Column decoder circuit for page reading of a semiconductor memory

Abstract: A column decoder circuit for page reading of a semiconductor memory includes a first level decoder stage (21), a second level decoder stage (22), and a plurality of bit selection stages (24), each comprising a plurality of selection branches (32a, 32b, 32c, 32d); wherein each selection branch is connected to a respective input of a multiplexer (33) and has a plurality of first level selector stages (35) and a second level selector stage (36, 48). Each second level selector stage (36, 48) comprises a first addressing selector (41) for addressing a first group of bit lines (30). Each bit selection stage (24) further comprises a second addressing selector (42) for addressing a second group of bit lines (30), current and next page selectors (45, 46) for selecting one of the first and second groups of bit lines (30).

Description: The present invention refers to a column decoder circuit for page reading of a semiconductor memory. It is known that the reduction in the times for reading semiconductor memories currently represents a problem of considerable importance. In particular, it is known that the data contained in a memory are not immediately available as soon as a reading operation is requested. In fact, it is necessary to wait for a latency time required for decoding the addresses of the memory words to be read (addressing of the memory), for selecting the memory cells, and transferring the information on a data bus. Column decoder circuits have thus been studied that enable simultaneous reading of a number of memory words, if these are set in consecutive memory locations (the reading of series of consecutive memory words, or "burst-mode" reading, is in fact a frequent case). The memory words read are supplied in succession on a data bus at every clock cycle, so as to eliminate or reduce the latency times. For greater clarity, reference is made to Figure 1, which shows a column decoder circuit 1 of a known type, in particular having 128 columns per bit and allowing simultaneous reading of four memory words having consecutive addresses (memory pages). The column decoder circuit 1, belonging to a semiconductor memory (not shown), comprises a first level decoder stage 2, a second level decoder stage 3, and a plurality of bit selection stages 4. In particular, the bit selection stages 4 are equal in number to the bits forming a memory word (for example, 16). The first level decoder stage 2 has an input connected to an address bus 5, from which it receives two first level address bits, and is connected at the output to a first level bus 7 to supply four first level signals YO 0 , ..., YO 3 , each having a selection value and a deselection value. The second level decoder stage 3 has an input connected to an address bus 5, from which it receives three second level address bits, and is connected at the output to a second level bus 8 to supply eight second level signals YN 0 , ..., YN 7 , each having a selection value and a deselection value. In particular, during reading, a single first level signal YO 0 , ..., YO 3 and a single second level signal YN 0 , ..., YN 7 at a time are set at the selection value, while all the other first level and second level signals have the deselection value. Each of the bit selection stages 4 has first level inputs, connected to the first level bus 7, second level inputs, connected to the second level bus 8, and third level inputs, connected to the address bus 5, from which they receive two third level address bits. In addition, each of the bit selection stages 4 is connected to a respective group of bit lines 10 of a memory array (not shown), and has an output connected to a data bus 11. Each group of bit lines 10 comprises, for example, 128 bit lines 10, identified in Figure 1 by a progressive number between 0 and 127, corresponding to a column address of a memory word. Figure 2 shows in greater detail a block diagram of a bit selection stage 4, which comprises first level selector stages 15, second level selector stages 16, column amplifiers 14 sense amplifiers"), and a multiplexer 12. Each first level selector stage 15 is connected to the first level bus 7 supplying the first level signals YO 0 , ..., YO 3 , and to four bit lines 10. In particular, the bit lines 10 connected to a same first level selector stage 15 have column addresses k, k+32, k+64, k+96 (k = 0, 1, ..., 31). Each second level selector stage 16 is connected to the second level bus 8, supplying the second level signals YN 0 ..., YN 7 , and to eight first level selector stages 15. The multiplexer 12 is connected to the second level selector stages 16 via the column amplifiers 14 and pass transistors 18. In addition, the multiplexer 12 is connected to the address bus 5, supplying the third level address bits, and has an output connected to the data bus 11. Figure 3 shows a circuit diagram of the bit selection stage 4 shown in Figure 2. According to the values of the first level signals YO 0 ..., YO 3 , each first level selector stage 15 selects and connects one of the bit lines 10 to the respective second level selector stage 16. For example, all the lines having column address k+32 (wherein k = 0, 1, ..., 31) are selected. Likewise, each second level selector stage 16, according to the values of the second level signals YN 0 ..., YN 7 , selects and connects a first level selector stage 15 to the respective column amplifier 14. In practice, four bit lines 10 having consecutive addresses are selected simultaneously, so that the content of four memory cells (not shown) connected to the selected bit lines 10 is available at the inputs of the multiplexer 12. If it is necessary to read one or more memory pages, in successive clock cycles, the multiplexer 12 sets the data present on its own inputs in sequence on the data bus 11. In known column decoder circuits, however, it is disadvantageous that simultaneous reading of four memory words is possible only if the address of the first memory word to be read can be divided by four. Otherwise, it would be necessary to have available, for each second level selector stage 16, a set of independent values of the second level signals YN 0 ..., YN 7 , while there are present a single second level decoder stage 3 and a single second level bus 8. Suppose, for example, that it is required to read memory words having column addresses 1, 2, 3 and 4. The bit lines 10 for the column addresses 1, 2 and 3 can be selected by setting the second level signal YN 0 at the selection value, which, however, selects the bit line 10 for the column address 0, instead of the one having column address 4 (Figures 2 and 3). Consequently, in this case it is possible to read only three valid memory words simultaneously. The aim of the present invention is to provide a column decoder circuit that is free from the drawbacks described above and, in particular, enables reading of memory pages independently of the initial column address. According to the present invention, a column decoder circuit is therefore provided for page reading of a semiconductor memory, as specified in Claim 1. For a better understanding of the invention, an embodiment thereof is now described, simply to provide a non-limiting example, with reference to the attached drawings, in which: Figure 1 illustrates a simplified block diagram of a column decoder circuit of a known type; Figure 2 illustrates a more detailed diagram of a block of the column decoder circuit of Figure 1; Figure 3 shows a simplified circuit diagram of the block of Figure 2; Figure 4 illustrates a simplified block diagram of a column decoder circuit according to the present invention; Figure 5 illustrates a detail of the block diagram of Figure 4; Figure 6 illustrates a more detailed diagram of a block of the column decoder circuit of Figure 4; Figure 7 illustrates a simplified circuit diagram of the block of Figure 6; Figures 8, 9 and 10 illustrate more detailed circuit diagrams of particulars of the block of Figure 6; and Figure 11 shows a table for signals present in the column decoder circuit of Figure 4. Figure 4 illustrates a column decoder circuit 20 belonging to a memory 19, in particular having 128 columns per bit and allowing simultaneous reading of four memory words having consecutive addresses (memory pages). The column decoder circuit 20 comprises a first level decoder stage 21, a second level decoder stage 22, a third level decoder stage 23, and a plurality of bit selection stages 24. In particular, the bit selection stages 24 are equal in number to the bits that form a memory word (for instance, 16). The first level decoder stage 21 receives from an address bus 25 two first level address bits ADD 6 , ADD 5 (Figure 5) and supplies to a first level bus 27 four first level signals YO 0 ..., YO 3 , each having a selection value (logic value '1') and a deselection value (logic value '0'). The second level decoder stage 22 receives from the address bus 25 three second level address bits ADD 4 -ADD 2 (Figure 5) and supplies to a second level bus 28 eight second level signals YN 0 ..., YN 7 , each having a selection value and a deselection value. The third level decoder stage 23 (not illustrated in detail and comprising the memory elements and a combinatorial circuit implementing the table of Figure 11, in a way that is obvious to the person skilled in the art) receives from the address bus 25 two third level address bits ADD 1 , ADD 0 (Figure 5) and supplies to a third level bus 29 third level signals YM 0 ..., YM 6 , each having a selection value and a deselection value. Each of the bit selection stages 24 has first level inputs connected to the first level bus 27, second level inputs connected to the second level bus 28, and third level inputs connected to the third level bus 29. The bit selection stages 24 are directly connected also to the address bus 25 and receive therefrom the two third level address bits ADD 1 , ADD 0 . In addition, each of the bit selection stages 24 is connected to a respective group of bit lines 30 of a memory array (not shown) and has an output connected to a data bus 31. Each group of bit lines 30 is formed, for example, by 128 bit lines 30, identified in Figure 4 with a progressive number between 0 and 127 corresponding to a column address of a memory word. Figure 6 shows a more detailed block diagram of a bit selection stage 24, which comprises a first, a second, and a third three-level selection branch 32a, 32b, 32c, and a two-level standard selection branch 32d. The three-level selection branches 32a, 32b, 32c and the standard selection branch 32d are connected, through respective column amplifiers 38 ("sense amplifiers"), to the inputs of a multiplexer 33, which has an output connected to the data bus 31. The three-level selection branches 32a, 32b, 32c have the same architecture, and thus only the first three-level selection branch 32a will be described in detail hereinafter, with reference to Figures 6-9. The first three-level selection branch 32a comprises a plurality of first level selector stages 35, a second level selector stage 36, and a third level selector stage 37. Each first level selector stage 35 (eight, in the present embodiment) is connected to the first level bus 27 supplying the first level signals YO 0 ..., YO 3 , and to four bit lines 30. The second level selector stage 36 is connected to the second level bus 28, supplying the second level signals YN 0 ..., YN 7 , and to eight first connection lines 36a, each of which is connected to a respective first level selector stage 35. In addition, the second level selector stage 36 is connected to a second connection line 37a and a third connection line 37b connected to the third level selector stage 37. The third level selector stage 37 is connected to the third level bus 29, supplying a pair of third level signals YM 0 ..., YM 6 , as illustrated in greater detail hereinafter, and is connected to a fourth connection line 37c connected to the respective column amplifier 38. As shown in detail in Figure 8, each of the eight first level selector stages 35 of the first selection branch 32a comprises four first level selectors 40, preferably formed by NMOS transistors, having their drain terminals connected to a respective bit line 30 and their source terminals connected together and to the first connection line 36a. In addition, each first level selector 40 has its gate terminal 40a connected to a respective line of the first level bus 27, so as to receive one of the first level signals YO 0 ..., YO 3 . With reference to Figure 9, the second level selector stage 36 of the first selection branch 32a comprises first and second intermediate selectors 41, 42 formed by NMOS transistors. In detail, the first intermediate selectors 41 have drain terminals connected to a respective connection line 36a and source terminals connected together and to the second connection line 37a. In addition, the second intermediate selectors 42 have drain terminals connected to seven of the eight first connection lines 36a and source terminals connected together and to the third connection line 37b. In this way, one of the first connection lines 36a is connected exclusively to the second connection line 37a, via the respective intermediate selector 41. The remaining seven first connection lines 36a are instead connected both to the second connection line 37a, via a respective first intermediate selector 41, and to the third connection line 37b, via a respective second intermediate selector 42 (Figures 7 and 9). Gate terminals 41a of the first intermediate selectors 41 are connected to respective Lines of the second level bus 28, so as to receive one of the second level signals YN 0 ..., YN 7 , and, furthermore, are each connected to the gate terminal 42a of a second, adjacent, intermediate selector 42 (Figure 9). Consequently, a first intermediate selector 41 and a second intermediate selector 42, which are adjacent and have respective gate terminals 41a, 42a in common, are simultaneously activated when the respective second level signal YN 0 ..., YN 7 it is brought to the selection value. In addition, the first intermediate selector 41, which receives at its gate terminal 41a the second level signal YN 0 , is coupled between the first connection line 36a, which is connected exclusively to the second connection line 37a, and the second connection line 37a (Figure 9). The third level selector stage 37 comprises a current page selector 45 and a next page selector 46 formed by NMOS transistors having drain terminals connected to the second connection line 37a and, respectively, to the third connection line 37b, source terminals connected together and to the fourth connection line 37c, and gate terminals 45a, 46a connected to two respective lines of the third level bus 29, supplying the third level signals YM 0 , YM 4 . Likewise, as shown in Figure 7, the second three level selection branch 32b receives the third level signals YM 1 , YM 5 , and the third three level selection branch 32c receives the third level signals YM 2 , YM 6 . With reference again to Figure 6, the standard selection branch 32d comprises eight first level selector stages 35, a standard selector stage 48 and a standard selector 50. The first level selector stages 35, which are identical to the first level selector stages 35 of the three level selection branches 32a-32c, and thus have already been described with reference to Figure 8, have respective outputs connected to the standard selector stage 48. As shown in Figure 10, the standard selector stage 48 comprises eight first intermediate selectors 41 having drain terminals connected to the output of a respective first level selector stage 35, source terminals connected together and to the corresponding column amplifier 38 via the standard selector 50, and gate terminals 41a connected to respective lines of the second level selection bus 28 to receive the second level signals YN 0 ..., YN 7 . The standard selector 50 formed by an NMOS transistor having its gate terminal connected to the third level bus 29, supplying a third level signal YM 0 ..., YM 6 , in particular a single third level signal YM 3 not supplied to the three level selection branches 32a, 32b, 32c. As shown in Figures 6-8, the bit lines 30 connected to a same first level selector stage 35 are identified by column addresses k, k+32, k+64, k+96 (k = 0, 1, ..., 31). In addition, the column addresses that identify the bit lines 30 can be divided into four classes, 0-3, corresponding, respectively, to the three-level selection branches 32a, 32b, 32c and to the standard selection branch 32d. Specifically, class 0 comprises the column addresses that are integer multiples of 4, i.e., equal to 4k (k = 0, 1, ..., 31), and are associated with values '00' of the third level address bits ADD 1 , ADD 0 ; class 1 contains the column addresses which, divided by 4, give as remainder 1, i.e., the column addresses 4k+1 (k = 0, 1, ..., 31), and are associated with values '01' of the third level address bits ADD 1 , ADD 0 ; class 2 contains the column addresses which, divided by 4, give as remainder 2, i.e., the column addresses 4k+2 (k = 0, 1, ..., 31), and are associated with values '10' of the third level address bits ADD 1 , ADD 0 ; and class 3 contains the column addresses which, divided by 4, give as remainder 3, i.e., the column addresses 4k+3 (k = 0, 1, ..., 31), and are associated with values '11' of the third level address bits ADD 1 , ADD 0 . All the bit lines 30 identified by column addresses belonging to class 0 are connected to the first three-level selection branch 32a; all the bit lines 30 identified by column addresses belonging to class 1 are connected to the second three-level selection branch 32b; all the bit lines 30 identified by column addresses belonging to class 2 are connected to the third three-level selection branch 32c; and all the bit lines 30 identified by column addresses belonging to class 3 are connected to the standard selection branch 32d. The operation of the column decoder circuit 20 is described below. For a clearer understanding, some of the terms that will be used hereinafter are now defined. By "physical page" is meant a group of four bit lines 30 identified by consecutive column addresses and connected in order, starting from the bit line 30 having the lowest column address, to the first, second, and third three-level selection branches 32a, 32b, 32c, and to the standard selection branch 32d. For example, the bit lines 30 identified by the column addresses 4, 5, 6 and 7 form a physical page (Figures 6 and 7). In addition, since the four words to be read may not belong to the same physical page, as defined above, by "current physical page" is meant the physical page to which the first of the four words to be read belongs. For example, if it is required to read memory words having addresses 5, 6, 7 and 8, the current physical page is the physical page comprising the bit lines identified by the column addresses 4, 5, 6 and 7 (Figures 6 and 7). When page reading of the memory is executed, i.e., when memory words having consecutive addresses are simultaneously read, initially reading is requested of a virtual page comprising the first four words to be acquired. The column address of the first memory word to be read is loaded on address bus 25. The virtual page coincides with the physical page if the column address of the first word to be read belongs to class 0. Otherwise, the virtual page comprises words having column addresses included in the current physical page and words having column addresses included in a subsequent physical page. For example, if the column address of the first word to be read is 5, the virtual page comprises the words having column addresses 5, 6, 7 and 8, the current physical page comprises the bit lines 30 having column addresses 4, 5, 6 and 7, and the subsequent physical page comprises the bit lines 30 having column addresses 8, 9, 10 and 11. According to the column address present on the address bus 25, one of the first level signals YO 0 , ..., YO 3 and one of the second level signals YN 0 , ..., YN 7 is set at the selection value, and all the other signals are kept at the deselection value. In this way, in each first level selector stage 35, the first level selector 40 is activated, which receives the first level signal YO 0 , ..., YO 3 set at the selection value, and the corresponding bit line 30 is connected to the respective second level selector stage 36 (to the standard selector stage 48 in the case of the standard selection branch 32d). For example, all the lines having column address k+32 (k = 0, 1, ..., 31) are selected. In addition, in each second level selector stage 36 the first intermediate sector 41 and the second intermediate sector 42 that receive the second level signal YN 0 , ..., YN 7 set at the selection value, are closed. Consequently, in each of the three-level selection branches 32a, 32b, 32c, a bit line 30 belonging to the current physical page and a bit line 30 belonging to the subsequent physical page are selected and are connected, the former to its own second connection line 37a, and the latter to its own third connection line 37b (Figures 6 and 7). At the same time, since in the standard selector stage 48 the first intermediate selector 41 that receives the second level signal YN 0 , ..., YN 7 set at the selection value, is closed, a bit line 30 belonging to the current physical page is selected and is connected to the standard selector 50. The third level address bits ADD 1 , ADD 0 of the first memory word to be read is loaded into the memory elements (not shown) of the third level decoder stage 23. Subsequently, the stored values are updated only when a variation in the group of first level address bits ADD 6 , ADD 5 occurs. The third level decoder stage 23 generates values of the third level signals YM 0 , ..., YM 6 according to the values assumed by 'the stored third level address bits ADD 1 , ADD 0 , as shown in the table given in Figure 11. In particular, in the third level selector stages 37, the current page selector 45 or the next page selector 46 are alternatively closed. If the column address of the first word of the virtual page belongs to class 0, in all the third level selector stages 37 the respective current page selector 45 is closed. If, instead, the column address of the first word of the virtual page belongs to class 1, class 2 or class 3, the next page selector 46 is closed in the third level selector stages 37, respectively, of the first three-level selection branch 32a, of the first and second three-level selection branches 32a, 32b, and of the first, second and third three-level selection branches 32a, 32b and 32c (Figures 7 and 11). The standard selector 50 of the standard selection branch 32d is, instead, always closed. In this way, the bit lines 30 having column addresses equal to the column addresses of the words forming the virtual page to be read are selected independently of the initial column address. During a latency interval, the data supplied via the selected bit lines 30, corresponding to the memory words of the first virtual page the reading whereof is requested, are stored in registers (known and not illustrated, present in the column amplifiers 38) and are made available to the inputs of the multiplexer 33. Thus, the multiplexer 33 feeds the data bus 31, in sequence and in successive clock cycles, with the data present at its own inputs. At the same time, a new virtual page can be read and supplied to the data bus 31 immediately after the first virtual page. The described column decoder circuit has the advantage of favouring simultaneous reading of a plurality of memory words. In fact, thanks to the presence of the third level selector stages 37, it is possible to perform page reading, even if the virtual page requested does not coincide with a physical page. In particular, four memory words can be read simultaneously even if the initial column address cannot be divided by four, provided the first level bits ADD 6 , ADD 5 remains unvaried. Use of page reading is thus rendered more flexible and effective and, consequently, the overall efficiency of the memory is improved. In addition, this improvement in efficiency is achieved with a limited increase in the number of components. Finally, it is clear that numerous modifications and variations can be made to the column decoder circuit described herein, without thereby departing from the scope of the present invention. In particular, the column decoder circuit can be adapted to memories having different characteristics. For example, by increasing the number of inputs of the first level and/or second level selector stages it is possible to decode column addresses for memories having a larger number of bit lines. In addition, the column decoder circuit may comprise a larger number of three-level selection lines, thus enabling reading of pages having a larger number of words.

Claims: A column decoder circuit for page reading of a semiconductor memory, comprising: a first level decoder stage (21) having an input connected to an address bus (25) and an output connected to a first level bus (27); a second level decoder stage (22) having an input connected to said address bus (25) and an output connected to a second level bus (28); and a plurality of bit selection stages (24), each comprising a plurality of selection branches (32a, 32b, 32c, 32d) and a multiplexer (33), said selection branches being connected to respective inputs of said multiplexer (33) and having a plurality of first level selector stages (35) and a second level selector stage (36, 48); each of said first level selector stages (35) being connected to said first level bus (27), to respective bit lines (30), and to a respective second level selector stage (36); each second level selector stage (36, 48) comprising first addressing means (41) for addressing a first group of bit lines (30); characterized in that each bit selection stage (24) comprises second addressing means (42) for addressing a second group of bit lines (30), and selection means (45, 46) for selecting one of said first and second groups of bit lines (30). The circuit according to Claim 1, characterized in that , in each bit selection stage (24), at least one selection branch (32a, 32b, 32c) comprises a third level selector stage (37) including said selection means (45, 46), said third level selector stage (37) being connected, through a first and a second connection (37a, 37b), to a respective on of said second level selector stage (36), and, through a third connection (37c), to said multiplexer (33). The circuit according to Claim 1 or 2, characterized by a third level decoder stage (23) having an input connected to said address bus (25) and an output connected to said third level selector stage (37) through a third level bus (29). The circuit according to Claim 3, characterized in that said third level selector stage (37) comprises a current page selector (45) and a next page selector (46); said current page selector (45) being connected between said first connection (37a) and said third connection (37c) and having a control terminal (45a) connected to said third level bus (29); said next page selector (46) being connected between said second connection (37b) and said third connection (37c) and having a control terminal (46a) connected to said third level bus (29). The circuit according to Claim 3 or 4, characterized in that said control terminal (45a) of said current page selector (45) and said control terminal (46a) of said next page selector (46) are connected to distinct lines of said third level bus (29); said lines of said third level bus (29) supplying third level signals (YM0, ..., YM6) generated by said third level decoder stage (23) for activating alternatively one of said current page selector (45) and said next page selector (46) The circuit according to any of Claims 2-5, characterized in that , in said at least one selection branch (32a, 32b, 32c), said second level selector stage (36) comprises a plurality of first selectors (41) connected to said first connection (37a) and a plurality of second selectors (42) connected to said second connection (37b). The circuit according to Claim 6, characterized in that , in said at least one selection branch (32a, 32b, 32c), at least some of said first level selector stages (35) are connected to a respective one of said first selector (41) and to a respective one of said second selector (42) of said second level selector stage (36). The circuit according to Claim 6 or 7, characterized in that said first and second selectors (41, 42) have respective control terminals (41a, 42a) connected to said first level bus (27); at least some of said first selectors (41) having a control terminal (41a) connected to the control terminal (42a) of a second adjacent selector (42). The circuit according to any of Claims 6-8, characterized in that each bit selection stage (24) comprises a respective standard selection branch (32d) having a plurality of further first level selector stages (35), a standard selector stage (48) and a standard selector (50); said standard selector stage (48) comprising a plurality of first selectors (41) which are connected between respective outputs of said further first level selector stages (35) and said standard selector (50) and have respective control terminals (45a) connected to said second level bus (28); said standard selector (50) being connected to a respective input of said multiplexer (33) and having a control terminal connected to said third level bus (39). A method for column decoding applied to a column decoder circuit for page reading of a semiconductor memory, comprising a first level decoder stage (21) having an input to an address bus (25) and an output connected to a first level bus (27); a second level decoder stage (22) having an input connected to said address bus (25) and an output connected to a second level bus (28); and a plurality of bit selection stages (24), each bit selection stage comprising a plurality of selection branches (32a, 32b, 32c, 32d) and a multiplexer (33), said selection branches being connected to respective inputs of said multiplexer (33) and having a plurality of first level selector stages (35) and a second level selector stage (36, 48); each of said first level selector stages (35) being connected to said first level bus (27), to respective bit lines (30), and to a respective second level selector stage (36); said method comprising the step of addressing (41) a first group of bit lines; characterized by the steps of addressing (42) a second group of bit lines (30) and selecting one of said first and second groups of bit lines (30).

Inventors: BALLUCHI DANIELE, BALLUCHI, DANIELE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2000
Main classification: G11C8
IPC codes: G11C7, G11C8