Bit line selection decoder, particularly for electronic memories

Abstract: A bit line selection decoder, particularly for electronic memories, comprising at least two bit lines, each of which can be selected by a respective switch, and a plurality of control lines that drive the switches. Its particularity resides in the fact that it comprises a decoder, in which the outputs drive the switches, and at least one first and one second bus of control lines that are arranged in input to the decoder and are adapted to address any one of the at least two bit lines.

Description: The present invention relates to a bit line selection decoder, particularly for electronic memories. It is known that in electronic memories it is sometimes necessary to select one or more of the bit lines of the electronic memory, for example in order to program it. The most obvious solution would be to place a transistor on each one of the bit lines and switch it on, by means of a control line, when it is necessary to select the respective bit line. Since an electronic memory has a large number of these bit lines, the control lines that switch on the transistors would be too many and would occupy a vast area of the silicon on which the memory is produced. A current solution is to divide the bit lines into groups including for example sixteen lines, and to select a particular group by means of a single bit line that is hierarchically higher and is in turn activated by a single transistor. The particular bit line is then selected within the particular group. An example of such a structure is shown in figure 1. A single control line YM0, which is hierarchically higher, activates the bit line A 1 , which leads to the sixteen-line group GR1. Each one of the sixteen lines of the group GR1 is activated by a respective control line YN0-YN15 by means of the respective transistor. It should be noted that in the illustrated example, the groups GR1-GR16 are sixteen (although they are not shown), and therefore the hierarchically higher control lines YM0-YM15 are likewise sixteen. In this manner it is possible to create a tree or pyramid structure with a plurality of hierarchy levels. Finally, the selected bit line can be either programmed by a programming circuit 2 or amplified by an amplifier 3. Although this solution is efficient, it has a large number of control lines for activating the bit lines; these control lines occupy a considerable area of the chip on which they are integrated. US-A-4,916,336 discloses a bit line selection decoder as defined in the preamble of claim 1. Accordingly, the aim of the present invention is to provide a bit line selection decoder capable of reducing the number of control lines that activate the individual bit lines. Within the scope of this aim, an object of the present invention is to provide a bit line selection decoder capable of reducing the chip area used for the selection of individual bit lines. Another object of the present invention is to provide a bit line selection decoder having a relatively low resistive path for the transistors. Another object of the present invention is to provide a bit line selection decoder that is highly reliable, relatively easy to manufacture, and at competitive costs. This aim and these objects are achieved by a bit line selection decoder, particularly for electronic memories, as defined in claim 1. The characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is an electrical diagram of a known bit line selection circuit; figure 2a is a view showing the circuit of figure 1 according to a different method; figure 2b is a view of another example of a known bit line selection circuit, shown according to the method of figure 2a; figure 2c is a view of another example of a known bit line selection circuit, shown according to the method of figure 2a; figure 2d is a view of another example of a known bit line selection circuit, shown according to the method of figure 2a; figure 3a is a view of the embodiment of the circuit of figure 2a with the decoder according to the present invention; figure 3b is a view of the embodiment of the circuit of figure 2b with the decoder according to the present invention; figure 3c is a view of the embodiment of the circuit of figures 2c and 2d with the decoder according to the present invention; figure 4a is a partial view of a possible embodiment of the decoder according to the present invention; figure 4b is another partial view of a possible embodiment of the decoder according to the present invention; figure 5 is a partial view of another possible embodiment according to the present invention, provided with two input buses; figure 6 is a view of an improved embodiment of the circuit of figure 5; figure 7 is a view of an embodiment of the circuit of figure 5, provided with three input buses. Figure 2a is a simplified diagram of the circuit of figure 1. The plurality of bit and control lines are shown as single buses. The first bus of the control lines comprises the hierarchically higher lines YM<0,15>, whereas the hierarchically lower bit lines are selected by the control lines YN<0,15>. The output is constituted by 256 (16x16) bit lines BL〈0,255〉, from which a single bit line is selected. Figure 2b is a diagram of another possible known bit line selection circuit. There are 32 hierarchically higher control lines YM<0,31> and 16 hierarchically lower control lines YN<0,15>. The output is constituted by 512 (32x16) bit lines BL<0,511>. Figure 2c is a diagram of another possible known bit line selection circuit. There are 32 hierarchically higher control lines YM<0,31> and 32 hierarchically lower control lines YN<0,31>. The output is constituted by 1024 (32x32) bit lines BL<0,1023>. Figure 2d is a diagram of another possible known bit line selection circuit with three hierarchical levels. There are four hierarchically higher control lines YP<0,3>, sixteen hierarchically intermediate control lines YM<0,15>, and sixteen hierarchically lower control lines YN<0,15>. The output is constituted by 1024 (4x16x16) bit lines BL<0,1023>. Figure 3a is a diagram of the embodiment, according to the present invention, of the selection circuit of figure 2a. The bus of the hierarchically higher control lines YM is divided into two buses: a first bus with eight lines YMH<0,7> and a second bus with two lines YML<0,1>. These buses constitute the input of a local decoder 4, the output whereof is constituted by sixteen connection points YM〈0,15〉. The connections to the transistors that activate a bit line are termed "points", since their length is negligible. In this manner, the number of control lines for the hierarchically higher lines is reduced by 16-(2+8)=6, since now only ten lines are required to activate sixteen transistors. Figure 3b is a diagram of the embodiment, according to the present invention, of the selection circuit of figure 2b. The input of the local decoder 4 is constituted by a first bus with sixteen lines YMH<0,15> and by a second bus with two lines YML<0,1>. The saving is evident. Instead of using thirty-two lines, as in the case of figure 2b, it is possible to use only (2+16)=18 control lines. The number of points YM<0,31> remains of course identical to that of figure 2b. Figure 3c is a diagram of the embodiment, according to the present invention, of the selection circuits of figures 2c and 2d. The decoder 4 now has three input buses: a first one YMH<0,7>, with eight control lines; a second one YMM<0,3>, with four control lines; and a third one YML<0,1>, with two control lines. The number of points, that is to say, of lines that drive the transistors is (2x4x8)=64. In this manner, the number of hierarchically higher control lines is reduced from 64 to 14, with a saving of 50 control lines. The partial diagram of a possible embodiment of the decoder 4 is shown in figure 4a. Said figure shows only two of the sixteen modules composing the decoder. The decoder has two input buses, a high one with eight bits YM<0,7> and a low one with two bits YN<0,1> ("0" in a field of "1"). Each module is composed of a NOR gate 5 supplied at 5V. The output of the NOR gate 5 is series-connected to a decoupling transistor 6 of the N-channel type, the gate whereof is connected to a 5V supply. The decoupling transistor 6 is furthermore connected to an inverter 7 supplied at 12V. The output of the inverter 7 is connected to the gate of a P-channel transistor 8 interposed between a 12V supply and the input of the inverter 7. The input of the inverter is furthermore connected to the ground by means of an N-channel transistor 9, termed drain stress transistor, which has the purpose of setting all the outputs of the decoder to logic level "1". This is required when the devices must be controlled to reject weak devices. When the output of the NOR gate 5 is at 5V, the input of the inverter 7 is at 4V, due to the drain/gate drop of the decoupling transistor 6. In this case, the output of the inverter 7 is brought to 0V and switches on the P-channel transistor 8, which brings the input of the inverter 7 to 12V. The P-channel transistor has the purpose of decoupling the inverter 7 without however causing said inverter to absorb current. The circuit is furthermore configured so as to carry the logic levels 0-5V to logic levels with a higher voltage 0-12V, since these levels are required by non-volatile memories. If the output of the NOR gate 5 is 0V, the input of the inverter 7 is equally at 0V, the output of the inverter 7 is at 12V, and the P-channel transistor 8 is off. If one wishes to control the device, the gate of the drain stress transistor 9 is set to "1", so that the output of the inverter becomes "1". It should be noted that this embodiment requires four transistors of the NOR gate, two transistors of the inverter, and two transistors that are shown (the drain stress transistor is not counted). Therefore, sixteen transistors are required for two modules (two outputs). Another possible embodiment of the decoder 4 is shown partially in figure 4b. As in the previous case, only two of the sixteen modules are shown in the figure. Each module comprises a NOR gate 5 supplied at 5V. The output of said NOR gate 5 is connected to the gate of a first N-channel transistor 10 and to the input of an inverter 11, supplied at 12V. The output of the inverter is connected to the gate of a second N-channel transistor 12. The drain of the first transistor 10 is connected to the source of a third transistor 13 of the P-channel type and its source is connected to the ground. The drain of the third transistor 13 is connected to a supply voltage of 12V. The source of the second transistor 12 is connected to the ground and its drain is connected to the source of a fourth transistor 14 of the P-channel type. The drain of the fourth transistor 14 is connected to a supply voltage of 12V. The drain of the first transistor 10 is furthermore connected to the gate of the fourth transistor 14 and the drain of the second transistor is connected to the gate of the third transistor 13. The output OUT of the module is constituted by the drain of the second transistor 12. When the output of the NOR gate 5 is at 5V, that is to say, at logic level "1", the output of the inverter 11 becomes 0V. In this manner, the first transistor 10 is activated, connecting the gate of the fourth transistor 14 to the ground. In this manner, the fourth transistor 14 brings the output of the module to 12V, whereas the second transistor 12 does not conduct. The third transistor 13 is switched off, since the output OUT is at 12V. Instead, when the output of the NOR gate 5 is at 0V, that is to say, at logic level "0", the output of the inverter 11 reaches 12V and switches on the second transistor 12, whereas the first transistor 10 is switched off. In this manner, the output OUT is set to 0V by the second transistor 12, which switches on the third transistor 13, which brings the gate of the fourth transistor 14 to 12V, so that said fourth transistor does not conduct. The embodiment of figure 4b is better than the embodiment of figure 4a in terms of efficiency and decoupling; however, since one module requires four transistors for the NOR gate, two for the inverter, and four transistors, which are shown, twenty transistors are required for two modules. Both the decoder of figure 4a and the decoder of figure 4b are not very efficient in terms of saving space on the chip, since to save six control lines one requires 128 transistors and the other requires 160 transistors. Figure 5 is a diagram of an improved and preferred embodiment of the decoder 4 according to the present invention. As in the previous cases, there are two buses with the control lines, a first one YN<0,1> composed of two control lines and a second one YM<0,7> composed of eight control lines. Both buses have a "1" prevalence, that is to say, all the control lines of a specific bus carry logic level "1" except for one, which selects the bit line and carries logic level "0". Thus, since the first bus YN<0,1> has only two lines, it always has one of the control lines at logic level "0" and the other control line at logic level "1". This decoder, too, is composed of modules. However, in this case each module produces two outputs of the decoder and therefore only eight modules for sixteen outputs are required for the present decoder. Figure 5 shows only two (100 and 101) of the eight modules. The connections between the buses and each individual module are identical and therefore only the connections to a single module are described. The first control line of the bus YN<0,1> drives a first N-channel transistor 15, the source whereof is connected to the ground. Likewise, the second control line of the bus YN<0,1> drives a second N-channel transistor 16, the source whereof is connected to the ground. The same holds for all the modules of the decoder. The drains of the first transistor 15 and of the second transistor 16 are connected by means of a third N-channel transistor 17, which is driven by a single control line of the second bus YM<0,7>. The third transistor 17, in each one of the modules, is driven by a single control line of the second bus YM<0,7>. The drains of the first transistor 15 and of the second transistor 16 are connected to a cross-coupled structure. More specifically, the drain of the first transistor 15 is connected to the source of a fourth transistor 18 of the P-channel type and the drain of the second transistor 16 is connected to the source of a fifth transistor 19, which is also of the P-channel type. The drains of the fourth transistor 18 and of the fifth transistor 19 are connected to the source of a sixth transistor 20 of the P-channel type, which is driven by the same control line of the bus YM<0,7> that drives the third transistor 17. The drain of the sixth transistor 20 is connected to a supply voltage of 12V. The source of the fourth transistor 18 is connected to the gate of the fifth transistor 19 and to a first output OUT1. The source of the fifth transistor 19 is connected to a second output OUT2 and to the gate of the fourth transistor 18. Furthermore, in order to obtain the drain stress effect, there is a seventh transistor 21 of the P-channel type that connects the two outputs OUT1 and OUT2. All the gates of the seventh transistors of the individual modules are driven by a single line by means of the activation signal DS. The operation of the decoder according to this embodiment is as follows. With reference to figure 5, assume that the first transistor 15 of the two modules 100 and 101 is controlled by a control line that carries logic level "0" and that the second transistor 16 of the two modules 100 and 101 is driven by a control line that carries logic level "1". Furthermore, assume that the third transistor 17 of the module 100 is driven by a control line that carries logic level "1" and that the third transistor 17 of the module 101 is driven by a control line that carries logic level "0". Likewise, the sixth transistor 20 of the two modules is driven by the same line that drives the third transistor 17. We first analyze the module 100. The first transistor 15 is off, whereas the second transistor 16 is on, connecting its drain to the ground. The third transistor 17 is also on and connects the drain of the first transistor 15 to the ground. In this manner, the first output OUT1 and the second output OUT2 are connected to the ground. The fourth transistor 18 and the fifth transistor 19 are on, but the sixth transistor 20 is not, and therefore the 12V supply does not affect the outputs of the circuit. We now consider the module 101. The first transistor 15 is off, whereas the second transistor 16 is on and connects its drain and the second output OUT2 to the ground. The third transistor 17 is off and does not connect the drain of the first transistor 15 to the ground. The fourth transistor 18 and the sixth transistor 20 are on and bring the first output OUT1 to 12V (logic level "1"). The fifth transistor 19 is off and the supply voltage of 12V does not affect the second output OUT2, which is connected to the ground. By changing the polarity of the control lines of the first bus YN<0,1>, that is to say, by applying a "1" on the first transistor 15 and a "0" on the second transistor, the outputs of each module are inverted. By applying the "0" to the third transistor 17 and to the sixth transistor 20 of the module 100, one of the outputs of the module 100 would reach 12V (of course, the third transistor 17 and the sixth transistor 20 of the remaining modules would have to be activated by a "1"). Figure 6 is a diagram of an improved embodiment of the decoder of figure 5. Each module of the decoder of figure 6 is practically identical to the modules of figure 5, except that there are an eighth transistor 22, of the N-channel type, placed between the first transistor 15 and the fourth transistor 18, and a ninth transistor 23, of the N-channel type, placed between the second transistor 16 and the fifth transistor 19. The gates of the transistors 22 and 23 are connected to a supply voltage that is equal to 5V. The transistors 22 and 23 have a cascoded structure that improves the resistance of the structure to breakdown, since the voltage at the drains of the first transistor 15 and of the second transistor 16 drop to 4V (5V minus the switch-on threshold of the transistors 22 and 23, approximately 1V). In this manner, the voltage at the drains never reaches 12V, which might damage the transistors 15 and 16. Figure 7 is a diagram of an embodiment with three control line buses. Apart from the two buses that have already been described earlier, the first one YN<0,1> and the second one YM<0,3>, which now has four control lines, a third bus W<0,3> is now also provided, that is also configured with "0" in a field of "1" and has four control lines. In this manner, the decoder has (2x4x4)=32 outputs or "points", and therefore requires sixteen modules, since each module generates two outputs. With respect to the configuration of figure 5, a tenth transistor 24 of the N-channel type is now provided that connects the outputs OUT1 and OUT2 of each module and is parallel-connected to the third transistor 17. Said tenth transistor 24 is driven by a single control line of the third bus W<0,3>. There is also an eleventh transistor 25 of the P-channel type that is series-connected to the sixth transistor 20. The operation of this embodiment can be deduced from the description of the operation of the embodiment of figure 5; however, it can be said that the output OUT1 or OUT2 is selected only in the modules in which the parallel-connected transistors 17 and 24 are driven by a logic level "0". In order to achieve complete decoding, the modules must receive in input all the possible combinations of the lines of the three buses. The control lines of the buses that are connected to each module are shown in the following table 1: MODULE BUS YM<0,3> BUS W<0,3> BUS YN<0,1> Module 1 YM0 W0 YN0 and YN1 Module 2 YM1 W0 YN0 and YN1 Module 3 YM2 W0 YN0 and YN1 Module 4 YM3 W0 YN0 and YN1 Module 5 YM0 W1 YN0 and YN1 Module 6 YM1 W1 YN0 and YN1 Module 7 YM2 W1 YN0 and YN1 Module 8 YM3 W1 YN0 and YN1 Module 9 YM0 W2 YN0 and YN1 Module 10 YM1 W2 YN0 and YN1 Module 11 YM2 W2 YN0 and YN1 Module 12 YM3 W2 YN0 and YN1 Module 13 YM0 W3 YN0 and YN1 Module 14 YM1 W3 YN0 and YN1 Module 15 YM2 W3 YN0 and YN1 Module 16 YM3 W3 YN0 and YN1 From the above table it is evident how it is possible to provide the progression for decoders having more buses and buses with a different number of control lines. Finally, in order to achieve the drain stress effect, the signals on the control lines and on the drain stress transistor 21 are set to logic level "0". In this manner, all the outputs of the decoder reach logic level "1". From the above description, it is evident that the present invention fully achieves the intended aim and objects. In particular, it is noted that for the embodiment of figure 5, only six transistors are required for two outputs of the decoder. Therefore, for a decoder having a bus with eight lines YM<0,7> and a bus with two lines YN<0,1>, the total number of transistors is 48, which is much less than the 128 transistors required for the decoder of figure 4a and the 160 transistors required for the decoder of figure 4b. In this manner, the number of transistors used does not have a strong negative effect on the saving in chip area achieved by reducing the number of control lines. Furthermore, with reference to figure 2d, one avoids having too many hierarchy levels, since the decoder according to the present invention allows to have a smaller number of hierarchical levels of bit lines. In this manner, one avoids increasing the dimensions of the bit line selection transistor, which slows the circuit, so as to keep the bit line circuit impedance low. Furthermore, since long control lines are avoided, as shown in figure 1, the capacitance of the "points" that drive the selection transistors remains limited, to the benefit of the speed of the circuit. Finally, in the case of a broken bit line matrix array, that is to say, when a single control line drives two selection transistors in respective symmetrical bit line circuits, since "points" are used instead of control lines for the activation of the selection transistors, the capacitance of said points is low and said points can be used to drive more than one transistor. In this manner, it is possible to increase the dimensions of the selection transistors to decrease the impedance of the bit line circuit. The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Thus, for example, the decoder can be provided dually, that is to say, by inverting the nature of the transistors, the logic levels of the buses and of the supply lines, and of course opposite selection placement. Finally, all the details may furthermore be replaced with other technically equivalent ones. In practice, the materials employed, as well as the shapes and the dimensions, may be any according to the requirements without thereby abandoning the scope of the protection of the appended claims. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A bit line selection decoder, particularly for electronic memories, comprising: at least two bit lines (BL), each of which can be selected by a respective switch; a plurality of control lines that drive said switches; a decoder (4) in which the outputs drive said switches; at least one first and one second bus (YM,YN) of control lines that are arranged in input to said decoder and are adapted to address any one of said at least two bit lines; characterized in that said decoder (4) comprises at least one module that comprises: a NOR gate (5) supplied by a first reference voltage (5V); a first transistor (6) of a first polarity that is series-connected to the output of said NOR gate and is driven by said first reference voltage (5V); an inverter (7) that is series-connected to said first transistor (6) and is supplied by a second reference voltage (12V); a second transistor (8) of a second polarity that is interposed between said second reference voltage (12V) and a node that is interposed between said first transistor (6) and said inverter (7), said second transistor being controlled by the output of said inverter. The decoder according to claim 1, characterized in that the product of the number of control lines of each one of said at least two buses is equal to the number of said at least two bit lines (BL). The decoder according to claim 1, characterized in that said switches are transistors. The decoder according to claim 1, characterized in that said module comprises a drain stress transistor (9) that connects said node to a third reference voltage. The decoder according to claim 1, characterized in that the number of said at least one module is equal to the number of outputs of said decoder. A bit line selection decoder, particularly for electronic memories, comprising: at least two bit lines (BL), each of which can be selected by a respective switch; a plurality of control lines that drive said switches; a decoder (4) in which the outputs drive said switches; at least one first and one second bus (YM,YN) of control lines that are arranged in input to said decoder and are adapted to address any one of said at least two bit lines; characterized in that said decoder (4) comprises at least one module that comprises: a NOR gate (5) supplied by a first reference voltage (5V); a first transistor (10) of a first polarity controlled by the output of said NOR gate (5); an inverter (11) supplied by a second reference voltage (12V) and receiving in input the output of said NOR gate (5); a second transistor (12) of said first polarity controlled by the output of said inverter (11); a third transistor (13) of a second polarity that is series-connected to said first transistor (10); a fourth transistor (14) that is series-connected to said second transistor (12); said third (13) and said first (10) transistors being interposed between said second reference voltage (12V) and a third reference voltage; said fourth (14) and said second (12) transistors being interposed between said second reference voltage (12V) and said third reference voltage; said third transistor (13) being controlled by a first node that is interposed between said second (12) and said fourth (14) transistors; and said fourth transistor (14) being controlled by a second node that is interposed between said first (10) and said second (12) transistors. The decoder according to claim 6, characterized in that the number of said at least one module is equal to the number of outputs of said decoder (11). A bit line selection decoder, particularly for electronic memories, comprising: at least two bit lines (BL), each of which can be selected by a respective switch; a plurality of control lines that drive said switches; a decoder (4) in which the outputs drive said switches; at least one first and one second bus (YM,YN) of control lines that are arranged in input to said decoder and are adapted to address any one of said at least two bit lines; characterized in that it comprises at least one module which comprises: a first output (OUT1) and a second output (OUT2); a first transistor (15) of a first polarity that connects said first output (OUT1) to a third reference voltage and is controlled by a first control line of said first bus; a second transistor (16) of said first polarity that connects said second output (OUT2) to said third reference voltage and is controlled by a second control line of said first bus; a third transistor (17) of said first polarity that connects said first output to said second output and is controlled by a control line of said second bus; a fourth transistor (18) of a second polarity that is series-connected to said first transistor (15), said first output being interposed between said first (15) and said fourth (18) transistors, said fourth transistor being controlled by said second output (OUT2); a fifth transistor (19) of said second polarity that is series-connected to said second transistor (16), said second output (OUT2) being interposed between said second (16) and said fifth (19) transistors, said fifth transistor being controlled by said first output (OUT1); a sixth transistor (20) of said second polarity that connects said fourth (18) and said fifth (19) transistors to a second reference voltage (12V), said sixth transistor (20) being controlled by said control line of said second bus. The decoder according to claim 8, characterized in that said first output (OUT1) and said second output (OUT2) are connected to a seventh drain stress transistor (21). The decoder according to claim 8, characterized in that it comprises: an eighth transistor (22) of said first polarity that is interposed between said first output (OUT1) and said first transistor (15) and is controlled by a first reference voltage (5V); a ninth transistor (23) of said first polarity that is interposed between said second output (OUT2) and said second transistor (16) and is controlled by said first reference voltage (5V). The decoder according to claim 8, characterized in that it comprises: at least one additional control line bus (W); at least one tenth transistor (24) of said first polarity that connects said first output (OUT1) and said second output (OUT2) and is controlled by a control line of said at least one additional bus (W); at least one eleventh transistor (25) of said second polarity that is series-connected to said sixth transistor (20) and is controlled by said control line of said at least one additional bus (W).

Inventors: PASCUCCI LUIGI, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: G11C8
IPC codes: G11C11, G11C8