This invention relates to a sequential logic circuit and, more particularly, to a multi-functional sequential logic circuit comprising semiconductor integrated circuits.
A block diagram of a typical sequential logic circuit is shown in FIG. 1. This circuit comprises of a preceding logic means 1, a logic memory means 2, and a succeeding logic means 3. A flip-flop circuit is usually utilized for the logic memory means 2, and a logic gate or a ROM is usually utilized for the preceding logic means 1 and the succeeding logic means 3. The preceding logic means 1 is supplied with two input signals, that is, an external INPUT 4 from an external circuit (not shown), and a logic state 5 from the logic memory means 2. With these two input signals, a logic calculation is executed so that the preceding logic means 1 generates a new logic state 6 and the content of the logic memory means 2 is rewritten with the generated new logic state. On the other hand, the logic state 5 is also given to the succeeding logic means 3. Then the succeeding logic means 3 outputs an OUTPUT 7 which is obtained from the logic state signal 5 through a predetermined logic calculation. Thus the sequential logic circuit decides the next logic state always depending on the present logic state and the external INPUT.
As each of the preceding logic means 1 and the succeeding logic means 3 is made up of a logic gate or a ROM, a logic calculation executed in each logic means is defined by an arrangement of the logic gate or the content of the ROM. In conclusion, the function of the whole sequential logic circuit is defined unequivocally by them. When a user wants to operate the sequential logic circuit with another function, a rearrangement of the logic gates or a change of the ROM is required. Even if the desired function slightly differs from the present function, the whole circuit need to be changed. The change requires considerable time and labor. In order to give a multi-function to the sequential logic circuit, the whole circuit becomes large and complex because an individual arrangement of logic circuits or an individual ROM have to be provided for each function.