Standard cell

Abstract: Disclosed is a large scale standard cell provided with at least three power source wires (10) and at least two circuit-element-arranging areas interposed therebetween, whereby the lateral length of the cell can be flexibly reduced to make delay owing to the wire resistance and capacity be avoidable and reduce the area of the cell or entire area of a circuit pattern composed therewith, and the vertical direction of the cell becomes optionally changeable in one cell row.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a standard cell laid out for LSI (large scale integrated circuit), and particularly to a large scale standard cell like a flip-flop standard cell (a so called bistable standard cell) whose length in the direction of the power source wire thereof can be controlled so as not to be larger than required. Description of the Prior Art A conventional standard cell used for a layout of a large scale integrated circuit, particularly a large scale cell like a flip-flop standard cell has a relatively large length in the direction of the power source wire thereof (hereinafter the direction is called lateral direction) as compared with cells other than the flip-flop cell. Thus, when a circuit pattern is composed by a combination of the flip-flop cell and the other, dead space tends to be produced in the circuit pattern owing to the uniformity of their lateral lengths. Accordingly, there is inconvenience in design of the circuit patterns as composed by the combination. Fig. 1 shows a composition of a conventional standard cell. As shown in the drawing, there is provided in the cell two power source wires 1, 1 extending substantially in parallel to the lateral direction (X axis) of the cell so that a group of circuit elements such as transistors and diodes for composing a logical circuit required are arranged in a space 2 defined between the two power source wires 1, 1. Particularly, when flip-flop standard cells composed by the circuit elements as mentioned above are arranged in one cell row , the lateral length of the row becomes considerably large. Fig. 2 shows a manner of a cell from which the clock skew tends to occur, and Fig. 3 shows waveforms produced by the clock skew shifting in timing from an original clock. In Fig. 2, when is driven clock input to a circuit 20 comprising a plurality of cells respectively having relatively long length by supplying thereto a clock signal i through a buffer 21, a clock signal (a) inputted to a cell A and a clock signal (b) inputted to a cell B are different from each other in their time constants of a wire resistance Ri and a wire capacity Ci between the buffer 21 and the cell A, and a wire resistance R and capacity C between the buffer 21 and the cell B. Thus, as shown in Fig. 3, rise times of the original clock signal i, clock signal (a) inputted through Ri₁ and Ci₁, and clock signal (b) inputted through R and C shift from one another owing to respective delays A caused by the difference of the time constant. The shift is generally called clock skew. Accordingly, there has been desired a suitable layout of the cells for avoiding the clock skew. The clock line of this clock skew becomes long and large, in the case that flip-flop cells are scattered. Accordingly, if all the flip-flop cells are arranged in one cell row, it is useful in order to make the clock skew small. However, if the flip-flop cells are conventional ones, the following problems occur. Fig. 4 shows an abstract layout of a portion of IC composed by conventional cells, in which are shown a cell row including only flip-flop cells 3 and other cell rows including cells 4 other than the flip-flop cell. As seen from the drawing, there are dead spaces 5 produced by the differences of lateral length of the rows of the flip-flop cells and the others. Accordingly, an area of the circuit pattern composed with the conventional cells by automatic place and route should become larger than required. Moreover, the conventional standard cell has only two power source wires 1, 1, thus it is impossible to connect cells inversely between the wires of the high potential side (V DD ) and the low potential side (v SS ) in one cell row. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a novel standard cell in a large scale cell such as a flip-flop (bistable) standard cell whose lateral length can be controlled so as not to be larger than required, whereby an area of the cell and an entire area of a circuit pattern composed with the cells by automatic place and route can be reduced. In order to achieve the above-mentioned object, a standard cell according to the present invention for automatic place and route is provided with at least three power source wires as set out in claim 1. Thus, the circuit elements such as transistors and diodes for composing a logical circuit can be arranged in at least two spaces respectively defined between each adjacent pair of the wires. Accordingly, when compared the lateral lengths of the standard cells according to the present invention and the prior art required for arranging the same number of the elements therein, the length of the former can be reduced to substantially half or less as compared with that of the latter because the elements can be arranged in at least two spaces in the case of the former. As seen from the foregoing, according to the present invention, there can be controlled the delay caused by increase of the wire resistance and capacity owing to the enlargement of the lateral direction of the cell. Moreover, since at least three power source wires are provided, the electrical potentials of respective wires can be arranged symmetrically in the vertical direction of the cell, thus it becomes possible to optionally change the vertical direction of the cell in one cell row. While, the vertical length of the cell increases as the number of the power source wires increases. These and other objects, the features and advantages of the present invention will be more apparent from the following description of preferred embodiments, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view for explaining a composition of a cell having two-wire type power source wires according to a prior art; Fig. 2 shows a circuit from which clock skew is generated by delay factors; Fig. 3 shows clock waveforms for explaining clock skew generated in the circuit in Fig. 2; Fig. 4 shows an abstract layout of a portion of IC composed by combination of cell rows respectively including flip-flop cells and other cells according to the prior art; Fig. 5 is a view for explaining a basic composition of an embodiment of a standard cell according to the present invention; Fig. 6 is a view for explaining a composition of an embodiment if a standard cell composed by combination of cell rows respectively including flip-flop cells having three-wire type power source wires according to the present invention and other cells having two-wire type power source wires; Fig. 7 is a view for explaining a composition of an embodiment of a connecting cell for connecting a cell having three-wire type power source wires according to the present invention and a cell having two-wire type power source wires; Fig. 8 is a view for explaining a composition of an embodiment of connection according to the present invention using the connecting cell in Fig. 7; Fig. 9 shows an unpreferable example of the connection of two kinds of cells different in the number of power source wires thereof using the connecting cell in Fig. 7; Fig. 10 shows a preferable example of the connection of two kinds of cells different in the number of power source wires thereof using the connecting cell in Fig. 7; Fig. 11 shows a cell having an input and an output terminal which are disposed away in the vertical direction from each other; Fig. 12 shows an example of connection of the cells in Fig. 11 according to the prior art; and Fig. 13 shows an example of connection of the cells in Fig. 11 according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENT Fig. 5 shows a basic composition of a standard cell according to the present invention. As shown in the drawing, the standard cell for automatic place and route according to the present invention is provided with at least three power source wires 10. And, the central one of the power source wires 10 is a low potential power source wire and the outside two power source wires are high potential power source wires. Accordingly, there can be obtained at least two spaces 11, 11 defined between each adjacent pair of the wires for arrangement of circuit elements such as transistors and diodes for composing a logical circuit. Thus, when compared the lateral lengths of the standard cells according to the present invention and the prior art as shown in Fig. 1 required for arranging the same number of the elements therein, the length of the former can be substantially reduced to half or less to that of the latter because the elements can be arranged in at least two spaces in the case of the former. While, the vertical length of the cell increases as the number of the power source wires increases. However, when the number of the power source wires becomes three, the vertical length of the cell is increased by only 1.5 times that of the prior art cell having two power source wires. Since at least three power source wires 10 are provided, the electrical potentials of respective wires can be arranged symmetrically in the vertical direction of the cell, thus it becomes possible to optionally change the vertical direction of the cell in one cell row. Fig. 6 shows an abstract layout of a portion of IC of an embodiment of the present invention. In the embodiment of the present invention, a plurality of flip-flop cells 7 are assembled together in one row 12, and other cells are assembled in the other rows 13, 13. Reference numerals 14 designate wiring areas respectively. As shown in the drawings, since three power source wires are provided in the flip-flop cells 7, if circuit elements for forming a logical circuit are suitably arranged therein, each the lateral length of the cells 7 can be optionally reduced into a desired length. Accordingly, the length of the row 12 comprising the flip-flop cells of an embodiment according to the present invention shown in Fig. 6 can be adjusted to substantially the same as that of the rows 13, 13, so there can be eliminated the dead space 5 as produced in the composition of the standard cell comprising a combination of a conventional flip-flop cell and other cells as shown in Fig. 4. Figs. 7 and 8 show an embodiment of a connecting cell 13 according to the present invention for connecting two kinds of cells having a different number of power source wires from each other, more specifically, the drawings show the case on which are connected cells having three power source wires and other cells having two power source wires, the wires consisting of aluminium. Further in the drawings, there are shown vias H and a connecting strip 16. When the two kinds of cells are connected at random as shown in Fig. 9, a plurality of the connecting cells 13 should be required and the entire lateral length of the connected cells increases by the length corresponding to the number of the connecting cells employed. Additionally, when the cells having two power source wires are interposed between the cells having three power source wires, wiring in the area becomes extremely difficult. While, as shown in Fig. 10, when the cells having the same number of wires are respectively assembled together, these two kinds of assembled cells can be connected with only one connecting cell 13. Next, there will be described a case on which are connected cells having an input and output terminal disposed away in the vertical direction of the cell from each other with reference to Fig. 11 and 12. In the drawing, the direction of the cell is designated by arrow. Since the direction of the conventional cell having two-wire type power source wires can not be changed vertically in one cell row, the cells should be so arranged that the directions thereof become the same. Accordingly, each wire 19 for connecting each pair of then input terminals 17 and the output terminals 18 of each adjacent pair of the cells should be relatively long. While, according to the present invention, the electrical potentials of respective power source wires can be optionally arranged to be vertically symmetrical. Thus, as shown in Fig. 13, the directions of the cells can be so arranged that each the pair of the input terminal 17 and the output terminal 18 to be connected can be adjacent to each other to shorten the wire 19. This method is applicable to other cases on which different kinds of cells are connected. As seen clearly from the foregoing explanation about an embodiment of the present invention, the area for arranging circuit elements therein can be increased in the vertical direction by increase of the number of the power source wires in a standard cell according to the present invention, thus the elements can be flexibly arranged in the cell, so that the lateral length of the cell can be reduced as compared with conventional cells on condition that the same number of the elements are respectively arranged. Since the electrical potentials of respective power source wires can be arranged vertically symmetrical in the cell according to the present invention, the direction of the cell can be changed vertically in one cell row. Accordingly, there can be shortened wires for connecting cells respectively having an input and an output terminal which are vertically disposed away from each other. Moreover, an entire area of a circuit pattern formed by automatic place and route can be also reduced. Various modification will become possible for those skilled in the art after receiving the teachings of the present invention disclosure without departing from the scope thereof.

Claims: A standard cell for automatic placement and routing of circuit elements of a large scale integrated circuit (LSI), characterised in that it comprises at least three power source wires (10A, 10B, 10C) substantially extending in one direction only (x-axis) of the cell, each adjacent pair (10A, 10B; 10B, 10C) of the power source wires being arranged in parallel and spaced apart relationship for forming a space (11A; 11B) therebetween in which the circuit elements of the standard cell are arranged. A standard cell as claimed in claim 1, wherein one of the power source wires (10B) is a low potential power source wire (V SS ) and the other power source wires (10A, 10C) are high potential power source wires (V DD ). A standard cell as claimed in claim 1, wherein the electric potentials of the at least three power source wires are arranged symmetrically with respect to the central power source wire (10B) towards both outsides thereof. An arrangement of at least two standard cells comprising: a first standard cell (C₁) having at least three power source wires (10) as claimed in any one of claims 1 to 3; a second standard cell (C₂) having at least two power source wires (15); and a connecting cell (13) for connecting the first and second standard cells. An arrangement of standard cells as claimed in claim 4, wherein a plurality of first standard cells are assembled together to form a first standard cell group (C₁ in Fig. 10), a plurality of second standard cells are assembled together to form a second standard cell group (C₂ in Fig. 10), and the first and second cell groups are connected by a single connecting cell (13).

Inventors: KUDO TSUNEAKI, WATANABE HIROYUKI, KUDO, TSUNEAKI, WATANABE, HIROYUKI
Applicants: TOSHIBA KK, TOSHIBA MICRO ELECTRONICS, KABUSHIKI KAISHA TOSHIBA, TOSHIBA MICRO-ELECTRONICS CORPORATION
Application year: 1989
Main classification: G11C5
IPC codes: H01L21, G11C5, G11C11, H01L27, G11C8