductive elements 116, 118, 120, and 122 of FIG. 7, and thus reduce the number of such elements by approximately half. Such elimination of switching transistors and elements reduces the layout area of the memory 10. Furthermore, where the elements are laser fuses, the reduction in the number of fuses allows an increase in the spacing between the fuses, and thus reduces the chance of error when the fuses are cut. Additionally, the elimination of such conductive elements decreases the resistances of the bit-line paths, and thus decreases the access times of the memory cells. 
     During the second test mode that occurs after selected redundant memory columns have been mapped to the addresses of defective matrix columns, CRS is inactive logic 1 so that only the mapped redundant columns are accessed simultaneously with the matrix columns. Thus, if the redundant column 0 is mapped to replace a defective matrix column, RCOL0 goes active logic 0 to select the redundant column 0 as discussed above in the description of the operation of the redundant-column select circuit 158. Likewise, if the redundant column 0 is not so mapped, RCOL0 remains inactive logic 1 such that the redundant column 0 is not selected. 
     FIG. 10 is a schematic diagram of a wafer test-mode power circuit 180 of the wafer test-mode circuit 38 of FIG. 3. The circuit 180 allows the memory 10 to be tested before the die on which the memory 10 is formed is scribed from the wafer (not shown). When either of the signals WTM0 or WTM1 is a logic 0, the circuit 180 generates an active logic 0 for a signal WFRB. The circuit 180 is further described in U.S. patent application Ser. No. 08,710,357, entitled INTEGRATED CIRCUIT THAT SUPPORTS AND METHOD FOR WAFER-LEVEL TESTING, filed Sep. 17, 1996 (Seed and Berry docket number 850063.456) and U.S. patent application Ser. No. 08/710,356, entitled INTEGRATED-CIRCUIT DIE SUITABLE FOR WAFER-LEVEL TESTING AND METHOD FOR FORMING THE SAME, filed Sep. 17, 1996 (Seed and Berry docket number 850063.458), which are incorporated by reference herein. 
     FIG. 11 is a schematic diagram of a logic circuit 250 of the control circuit 36 of FIG. 3. The circuit 250 generates the signals CRS and CRS of FIGS. 5b and 9. In operation, during the first test mode when WFRB (FIG. 10) is active logic 0 and a signal FON (FIG. 12) is active logic 1, a NAND gate 252 generates logic 0 at its output. A first inverter 254 generates an active logic 1 for CRS, and a second inverter 256 generates an active logic 0 for CRS. During the second test mode when WFRB is inactive logic 1 and FON is inactive logic 0, the circuit 250 generates an inactive logic 0 for CRS and an inactive logic 1 for CRS. 
     FIG. 12 is a schematic diagram of test-mode logic circuit 258 of the control circuit 36 of FIG. 3. The circuit 258 generates FON (FIG. 11) and other signals from test-mode signals TM0-TM2, wafer test-mode signals WTM0-WTM1, and WFRB. The circuit 258 is further described in U.S. patent application Ser. No. 08/587,708, entitled DEVICE AND METHOD FOR DRIVING A CONDUCTIVE PATH WITH A SIGNAL, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/588,762, entitled DATA-INPUT DEVICE FOR GENERATING TEST SIGNALS ON BIT AND BIT-COMPLEMENT LINES, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/589,141, entitled WRITE DRIVER HAVING A TEST FUNCTION, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/589,140, entitled MEMORY-ROW SELECTOR HAVING A TEST FUNCTION, filed Jan. 19, 1996; U.S. patent application Ser. No. 08/588,740, entitled DEVICE AND METHOD FOR ISOLATING BIT LINES FROM A DATA LINE, filed Jan. 19, 1996; and U.S. patent application Ser. No. 08/589,024, entitled LOW-POWER READ CIRCUIT AND METHOD FOR CONTROLLING A SENSE AMPLIFIER, filed Jan. 19, 1996, which are incorporated by reference herein. 
     FIG. 13 is a schematic diagram of one embodiment of a redundant-row decoder 260 of the redundant-row decode and select circuit 28 of FIG. 3. In one embodiment, the memory 10 includes four redundant rows and the circuit 28 includes two circuits 260, one for each pair of the four redundant rows. The redundant-row decoder 260 includes a decoder 262a, which is associated with the redundant row 0, and a decoder 262b, which is associated with the redundant row 1. The remaining redundant-row decoder 260 is associated with the redundant rows 3 and 4. For clarity, the decoder 262a is described in detail, it being understood that the decoder 262b is structured and operates in a similar manner. 
     The decoder 262a includes an enable circuit 264a and a select circuit 266a. When the redundant row 0 is mapped to the address of a defective matrix row, the enable circuit 264a enables the select circuit 266a to activate the redundant row 0 when the defective matrix row is addressed. 
     The enable circuit 264a includes a selectively conductive element 268a, which is made conductive when the redundant row 0 is not mapped to replace a defective matrix row, and which is made nonconductive when the redundant row 0 is so mapped. The circuit 264a also includes an NMOS transistor 270a, a latch 272a, which includes an NMOS transistor 274a and an inverter 276a, and an inverter 278a. 
     The select circuit 266a includes 8 pass gates or switches 280a (only one is shown for clarity), which receive row-address true signals Rat 1  -Rat 8 , and 8 switches 282a (only one is shown for clarity), which receive row-address complement signals Rac 1  -Rac 8 . Eight selectively conductive elements 284a (only one is shown for clarity) are each serially coupled between a corresponding one of the switches 280a and a corresponding one of the nodes NFA 1  -NFA 8 . Eight selectively conductive elements 286a (only one is shown for clarity) are serially coupled between a corresponding one of the switches 282a and a corresponding one of the nodes NFA 1  -NFA 8 . Eight switches 287a (only one is shown for clarity) each have a control terminal coupled to the output of the inverter 278a, and are coupled between a corresponding one of the nodes NFA 1  -NFA 8  and Vss. The nodes NFA 1  -NFA 3  are coupled to the respective inputs of a NAND gate 288a, nodes NFA 4  -NFA 6  are coupled to the respective inputs of a NAND gate 290a, and the nodes NFA 7  -NFA 8  are coupled to the respective inputs of a NAND gate 292a. The outputs of the NAND gates 288a, 290a, and 292a are coupled to the respective inputs of a NOR gate 294a, which has an output coupled to an input of a NOR gate 296a. The NOR gate 296a generates the active-low, redundant-row-0 select signal RRWDC-0. A second input of the NOR gate 296a is coupled to receive CRS (FIG. 11). 
     In operation during a read or write cycle when the redundant row 0 is not mapped to replace a defective matrix row, the element 268a is conductive and CRS is inactive logic 0. The inverters 276a and 278a generate logic 0 and logic 1 respectively to inactivate the switches 280a, and to activate the switches 287a. These active switches drive the inputs of the NAND gates 288a, 290a, and 292a to logic 0. The NAND gates 288a, 290a, and 292a generate logic 0s at their outputs, thus causing the NOR gate 294a to output logic 0. Because both CRS and the output of the NOR gate 294a are logic 0, RRWDC-0 is inactive logic 1. Thus, the circuit 262a does not select the redundant row 0. 
     In operation during a read or write cycle when the redundant row 0 is mapped to replace a defective matrix row, the element 268a is nonconductive and CRS is inactive logic 0. The inverters 276a and 278a generate logic 1 and logic 0 respectively to activate the switches 280a, and to inactivate the switches 287a. The appropriate ones of the elements 284a and 286a are made nonconductive such that when the values of Rat 1  -Rat 8  and Rac 1  -Rac 8  correspond to the defective matrix row, the NAND gates 288a, 290a, and 292a receive logic 1 at each of their inputs. For example, if the defective matrix row corresponds to Rat 1-Rat   8  all equal logic 1 and Rac 1  -Rac 8  all equal logic 0, then all eight elements 284a are made conductive, and all eight elements 286a are made nonconductive. Thus, when the defective row is addressed, the NAND gates 288a, 290a, and 292a receive logic 1 s at all of their inputs and generate logic 0s at their outputs, and RRWDC-0 is active logic 0 to select the redundant row 0. 
     During the first test mode as discussed above in conjunction with FIGS. 5b, 8, and 9, where all the matrix rows and redundant rows, whether mapped or not, are simultaneously accessed, CRS is active logic 1, which forces the output of the NOR gate 296a to logic 0. Thus, RRWDC-0 is active logic 0 regardless of whether the redundant row 0 is mapped or not. 
     During the second test mode where all matrix rows and mapped redundant rows, but not unmapped redundant rows, are simultaneously selected, CRS is inactive logic 0, and the signals Rat 1  -Rat 8  and Rac 1  -Rac 8  are all logic 1. Because Rat 1  -Rat 8  and Rac 1  -Rac 8  are all logic 1, if the redundant row 0 is mapped to any defective matrix row, the circuit 262a operates as discussed above to select the redundant row 0. 
     FIG. 14 is a schematic diagram of a multiplexer circuit 300, which is similar to the redundant-switch control circuit 164 (FIG. 9), and which may be used for other applications in the memory 10 or in other circuits where a multiplexer is needed. The multiplexer 300 includes k input terminals, which receive input signals IN 0  -IN k-1 , k selectively conductive elements F 0  -F k-1 , which are each coupled between a corresponding input terminal and a node 302, a selectively conductive pull-down element F p , which is coupled between the node 302 and a reference voltage such as Vss, and an inverter 304, which is coupled between the node 302 and an output terminal 305. 
     During operation when none of a plurality of parallel-connected multiplexers 300 are used, the elements F 0  -F k-1  and F p  of every multiplexer 300 are made conductive, and IN 0  -IN k-1  are each at the same logic level, here logic 0, such that there is no signal conflict at the node 302. That is, each input signal IN 0  -IN k-1  and Vss drive the node 302 to the same logic level, not to different logic levels. In another embodiment of the invention, the element F p  is coupled to Vcc, and each signal IN 0  -IN k-1  is logic 1. Alternatively, one may make all of the elements F 0  -F k-1  nonconductive and the element F p  conductive, or may make the elements F 0  -F k-1  conductive, the element F p  nonconductive, and IN 0  -IN k-1  equal to the same logic level. 
     During operation when at least one of the parallel-connected multiplexers 300 is used to couple a selected one of the input signals IN 0  -IN k-1  to the output 305, all the elements F 0  -F k-1  of the used multiplexer 300 are made nonconductive except the element that is coupled to the selected input signal. F p  of the used multiplexer 300 is also made nonconductive. For example, if one desires a used multiplexer 300 to couple IN, to the output 305, then all the elements F 1  -F k-1  and F p  are made nonconductive, and F 0  is made conductive. Thus only the selected signal IN 0  propagates through to the output 305. With respect to each of the unused ones of the parallel-connected multiplexers 300, all the elements F 0  -F k-1  are made nonconductive, and the element F p  is made conductive. 
     An advantage of the multiplexer 300 is that it has fewer components than similar conventional multiplexers. For example, the multiplexer 300 does not require components such as transistors to isolate the input signals from the node 302. Such a reduction in components often reduces the overall layout area of the memory device 10, and increases the speed of the multiplexer 300 because there are no transistor or gate delays. 
     FIG. 15 is a block diagram of a computer system 306, which incorporates the memory 10 of FIGS. 2 and 3. The computer system 306 includes computer circuitry 308 for performing computer functions, such as executing software to perform desired calculations and tasks. The circuitry 308 typically includes a processor 310 and the memory 10, which is coupled to the processor 310. One or more input devices 312, such as a keypad or a mouse, are coupled to the computer circuitry 308 and allow an operator (not shown) to manually input data thereto. One or more output devices 314 provide to the operator data that is generated by the computer circuitry 308. Examples of such output devices 314 include a printer and a video display unit, such as a cathode-ray tube (CRT) display. One or more data-storage devices 316 are coupled to the computer circuitry 308 to store data on or retrieve data from external storage media (not shown). Examples of the storage devices 316 and the corresponding storage media include drives that accept hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). Typically, the computer circuitry 308 includes address, data, and control busses that are respectively coupled to the address, data, and control busses of the memory 10. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.