DUAL PORT TYPE SEMICONDUCTOR MEMORY

Abstract: A dual port type semiconductor memory including a dy­ namic type random access memory (RAM) comprising a plurality of word lines (W l ... W n ), a plurality of pairs of bit lines (BL, BL ), and a dynamic type RAM connected between the word lines and the bit lines at each intersection of the word lines and bit lines. A first column decoder (CD₁) is con­ nected to a writing-reading out bus (B₁) through a pair of gates (G₁) consisting of two transistors directly connected to each bit line, and a second column decoder (CD₂) is con­ nected to a reading out bus (B₂) through a pair of gates (G₂) consisting of four transistors. Gates of two of the four tran­ sistors are connected to each bit line (BL, BL ). The other two or the four transistors are connected to the reading out bus (B₂).

Description: The present invention relates to a semiconductor memory, more particularly, to a dual port type semiconductor memory particularly for use in a memory of a computer system. Recent research and development in the field of semiconductor memories has concentrated on the achievement of greater miniaturisation and integration. Few studies, however, have been made on the achievement of greater efficiency of utilisation of semiconductor memories. One well-known technique for achieving greater efficiency of utilisation of semiconductor memories is the use of a dual port type semiconductor memory using a static type RAM. A static type RAM has the feature of "non-destructive read out". That is to say, even if data is read out from a memory cell, the data stored in the memory cell is not erased. Since the static type RAM has the feature as mentioned above, it is possible simultaneously to read out two data from the same memory cell. A static type RAM, however, has many elements compared with a dynamic type RAM, usually six as opposed to two. Therefore, a static type RAM is disadvantageous in minimising and increasing the integration of memories. US-A-4044340 illustrates a single port memory having a read/write means to enable data to be written to and read from the memory. It provides no means, however, to enable anything other than the single read/write operation to be performed at any one time. In accordance with the present invention, a semiconductor memory includes a dynamic type random access memory comprising a plurality of word lines; a plurality of pairs of bit lines; a plurality of dynamic memory cells each cell being connected between respective word lines and bit lines at each intersection of the word lines and the pairs of bit lines respectively; a plurality of first column decoders each connected to a common writing-reading out bus, and to a respective pair of bit lines, wherein each of the first column decoders is connected to the writing-reading out bus through a pair of gates comprising two transistors connected to the respective bit lines to enable data to be transfered either from the bit lines to the writing-reading out bus or from the writing-reading out bus to the bit lines; and a plurality of sense amplifiers each connected to a respective pair of bit lines to store temporarily data in a selected memory cell to prevent destruction of the data during a reading operation; and is characterised in that the memory comprises a dual port memory, the memory further comprising a plurality of second column decoders each connected to a common reading out bus, and to a respective pair of bit lines, wherein each of the second column decoders is connected to the reading out bus through a pair of gates comprising four transistors, gate electrodes of two of the four transistors being connected to the bit lines and the other two of the four transistors being connected to the reading out bus to enable data to be transferred only from the bit lines to the reading out bus. Up to now no dual port type semiconductor memory of this type using a dynamic type RAM has been realised. The invention provides a dual port type semiconductor memory including a dynamic type RAM enabling a high efficiency of utilisation. It also provides a dual port type semiconductor memory featuring high miniaturisation and integration. Thus, it is possible to use a dynamic type RAM as dual port type semiconductor memory, which is advantageous to minimise and to increase the integration of the dual port type semiconductor memory. An example of a dual port type memory in accordance with the present invention will now be described and contrasted with a known memory with reference to the accompanying drawings, in which:- Figure 1 is a basic circuit diagram of a portion of a conventional dual port type conductor memory using a static type RAM; Fig. 2 is a basic circuit diagram of a portion of a dual port type semiconductor memory using a dynamic type RAM according to an embodiment of the present invention; and, Fig. 3 is a block diagram of the overall structure of a dual port type semiconductor memory using a dynamic type RAM according to an embodiment of the present invention. Before describing the preferred embodiments, an explanation will be given of a conventional dual port type semiconductor memory using a static type RAM for reference. Referring to Fig. 1, MC represents a memory cell formed by a static type RAM in the form of a flip-flop circuit. The memory cell MC is connected between word lines W, consisting of lines W R , W R , and W W , and two pairs of bit lines BL W , BL R and BL R , BL W at each intersection of these word lines and bit lines. In writing to the memory cell MC, the word line W W and the bit lines BL W , BL W are selected and writing data D W , D W are applied to the memory cell MC by commands transmitted from a central processing unit (CPU, not shown). In reading out from the memory cell MC, the word line W R and the bit line BL R are selected and the read-out data D R is obtained from the memory cell MC by commands transmitted from the CPU through an address line (not shown). If the memory cell MC is accessed during the above-mentioned reading out process by commands transmitted from the CPU through another address line (not shown), the word line W R and the bit line BL R are selected and inverted reading out data D R is obtained from the memory cell MC. The dual port type semiconductor memory using the static type RAM can be accessed simultaneously by means of the above-mentoned writing and reading out process. As a result, the efficiency of utilization for the semiconductor memory is increased. A dual port type semiconductor memory using a dynamic type RAM according to an embodiment of the present invention will now be described. Referring to Fig. 2, a plurality of word lines are shown by the reference letters W₁ , W₂ --- W n . A pair of bit lines is shown by the reference letters BL and BL . A plurality of single transistor type dynamic memory cells are shown by the same reference letters MC'. Each of the memory cells MC' is connected between a word line and bit line at each intersection of the word lines and bit lines, as shown in the drawing. Each of the memory cells MC' consists of a transistor Tr, a capacitor C, and voltage source V CC . To use the capacitor C to store data, the data is rewritten periodically to prevent it from being extinguished by unavoidable discharge in what is generally called a "refresh" operation. The refresh operation can be performed by a sense amplifier SA. The sense amplifier SA usually comprises a flip-flop circuit and can apply the refreshed data as before to the memory cell MC' before the data stored in the capacitor C of the memory cell MC' is extinguished. Accordingly, the sense amplifier SA can store data for a short term. Therefore, the sense amplifier SA has the function of "non-destructive read out" as well as the feature of the static type RAM as mentioned above. Moreover, the sense amplifier SA can function to hold the data to all memory cells MC' connected to the sense amplifier SA in the case of both writing and reading of the data. The dual port type semiconductor memory using a dynamic type RAM according to the present invention, moreover, comprises at least two column decoders (CD₁ , CD₂). A first column decoder CD₁ is connected to the writing-reading out bus B₁ through a pair of gates G₁. A second column decoder CD₂ is connected to the reading out bus B₂ through a pair of gates G₂. Assuming that the CPU requests simultaneously read out of the data D from the upper memory cell MC' and the data D' from the lower memory cell MC' through different address routes (not shown), the first column decoder CD₁ is selected through a column address bus A₁ for one address route, and the second column decoder CD₂ is selected through a column address bus A₂ for the other address route. Consequently, the desired data is applied to the CPU through each of the address routes. The first column decoder CD₁ can write the data to the memory cell MC' because a pair of gates G₁ is directly connected between the bit lines BL, BL and the writing-reading out bus B₁ not through a gate of the transistor. Accordingly, the writing access can be performed only through the first column decoder CD₁. When the data is written to one of the memory cells MC' at the optional word line W n , at the same time, if the CPU requests to read out data from another memory cell MC' (not shown), selection of the memory cell and reading out of the data from it can be performed by another column decoder (not shown). Since a pair of gates G₂ is controlled by the second column decoder CD₂ and is connected to the bit lines BL, BL through a gate of the transistor, the data on the bit lines are not affected by the action of the pair of gates G₂. Namely, when the data is read out from the memory cell through a pair of gates G₂ , the data on the bit lines are not destroyed due to the influence of the reading out bus B₂. Meanwhile, since a pair of gates G₁ is controlled by the first column decoder CD₁ and is directly connected to the bit lines BL, BL through a drain and a source of the transistor not through the gate as mentioned above, the data on the bit lines are affected by the writing-reading out bus B₁. In order to prevent the influence from bus B₁ , when the data is read out from the memory cell through the first column decoder CD₁ , the memory cell is refreshed by an external amplifier (WA, shown in Fig. 3) connected to the bus B₁ as well as by the conventional external amplifier at the dynamic type RAM. Figure 3 is a block diagram of the overall structure of a dual port type semiconductor memory using a dynamic type RAM. Referring to Fig. 3, reference SAG shows a group of the sense amplifiers (SA₁ to SA n ), and MCA shows a group of the memory cells MC', i.e., a memory cell array. Reference letter DCG₁ shows a group of first column decoders CD₁₁-CD 1n including pairs of gates (G₁ to G 1n ), and DCG₂ shows a group of second column decoders CD₂₁-CD 2n including pairs of gates (G₂₁ to G 2n ). A row decoder can select one of the word lines (W₁ , W₂ --- W n ) based on a row address commnnd RA transmitted from the CPU. One of the first column decoders (CD₁₁ to CD 1n ) in the DCG₁ is selected by a column address command transmitted from the CPU through a column address bus A₁. Similarly, the second column decoder (CD₂₁ to CD 2n ) in the DCG₂ is selected through a column address bus A₂. The read out data from the DCG₁ is applied to a first reading out buffer amplifier RB₁ through the bus B₁. The read out data D R1 is obtained from the output of the buffer RB₁. Since the first column decoder has the function of writing as mentioned above, the writing data D W is applied to the bus B₁ through a writing amplifier WA. Meanwhile, the read out data from the DCG₂ is applied to a second reading out buffer amplifier RB₂ through the bus B₂. The read out data D R2 is obtained from the output of the buffer RB₂.

Claims: A semiconductor memory including a dynamic type random access memory (RAM) comprising a plurality of word lines (W₁... W n ); a plurality of pairs of bit lines (BL, BL ); a plurality of dynamic memory cells (MC') each cell being connected between respective word lines and bit lines at each intersection of the word lines and the pairs of bit lines respectively; a plurality of first column decoders (CD₁) each connected to a common writing-reading out bus (B₁), and to a respective pair of bit lines, wherein each of the first column decoders (CD₁) is connected to the writing-reading out bus (B₁) through a pair of gates (G₁) comprising two transistors connected to the respective bit lines to enable data to be transfered either from the bit lines to the writing-reading out bus (B₁) or from the writing-reading out bus (B₁) to the bit lines; and a plurality of sense amplifiers (SA) each connected to a respective pair of bit lines to store temporarily data in a selected memory cell to prevent destruction of the data during a reading operation; characterised in that the memory comprises a dual port memory, the memory further comprising a plurality of second column decoders (CD₂) each connected to a common reading out bus (B₂), and to a respective pair of bit lines, wherein each of the second column decoders (CD₂) is connected to the reading out bus (B₂) through a pair of gates (G₂) comprising four transistors, gate electrodes of two of the four transistors being connected to the bit lines and the other two of the four transistors being connected to the reading out bus (B₂) to enable data to be transferred only from the bit lines to the reading out bus (B₂). A dual port type semiconductor memory according to claim 1 wherein the first column decoders (CD₁) are connected to a common first column address bus (A₁). A dual port type semiconductor memory according to any of the preceding claims, wherein the second column decoders (CD₂) are connected to a common second column address bus (A₂). A dual port type semiconductor memory according to any of the preceding claims, wherein each memory cell (MC') comprises a single transistor.

Inventors: BABA, FUMIO MIYAMAEDAIRA GURIN HAITSU 48-303
Applicants: FUJITSU LIMITED
Application year: 1983
Main classification: G11C11
IPC codes: G11C8, G11C11