Logic Circuit

1. Field of the Invention

The present invention relates to a logic circuit which is equipped with a shift function and a count function.

2. Description of the Prior Art

When inputting an initial value into a counter and then taking out the counted result after a predetermined number of count operating, the use of a small scale integrated circuit (SSI) or the like, in which the bits of data handled are input in parallel as the initial value to the counter and output in parallel as the counted value, does not pose any particular problems. In a large scale integrated circuit (LSI), however, such parallel input of the initial value into the counter from the outside and parallel output of the count value are difficult because of an increase in the number of bits of data handled and a limitation on the number of pins that can be used. Therefore, it is general practice in the prior art to supply the initial value data in serial form from the outside, convert the serial data, by a serial-parallel converting register, to parallel form for input to the counter and convert the count value of the counter in parallel form, by a parallel-serial converting register, to serial form for output.

FIG. 1 illustrates the arrangement of a conventional 4-bit counter/shift register employing a shift register and a counter. In FIG. 1, on the left-hand side of the broken line is a shift register part and on the right-hand side is a counter part. Reference numerals 1-1, 1-2, 1-3 and 1-4 indicate circuits for loading data in parallel; 2-1, 2-2, 2-3 and 2-4 designate cascade-connected flip-flops (FF), each of which is usually a delay (D) flip-flop or delay (D) latch; 3-1, 3-2, 3-3 and 3-4 identify gate circuits for loading data into the counter part and causing it to perform the count operation; 4-1, 4-2, 4-3 and 4-4 denote cascade-connected flip-flops (FF), each of which is usually a J-K flip-flop; and 5 indicates a carry output circuit.

The circuit of FIG. 1 perfroms in following manner. First, the initial value of each bit of the counter is entered in the shift register part. This is achieved by raising the level of a signal on a SHIFT LOAD terminal to a high level "H" in order to enter the initial value of the counter into the bit of each of the shift registers from a SERIAL DATA IN terminal. Needless to say, it is also possible, in this circuit, to set each bit of the counter to a low level "L" by a signal on a 7 COUNT terminal. Next, by raising the level of a signal at a COUNT LOAD terminal to the high level "H", the initial value is loaded into the counter part from the shift register part.

The counting operation is carried out in the following manner. First, the level of the signal at a COUNT LOAD terminal is lowered to the low level "L" and, at this time, a carry signal is applied to a CARRY IN terminal. When the signal at the CARRY IN terminal is at the high level, a carry exists and when the level of this signal is low, no carry exists.

Upon completion of the counting operation, the count result is loaded into the shift register part. This is performed by lowering the level of the signal at the SHIFT LOAD terminal to the low level L. Thereafter, data of each bit in the shift register part is outputted from a SERIAL OUT terminal.

FIG. 2 is a circuit diagram showing a specific example of the circuit arrangement of the counter/register depicted in FIG. 1. In FIG. 2, parts corresponding to those in FIG. 1 are identified by the same reference numerals and characters. The load circuits 1-1 to 1-4 are each constructed with three gates, and accordingly twelve gates are needed. The delay flip-flops 2-1 to 2-4 are each constructed with six gates; hence, 24 gates are required. The gates 3-1 to 3-4 are constructed with 19 gates in all. The J-K flip-flops 4-1 to 4-4 are each constructed with eight gates, and consequently 32 gates are required. Therefore, the counter/shift register of FIG. 1, except the carry output circuit and so forth, requires a total of about 87 gates.

As described above, the prior art circuit employs a register with the same number of bits as the counter for the serial-input, count and serial-output operations. Accordingly, for n-bit counting, the entire circuit arrangement including the register requires 2n flip-flops, along with a large number of gates. It is wasteful to include, for each bit, two flip-flops, one for register use and the other for counter use, and since the register and the counter do not overlap in their operational timing, it is possible to perform the both functions with one flip-flop per bit.

Accordingly, it is an object of the present invention to provide a logic circuit in which a counter is equipped with the function of a shift register or a shift register is equipped with a function of the counter, thereby overcoming the above-mentioned defect in the prior art.

Briefly stated, the logic circuit of the present invention comprises a plurality of cascade-connected flip-flops, each having connected to its input a logical gate group composed of gates which are opened and closed by a shift signal and a count signal. The logic circuit is constructed so that the flip-flop of each bit serves as a shift register or counter in response to the above-mentioned signals.

FIG. 1 is a block diagram showing the construction of a conventional counter/shift register employing a shift register and a counter;

FIG. 2 is a circuit diagram illustrating a specific example of the basic circuit arrangement of the counter/shift register shown in FIG. 1;

FIG. 3 is a block diagram showing the construction of a 4-bit delay flip-flop;

FIG. 4 is a circuit diagram illustrating the construction for a logical gate of a first bit;

FIG. 5 is a circuit diagram illustrating the construction for a logical gate of a second bit;

FIG. 6 is a circuit diagram illustrating the construction for a logical gate of a third bit;

FIG. 7 is a circuit diagram illustrating the construction for a logical gate of a fourth bit;

FIG. 8 is a circuit diagram showing the arrangement of a 4-bit counter/shift register including the logical gates shown in FIGS. 4 to 7;

FIGS. 9 and 10 are circuit diagrams illustrating a logical gate with a construction different from those in FIGS. 4 to 7; and

FIG. 11 is a circuit diagram showing the construction of a logical gate generalized.

In FIG. 3 there is illustrated the construction of a circuit (hereinafter referred to simply as a counter) which comprises four stages of flip-flops A to D and functions as both a shift register and a counter, and in which the flip-flops A to D are driven by a common clock signal CK and reset (cleared) by a common clear signal CLR. Connected to the input stages of the flip-flops A to D there are respectively included logical gates G.sub.A to G.sub.D shown in FIGS. 4 to 7. In FIG. 3, reference character Sin indicates shift data (an initial value to be preset in the counter) which are inputted into the counter in a serial fashion; SHT designates a shift mode signal which signifies a shift operation when it is at the high level "H"; and CNT identifies a count mode signal, which signifies a count operation when it is at the high level "H". The shift mode signal SHT and the count mode signal CNT bear the following relationship:

The logical gate G.sub.A of a first bit A is set to fulfil the following conditions. During the shift, since the shift data Sin are inputted, the data input D.sub.A input into the flip-flop A is as follows:

The expression (2) means that when logical gate G.sub.A is in the shift mode, the data input D.sub.A is the shift data Sin. During counting the element of the first bit is required only to perform a toggle operation, so that the data input D.sub.A to the flip-flop A is as follows:

The expression (3) means that when the logical gate G.sub.A is in the count mode, the data input D.sub.A is Q.sub.A, which is the Q output from the flip-flop A and the inverted output or output of the Q output Q.sub.A from the flip-flop A. This applies to the other bit outputs of the flip-flops B, C and D, that is, each have a Q and Q output. Accordingly, in order for the logical gate G.sub.A to be equipped for both the count and the shift function, it suffices, from the expressions (2) and (3), that the data input D.sub.A is as follows:

FIG. 4 shows an embodiment of the expression (4). In FIG. 4, an AND gate AG.sub.1 corresponds to the expression (2), an AND gate AG.sub.2 corresponds to the expression (3) and an OR gate OG.sub.1 embodies the expression (4).

In the logical gate G.sub.B of a second bit B, during shifting, the output Q.sub.A of the preceding stage is applied to an input D.sub.B, the data input D.sub.B to the flip-flop B is as follows:

During counting, when Q.sub.A =1 (in the case of a carry from the first bit A), since the logical gate GB is required only to perform the toggle operation, the data input DB is as follows:

During counting, when Q.sub.A =0 (in the case of no carry from the first bit A), it is necessary that the status be invariable, and in this case, the data input D.sub.B is as follows:

Combining the expressions (6) and (7), it folows that

Accordingly, in order that the logical gate GB may be equipped with the shift and the count function, the data input D.sub.B which results from combining the expressions (5) and (8), is as follows:

Using the expression (1), the expression (9) is simplified as follows: ##EQU1## FIG. 5 shows an embodiment of the expression (10). In FIG. 5, AND gates AG.sub.1, AG.sub.2 and AG.sub.3 respectively correspond to the right hand first, second and third terms of the expression (10), and an OR gate OG.sub.1 generates the logical sum of the first to third terms of, and thus complets execution of, the expression (10).

Also with respect to third and fourth bits C and D, their logical gates are similarly constructed, with only the number of carry conditions increasing. A logical gate G.sub.C of the third bit C is set so that data input D.sub.C to the flip-flop C satisfies the following condition: ##EQU2## FIG. 6 is an embodiment of the expression (11). In FIG. 6, AND gates AG.sub.1 and AG.sub.4 respectively correspond to the right hand first to fourth terms, where the (third term equals CNT.multidot.Q.sub.A .multidot.Q.sub.C, and the fourth term equals CNT]Q.sub.B .multidot.Q.sub.C, of the expression (11), and an OR gate OG.sub.1 generates the logical sum of the first to fourth terms and thus completes the execution of the expression (11). Likewise, a logical gate G.sub.D of the fourth bit D is constructed so that data input D.sub.D to the flip-flop D satisfies the following condition: ##EQU3## FIG. 7 shows an embodiment of the expression (12). In FIG. 7, AND gates AG.sub.1 and AG.sub.5 correspond to the right hand first to fifth terms of the expression (12) and an OR gate OG.sub.1 generates the logical sum of the first to fifth terms and thus completes the execution of the expression (12).

FIG. 8 illustrates a 4-bit counter/shift register in which the logical gates shown in FIGS. 4 to 7 are connected in the bit arrangement of FIG. 3. Since the signals shift SHT and count CNT bear the relationship that SHT=CNT, the signal CNT/SHT which is at the high-level in the shift mode is applied to an inverter INV; in this case, the shift signal SHT is used as the input to the inverter INV and the count signal CNT is used as the of the inverter INV. When the shift signal SHT is at the high-level, gates G.sub.11, G.sub.21 and G.sub.31 and G.sub.41 are opened and the serial data Sin inputted in synchronism with the clock CK are applied via the AND gate G.sub.11 and an OR gate G.sub.13 to the input terminal D.sub.A of the flip-flop A and read out in sequential order. The output Q.sub.A from the flip-flop A is applied via the AND gate G.sub.21 and an OR gate G.sub.24 to the input terminal D.sub.B of the flip-flop B and read therein by the clock input. Also in the flip-flops C and D, the same operations as mentioned above are performed. Accordingly, when inputting the serial data Sin of, for example, four bits, the bits are stored in the flip-flops D, C, B and A. In this way, the counter is initialized. Next, when the count shift signal CNT/SHT is at the low-level, the signal shift SHT is at the low-level and the count signal CNT is at the high-level, the aforesaid gates G.sub.11, G.sub.21, G.sub.31 and G.sub.41 are closed while the AND gates G.sub.12, G.sub.22, G.sub.32, G.sub.33, G.sub.42, G.sub.43 and G.sub.44 are opened. In the input gate groups of the flip-flops A to D of the first to fourth stages, the arithmetic operations of the right hand second term of the expression (4) and the right hand second and third terms of the expressions (10), (11) and (12) are carried out; namely, a count-up operation takes place. Next, when the count shift signal CNT/SHT is at the high-level again, the next serial data Sin are sequentially entired into the flip-flops A to D. In the absence of the next data, the data Sin is made a continuation of 1 or 0. When the count shift signal CNT/SHT is at the high level the count results are sequentially outputted from a terminal Sout. An AND gate AG.sub.6 obtains the logical product of Q outputs from the respective flip-flops in order to yield a carry output CRY for the first to fourth bits. The carry output CRY is used when connecting a number of counter stages.

In the circuit of FIG. 8, since the delay flip-flops A to D are each constructed with by six gates, the number of gates required in this part is 24, and since about 20 gates are needed in the other parts, approximately 44 gates in all are required. Accordingly, the number of gates used in the circuit of the present invention is about 1/2 that needed in the prior art circuit arrangement of FIG. 2.

FIGS. 9 and 10 show modified forms of the logical gate G.sub.C and a similar construction also applies to the other logical gates. In FIG. 9, a NOR gate is substituted for the OR gate OG.sub.1 in FIG. 6. With this arrangement, the output from the NOR gate NOR is reversed in level from the output from the OR gate OG.sub.1, therefore it is necessary to handle the data input D.sub.C as D.sub.C, by reversing the outputs Q.sub.C and Q.sub.C (Q.sub.C .fwdarw.Q.sub.C, Q.sub.C .fwdarw.Q.sub.C), apply the clear signal CLR to a preset terminal Tp and, conversely, apply a preset signal PRE to a clear terminal Tc. In FIG. 10, the AND gates AG.sub.1 to AG.sub.4 in FIG. 6 are replaced by NAND gates NG.sub.1 to NG.sub.4 and the OR gate OG.sub.1 in FIG. 6 is replaced by an AND gate AG.sub.7. In this case, the output from the AND gate AG.sub.7 is reversed in level from the output from the OR gate OG.sub.1, and accordingly the same output reversal operations as those in FIG. 9 are performed.

A counter/shift register of more than four bits can be formed by using a plurality of such 4-bit counter/shift registers and the number contained in bits of one counter shift register can also be changed. A counter/shift register with an arbitrary number of bits can be constructed as follows. A logical gate G.sub.n of an nth bit is formed as shown in FIG. 11 since the data input D.sub.n is such as given by the following expression:

where CRY=Q.sub.1 .multidot.Q.sub.2 . . . Q.sub.(n-1). The right hand first, second and third terms of the expression (13) can be embodied respectively by gates G.sub.1 by substituting SHT, G.sub.2 and G.sub.3 in FIG. 11 (CNT can be omitted, as mentioned previously) and their logical sum can be embodied by a gate G.sub.4 in FIG. 11. The gates G.sub.1 to G.sub.3 are AND gates, each having the AND function, or NAND gates and the gate G.sub.4 is an OR gate having the OR function or a NOR gate, as described previously. Generalizing the present invention as shown in FIG. 11, a counter/shift register of a desired number of bits can be created, however by connecting a plurality of stages of such circuits, it is possible to construct a counter/shift register in which the number of bits is an integral multiple of the number of stages. Further, the flip-flops used need not always be D flip-flops but may also be J-K flip-flops.

The logical circuit of the present invention described above has the count function and the shift function, and hence is a general purpose circuit. That is, the logical circuit of the present invention is capable of performing an independent operation of a counter, an independent operation of a shift register and a compound operation comprised of serial inputting of data and serial outputting of data after counting. In addition, the logical circuit can be constructed with the desired number of bits and the number of flip-flops is limited to that required for constructing the counter and, further, the number of gates used is smaller than that in the case of constructing the counter and the register separately. Accordingly, the present invention is of particular utility when applied to the master slice system large scale integration LSI which is limited in the available number of gates.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.