JP3065671B2 - Logic circuit design support equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[Object of the Invention]

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a logic circuit design support apparatus for reducing the circuit scale of a logic circuit, and more particularly to a logic circuit for accurately determining the necessity or unnecessary of circuit elements and realizing a smaller circuit scale. The present invention relates to a circuit design support device.

[0003]

2. Description of the Related Art In the design of a logic circuit, there is a case where an output of a circuit element is not connected anywhere and a circuit element whose output signal does not propagate anywhere is included in the logic circuit. Such a circuit element often appears when a logical circuit design support apparatus is used to perform a hierarchical design or a design using a circuit library.

FIG. 3 shows 4-bit input data A (0-3).
Is incremented by 1 to output a data B (0-3) and a carry C, and a logic circuit diagram of a 4-bit incrementer, which is assumed to be registered in a circuit library. For example, when using the logic circuit of FIG. 3 to incorporate a 4-bit incrementer that does not use a carry output into a certain circuit, the output of the circuit element 31 (two-input AND circuit) is not connected anywhere, and therefore, the circuit The element 31 may or may not exist as a logic circuit, and is an unnecessary circuit element as a logic circuit.

In general, it is considered that the smaller the size of a logic circuit is, the better it is realized. Therefore,
When there is such an unnecessary circuit element, the unnecessary circuit element is often deleted at the design stage to realize the circuit.

As a method conventionally used to delete the unnecessary circuit element, there is a method of deleting the unnecessary circuit by a procedure shown in FIG. First, when circuit data is read in step S41, a circuit element whose output is not connected to any part is searched for in the logic circuit in step S42, and the circuit element is deleted from the logic circuit. Next, in step S43, it is determined whether or not a new circuit element whose output is not connected to anything has newly occurred due to the deletion of the circuit element. If there is, the process returns to step S42 to delete the circuit element. . As described above, the processing in step S42 is continued until there is no circuit element whose output is not connected to any part. Finally, in step S44, circuit data is output.

However, in this conventional method, not all unnecessary circuit elements in a logic circuit cannot be deleted, and certain types cannot be removed. For example,
If the output of one circuit element is connected only to its own input and not to the other, it is clear that this circuit element is equivalent to a logic circuit whether or not it is present. However, such a circuit element, that is, a circuit element that forms a loop cannot be recognized as an unnecessary circuit element by the conventional method, and therefore cannot be deleted even though it is actually unnecessary. FIG. 5 shows an example of a logic circuit including circuit elements which are unnecessary circuit elements due to the formation of a loop but cannot be recognized as unnecessary circuit elements by the conventional method. In the figure, a signal output from an output terminal Q of the F / F 54 is a three-input NAND circuit 52.
The output of the three-input NAND circuit 52 is supplied to the input terminal D of the F / F 54 to form a loop, and the output terminal Q of the F / F 54 is Not connected. Therefore, three-input NA
The ND circuit 52 and the F / F 54 are unnecessary circuit elements.

In addition, there are circuit elements which are not necessary logically, but are necessary as an actual circuit. For example, it is a circuit element used to balance the load, which is used as a load for adjusting the timing. In the conventional method, such a circuit element is deleted even though it is a necessary circuit element. Alternatively, if such a circuit element is to be left, an unnecessary circuit element cannot be deleted.

[0009]

As described above, in the conventional logic circuit design support apparatus, a circuit element whose output is not connected to any part is recognized as an unnecessary circuit element and is deleted, so that a loop is formed. Unnecessary circuit elements and the like that have been formed cannot be deleted, and circuit elements that are actually required due to timing adjustment and the like are deleted as logically unnecessary circuit elements.

The present invention solves the above problems,
It is an object of the present invention to provide a logic circuit design support device that accurately determines whether circuit elements are necessary or unnecessary and realizes a smaller circuit scale.

[Structure of the Invention]

[0012]

SUMMARY OF THE INVENTION In order to solve the above problems, a feature of the present invention is to provide a logic circuit design support apparatus for designing a logic circuit of a smaller circuit scale by deleting unnecessary circuit elements in the logic circuit. As shown in FIG. 1, a designation means 1 for designating a circuit element, and a signal propagation path from an output of the circuit element to an external output terminal of the logic circuit or the designation means 1 for all circuit elements in a logic circuit. Detecting means 3 for detecting whether there is a signal propagation path to the input of the designated circuit element, and deleting means for deleting a circuit element which has been determined by the detecting means 3 to have no signal propagation path 5 is provided.

[0013]

In the logic circuit design support apparatus of the present invention, the designating means 1 designates a circuit element to be left, and the detecting means 3 outputs all circuit elements in the logic circuit from the output of the circuit element to the outside of the logic circuit. It is detected whether there is a signal propagation path to an output terminal or a signal propagation path to an input of a circuit element specified by the specifying means 1, and if the detecting means 3 does not have any of the signal transmitting paths, The determined circuit element is deleted by the deletion means 5.

Therefore, a circuit element whose output is not connected to any part is deleted because there is no signal propagation path from the circuit element to the external output terminal of the logic circuit. Regarding the formed circuit element, the circuit element having no signal propagation path to the external output terminal of the logic circuit is deleted. Furthermore, circuit elements that are logically unnecessary but are to be retained for load balancing or the like can be excluded from deletion by specifying them in advance by the specifying unit 1.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention. This figure shows a flowchart of a circuit scale reduction process of the logic circuit design support device according to the present invention. Note that the logic circuit design support apparatus of the present embodiment is realized on a computer as an application program for supporting the logic design of a logic device such as an LSI.

First, in step S21, circuit data is read, and then, in step S22, a set S of circuit elements whose outputs are connected to external output terminals of the logic circuit among circuit elements in the logic circuit is obtained. Further, steps S23 and S24
Then, all the circuit elements output to the input of the circuit elements included in the set S are added to the set S. This process is called a new set S
Repeat until there are no more circuit elements to add. Eventually, the set S becomes a set of logically necessary circuit elements in the logic circuit, and circuit elements not included in the set S become logically unnecessary circuit elements. Therefore, in step S25, all circuit elements not included in the set S are deleted from the logic circuit, and finally, in step S26, circuit data is output.

In the case where there is a circuit element which is not logically necessary but which should be kept in an actual circuit, in FIG. 2, "external output terminal" in step S22 is replaced with "external output terminal and What is necessary is just to replace the input with the input of the desired circuit element, and the processing can be realized only by simple extension.

The logic circuit may be represented not only at a so-called gate level such as an AND circuit or an OR circuit, but also at a switch level using a transistor or the like. Further, the same processing can be performed even when the data is represented by a so-called register transfer level based on data transfer between registers.

[0020]

As described above, according to the present invention, the detection means determines whether or not there is a signal propagation path from the output of the circuit element to the external output terminal of the logic circuit for all circuit elements in the logic circuit. The circuit element that has been detected and the detection means determines that the signal propagation path does not exist is deleted by the deletion means, so that the circuit element forming the loop also receives the signal to the external output terminal of the logic circuit. Circuit elements that do not have a propagation path will be deleted, logically necessary,
It is possible to provide a logic circuit design support apparatus that can accurately determine unnecessary and realize circuits having the same logical function as smaller circuits.

The designating means designates a circuit element to be retained, and the detecting means designates a signal propagation path from an output of the circuit element to an external output terminal of the logic circuit or a designating means for all circuit elements in the logic circuit. Since it is determined whether or not there is a signal propagation path to the input of the circuit element specified in the above, the circuit element which is determined by the detection means to have no signal propagation path is deleted by the deletion means. In addition, it is possible to provide a logic circuit design support apparatus that can remove a circuit element that is logically unnecessary but is to be retained for load balancing or the like from a deletion target, and that can more flexibly realize a circuit scale reduction function. .

[Brief description of the drawings]

[Fig. 1] Claim correspondence diagram

FIG. 2 is a flowchart of a circuit scale reduction process of the logic circuit design support device according to the present invention;

FIG. 3 is a logic circuit diagram of a 4-bit incrementer (first example of a logic circuit including unnecessary circuit elements)

FIG. 4 is a flowchart of a circuit size reduction process of a conventional logic circuit design support apparatus.

FIG. 5 is a logic circuit diagram including a loop (a second example of a logic circuit including unnecessary circuit elements).

[Explanation of symbols]

 1 Designating means 3 Detecting means 5 Deleting means 31, 52, 54 Unnecessary circuit elements

Claims (1)

(57) [Claims] 1. A logic circuit design support apparatus for designing a logic circuit having a smaller circuit scale by deleting unnecessary circuit elements in a logic circuit, a designating means for designating a circuit element, and all circuits in the logic circuit Detecting means for detecting whether a signal propagation path from the output of the circuit element to the external output terminal of the logic circuit or the input of the circuit element specified by the specifying means exists for the element; And a deletion unit for deleting a circuit element determined to have no signal propagation path by the means.