SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device includes word lines (WLO, . . .), bit lines (BLO, . . .) memory cells (a. b, . . .) each connected between the word lines and the bit lines at each intersection thereof, sense amplifiers (SAO, . . .) each con­ nected to each pair of bit lines for amplifying a difference in potential between said lines, bit line reset circuits (BRO′,. . .) each connected to each pair of the bit lines, the difference in potential being held during the read/write cycles, and transfer mode setting circuit for optionally selecting one word line and thereafter another word line, and for simul­ taneously reading out data in each memory cell connected to the one word line to each bit line and thereafter simu­ laneously writing data in each bit line amplified by the sense amplifier to each corresponding memory cell connected to the another word line.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device. More particularly, it relates to a semiconductor memory device enabling high speed data transmission between memory cells. 2. Description of the Related Art In a semiconductor memory device constituted by, for example, a plurality of dynamic type random access memory cells (RAM) each of which comprises a transfer gate transistor and a capacitor, data transmissions are performed between each of the memory cells to move an image on a display apparatus, for example, a cathode-ray tube display (CRT). This movement of the image is, in general, called "scrolling". In a "scrolling" action, the image is moved upward or downward on the display surface. This method is performed in such a way that, first, each word line of the dynamic type RAM is corresponded to each horizontal scanning line of the CRT; second, in the case of upward "scrolling", the data of each memory cell connected to the second word line is transferred to each corresponding memory cell connected to the first word line, and the data of each memory cell connected to the third word line is transferred to each corresponding memory cell connected to the second word line. A similar operation is repeatedly performed to each memory cell connected to the final word line. In the conventional method, however, it is difficult to achieve high speed "scrolling" because data transmission is sequentially performed for every memory cell. That is, first, data of one memory cell is read out; second, the read out data is written in an external register; third, the data in the external register is read out; and finally, the read out data is written in the other memory cell. These read out/write cycles are performed for each and every memory cell. Document US-A-4 193 127 discloses a memory structure in which the information of a first row of memory cells can be read and simultaneously transferred to a second row of memory cells. For each column of cells, two pairs of bit lines are provided, and the access circuits are complicated. SUMMARY OF THE INVENTION The primary object of the present invention is to provide a semiconductor memory device enabling high speed data transmission between each of the memory cells, thereby achieving a high speed "scrolling" of an image on a display apparatus. Another object of the present invention is to provide a semiconductor memory device enabling a reduction of power consumption when "scrolling" an image on a display apparatus. In accordance with the present invention there is provided a semiconductor memory device which is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 is a schematic circuit diagram of a conventional semiconductor memory device, Fig. 2 is a circuit of a memory cell shown in Fig. 1, Fig. 3 is a circuit of a sense amplifier shown in Fig. 1, Fig. 4 is a circuit of a bit line reset circuit shown in Fig. 1, Fig. 5 is a timing chart for explaining data transmission to each of the memory cells shown in Fig. 1, Fig. 6 is a schematic circuit diagram of a semiconductor memory device according to an embodiment of the present invention, Fig. 7 is a timing chart for explaining data transmission according to the embodiment shown in Fig. 6, Fig. 8 is a schematic circuit diagram of a semiconductor memory device according to another embodiment of the present invention, Fig. 9 is a timing chart for explaining data transmission according to the embodiment shown in Fig. 8, Fig. 10 is a bit line reset circuit according to the present invention, and Fig. 11 is a detailed partial circuit of a word decoder shown in Fig. 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments, an explanation will be given of a conventional semiconductor memory device using a dynamic type RAM. Referring to Fig. 1, WD represents a word decoder, WL0, WL1, ... WL511 word lines, CD a column decoder, CL0, CL1, ... CL511 column selection lines, BL0, BL0 , BL1, BL1 , ... BL511, BL511 bit lines, DB, DB data busses, a , b , c ... memory cells, SA0, SA1, ... SA511 sense amplifiers, and BR0, BR1, ... BR511 bit line reset circuits. Each memory cell ( a , b , c ...) comprises a metal-oxide-semiconductor (MOS) transistor Q1 as the transfer gate and a capacitor C₀ as a memory element, as shown in Fig. 2. That is, in this embodiment, the memory cell is one transistor-capacitor type. As is known, when the transistor Q1 is turned ON, the potential of the capacitor C₀ is changed to the potential of the bit line BL so that a read or write operation is performed. Each sense amplifier SA0 to SA511 comprises two MOS transistors Q2 and Q3 cross-connected as shown in Fig. 3. When a sense amplifier enable (SAE) signal is input to both transistors, so that they are activated by the SAE signal, the difference of potential between the bit lines BL and BL is amplified. Each of the bit line reset circuits BRO to BR511 comprises three MOS transistors Q₄ , Q₅ and Q₆ as shown in Fig. 4. When a bit line reset signal ø BR is input to these transistors, they are turned ON so that the bit lines BL and BL are short-circuited by the transistor Q₆ and the potentials of both bit lines are reset to the power source potential V CC . Figure 5 is a timing chart for explaining data transmission between each of the memory cells, for example, from the memory cell b connected to the word line WL1 to the memory cell a connected to the word line WL0. In Fig. 5, V CC represents the power source potential, and V SS the earth potential. One data transmission operation includes a read-out cycle and a write cycle. When the word line WL1 is at a high level (selection state) in the read-out cycle, the data stored in the memory cell b is read out to the bit line so that a small difference in potential occurs between bit lines BL0 and BL0 . This small difference in the potential is amplified by the sense amplifier SA0 so that a large difference of the potential is applied between the bit lines BLO and BL0 . When the column selection line CL0 is at the high level, the amplified bit line data is output to the data busses DB and DB . The data in the busses DB and DB is stored in each data register (not shown). Next, the word line WL1 and the column selection line CL0 are set to a low level (non-selection state), and the potentials of both bit lines BL0 and BL0 are reset (pre-charged) to the power source potential V CC by the bit line reset circuit shown in Fig. 4. The read-out cycle is completed by these procedures. When the word line WL0 is selected to a high level (selection state) in the write cycle, first, the data of the memory cell a is read out so that the bit line BL0 becomes high level and the bit line BL0 becomes low level, then, the column selection line CL0 becomes high level. When the data stored in each register (not shown) at the previous read-out cycle is applied to the data bus, the data bus DB becomes low level and the data bus DB becomes high level, so that the potential of the bit line BL0 is inverted to low level and that of the bit line BL0 is inverted to high level. Consequently, the data in the memory cell a is replaced (written) by the data in the memory cell b . Next, the column selection line CL0 is set to low level and the write cycle is completed by these procedures. In the above-explained conventional method, the data transmission necessitates two cycles, in order to transfer the data from the memory cell b to the memory cell a . That is, one cycle is constituted by reading out from cell b and writing to the register, and the other cycle is constituted by reading out from the register and writing to cell a . For example, when 512 memory cells (256 kRAM) are connected to the one word line, 1024 cycles (2 cycles x 512) are necessary for "scrolling" one word line. Moreover, when there are 512 word lines in the display surface, the necessary cycle then become (1024 x 512) cycles. A semiconductor memory device using a dynamic type RAM according to an embodiment of the present invention will be now described. Referring to Fig. 6, a transfer enable signal generating circuit GEN is added to the conventional semiconductor memory device shown in Fig. 5. This circuit GEN is used for the transfer mode. In this case, high speed "scrolling" according to the present invention is performed by selecting an arbitrary word line in the transfer mode. Moreover, new bit line reset circuits BR0', BR1' ... BR511' shown in Fig. 10 are provided instead of the conventional bit line reset circuits. A transfer signal TR is input to the generating circuit GEN. This transfer signal TR is generated by a "scrolling" command applied from a keyboard (not shown) by an operator. When the transfer signal TR is low level, the generating circuit GEN generates a transfer enable signal TE in the form of a pulse. When the transfer enable signal TE is low level and is input to each bit line reset circuit BR0' to BR511', the operation of each bit line reset circuit is inhibited by this low level signal. That is, as shown in Fig. 10, the low level transfer enable signal TE is input to both gates of the transistors Q₇ and Q₈ , and transistors Q₇ and Q₈ are cut off by this signal TE. In this case, alternatively, the transfer enable signal TE may be input to a reset signal ø BR generating circuit (not shown), and such reset signal generating circuit can inhibit generation of the signal ø BR when the transfer enable signal TE is low level. Moreover, the operation of the column decoder CD is inhibited during the low level state of the transfer enable signal TE, and in this case, no columns are selected. Figure 7 is a timing chart for explaining data transmission according to the first embodiment of the present invention. In Fig. 7, when the transfer enable signal TE is set to low level and the word line WL1 is selected to high level (selection state) in the read-out cycle, a small difference in potential occurs between the bit line BL0 and BL0 . This small difference in the potential also simultaneously occurs in all other bit lines corresponding to the data of each memory cell b , d ... on the word line WL1. Each small difference in the potential is amplified by each sense amplifier SA0, SA1 ... SA511, so that a large difference in the potential is applied between each pair of bit lines. In Fig. 7, only one pair of bit lines BL0 and BL0 are shown, to facilitate the explanation. As mentioned above, after all data in the memory cells connected to the word line WL1 are read out to the corresponding bit line, the word line WL1 is brought to low level (non-selection state). The read-out cycle is completed by these procedures. In this case, each sense amplifier SA0 to SA511 is activated by the sense amplifier enable signal SAE so that the potentials of both bit lines BL0 and BL0 are held as they stand for a short interval. When the word line WL0 is brought to high level in the write cycle, all data in the bit lines is simultaneously written to the corresponding memory cells a, c, ... on the word line WL0. That is, when the word line to be selected is selected by the word decorder WD, all data in the bit lines is simultaneously written to the corresponding memory cells connected to the selected word line. Although only the bit lines BL0 and BL0 are shown in Fig. 7, similar timing charts would apply regarding the other bit lines. When the writing is completed, the word line WL0 is set to low level. Moreover, the transfer enable signal TE is set to high level so that each bit line reset circuit BR0' to BR511' shown in Fig. 10 is activated by this signal TE. The bit lines BL0 and BL0 are reset (pre-charged) to the power source potential V CC and the write cycle is completed by these procedure. As mentioned above, in the read-out cycle, no "pre-charging" of the bit lines is performed by each bit line reset circuit BR0' to BR511' in the interval time between the read cycle and the write cycle, and no column selections are performed by the column decoder CD. These methods are features of the present invention and are different from the conventional method shown in Figs. 1 and 5. Accordingly, although the data transmission per one word line necessitates an order of 1024 cycles in the conventional method, it necessitates only two cycles per one word line according to the present invention. Consequently, considerable high speed "scrolling" can be achieved according to the present invention. Moreover, power consumption for "scrolling" can be considerably reduced by applying the transfer enable signal TE to the column decoder CD, thereby inactivating the same. Figure 8 is a schematic circuit diagram according to another embodiment of the present invention. Referring to Fig. 8, a transfer word line drive circuit DV is added to the circuit of the first embodiment shown in Fig. 6. In this other embodiment, the word line of the write cycle is selected after a short delay time as shown in Fig. 9. The timing of selection of the word line is determined by a transfer word drive signal TWD generated from the drive circuit. This signal TWD is generated based on the timing of the transfer enable signal TE. Figure 9 is a timing chart for explaining data transmission according to this other embodiment of the present invention. In Fig. 9, when the word line WL1 is brought to high level in the read-out cycle, the word line WL0 to be selected is also brought to high level after a short delay time from the selection of the word line WL1. In this case, the word line WL1 is not brought to low level before the selection of the word line WL0. When the word line WL0 is brought to high level, the same procedure as shown in Fig. 7 is performed in the write cycle. Accordingly, both word lines WL1 and WL0 are simultaneously brought to low level when the write cycle is completed. As mentioned above, in this embodiment, the data transmission time per one word line can be reduced to less than that of the two cycles shown in the first embodiment. As is obvious from Figs. 7 and 9, the interval time, which holds the potential of bit lines BL0 and BL0 , is necessary in the first embodiment because no word line drive circuit is provided. Alternatively, no interval time is necessary in the second embodiment because a word line drive circuit is provided. Consequently, the data transmission time per one word line can be considerably reduced, and high speed "scrolling" can be achieved according to the second embodiment. Figure 11 is a detailed partial circuit of the word decorder WD according to the embodiment shown in Fig. 8. Here, the word drive lines WD0 and WD1 are connected to the transfer word line drive circuit DV. The transfer word drive signal TWD is applied to these lines from the drive circuit DV. Each NOR GATE (0 to 511) is connected to the corresponding word line (0 to 511) through gate transistors. When the NOR GATE 1, for example, is selected by address signals A, A and a reset signal ø RST , the gate transistor Q₁₂ is turned ON and the word line WL1 is driven by the word drive line WD0. When the data is transferred to the adjacent word line WL0, the drive signal TWD is applied on the drive line WD1 so that the word drive line WD1 is driven, then, the word line WL0 is driven by the word drive line WD1 since the gate transistor Q₁₄ is already turned ON. The data transmission from every work line is not limited to adjacent word lines. For example, the word line WL5 is selected and the data of the memory cells connected to the word line WL5 is read out, then the word line WL0 is selected and the data of the memory cells connected to the word line WL5 is written to the memory cells connected to the word line WL0. That is, a so-called jump type data transmission is possible according to the present invention. This method is advantageous for, for example, the "scrolling" of characters.

Claims: A semiconductor memory device comprising : a plurality of word lines (WL0, WL1, ...); a plurality of bit lines (BL0, BL0 , BL1, BL1 , ...); a plurality of memory cells (a, b, c, ...) respectively connected between said word lines and said bit lines at intersections of said word lines and bit lines; a plurality of sense amplifiers (SA0, SA1, ...), each connected to a pair of bit lines, for amplifying a difference in potential between respective said bit lines; and a plurality of bit line reset means (BR0', BR1', ...) each connected to a pair of said bit lines, for resetting the bit lines; characterized in that it further comprises transfer control means for producing a transfer enable signal (TE) and for arbitrarily selecting a first word line and shortly thereafter selecting a second word line, such that the selections of both said word/lines overlap for a certain time period, and for reading out, during one combined read/write memory cycle defined by said transfer enable signal (TE), data in each memory cell connected to said first word line onto each bit line and thereafter writing said data on each bit line amplified by one corresponding amplifier of said sense amplifiers (SA0, SA1, ...) to each corresponding memory cell connected to said second word line; and inhibiting means (Q7,Q8) responsive to said transfer enable signal (TE) for inhibiting said bit line reset means to become operative during said combined read/write memory cycle (Figs. 8-10). A semiconductor memory device as claimed in claim 1, wherein said transfer control means comprises a transfer enable signal generating circuit (GEN), connected to said bit line reset means (BR0', BR1',...), which generates said transfer enable signal (TE) in response to an input transfer signal ( TR ) to hold the potential between each of said bit lines. A semiconductor memory device as claimed in claim 2, wherein said transfer control means further comprises a transfer word line drive circuit, operatively connected to said word lines and said transfer enable signal generating circuit (GEN), which generates a transfer word drive signal (TWD) to select said first and second word lines. A semiconductor memory device as claimed in claim 3, wherein said transfer control means further comprises a word decoder (WD), connected to said word lines and said transfer word line drive circuit, which selects said first and second word lines based on said transfer word drive signal (TWD). A semiconductor memory device as claimed in claim 2, wherein said transfer enable signal (TE) has a low level or a high level and said inhibiting means comprises MOS transistor (Q7, Q8) connected to each bit line, each MOS transistor having a gate receiving said transfer enable signal (TE) and each of said MOS transistors being cut off when the low level of said transfer enable signal is input to each of said gates. A semiconductor memory device as claimed in claim 1, further comprising decoder means (WD) for providing a first selection signal for the first word line in response to a first address and for providing a second selection signal for an adjacent word line in response to the first address and an adjacent word line selection signal. A semiconductor memory device as claimed in claim 1, further comprising decoder means (WD) for providing a first selection signal for the first word line in response to a first address and for providing a second selection signal for the second word line in response to a second address.

Inventors: TAKEMAE, YOSHIHIRO
Applicants: FUJITSU LIMITED
Application year: 1985
Main classification: G11C8
IPC codes: H01L27, G11C7, G11C8, G11C11