CIRCUIT FOR COMPARING MAGNITUDES OF BINARY SIGNALS

Abstract: The circuit comprises first and second n-channel MOS trans­ istors (1, 2) having source electrodes connected together and drain electrodes connected to source electrodes of a third and fourth n-channel MOS transistors (4, 6) respectively, which have their drain electrodes connected together and to an output port (8). The first and second binary signals are applied to the gate electrodes of the first and third transistors. Inverting amplifiers (10, 12) responsive to the first and second signals apply third and fourth signals at respective different logical levels to the gates of the second and third transistors (2, 6). A fifth n-channel MOS transistor (7) has its drain electrode connecting to the drain electrode of the first transistor (1) and the source electrode of the third transistor (4). The third binary signal is applied to its gate electrode and the source electrode is grounded. A sixth p-channel transistor (5) supplies voltage to the drain electrode of the second transistor (2) and the source electrode of the fourth transistor (6).

Description: BACKGROUND OF THE INVENTION The present invention relates to a circuit for comparing the magnitudes of two n-bit binary signals each being constituted by an n-bit binary number (n = 1, 2, 3 ...). A circuit for comparing binary signals of the kind described has customarily been constructed by combining a plurality of logic gates such as AND gates and NAND gates. One known comparing circuit is a µPD4063BC/4063BG which is disclosed in "CMOS DIGITAL IC 1986" published December 10, 1985, pp. 147 to 151. This binary signal comparing circuit produces a logical 1-bit signal of either ONE or ZERO depending upon whether one of the two binary signals is greater or smaller than the other. A drawback with that comparing circuit is the number of logic gates that are required to compare 4-bit binary numbers and, hence, such a circuit cannot be made without needing a prohibitive number of transistors. The result is an increase in current consumption as well as in chip area required. In addition, when the number of bits of each binary signal is increased, wiring becomes complicated making the design of an integrated circuit difficult. IBM Technical Disclosure Bulletin, Vol. 28, No. 8, January 1986, page 3535 describes an "address compare circuit" for determining whether two n-bit addresses are the same or not. For each of the n bits the respective address bits An and Bn are compared by using four CMOS transistors connected to a voltage source through a load and to ground through a clock device. Of the four CMOS transistors two are n-channel devices and two are p-channel devices. Two series circuits are formed between the load and the clock device, the first comprising an n-channel device connected to the load and a p-channel device connected to the clock device, and the second being the other way round with the p-channel device connected to the load and the n-channel device connected to the clock device. The An input is applied to the gates of the two transistors connected to the load and the Bn input is applied to the gates of the two transistors connected to the clock device. The transistors for all the n bits are connected in parallel between the same load and clock device, and the output is taken from the junction between the load and the transistors. If any one of the bits An differs from the corresponding bit Bn, then a conductive path is formed through the transistors and the output is pulled low. Only if all the bits of A are the same as all the bits of B is the output high. Thus the circuit can only detect equality, and can not compare the magnitudes of A and B to determine which is the larger. SUMMARY OF THE INVENTION The present invention in its two aspects is defined in the independent claims 1 and 7, to which reference should now be made. Advantageous features of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will be described by way of example in the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a circuit diagram schematically showing a first embodiment of the comparing circuit in accordance with the present invention; Fig. 2 is a circuit diagram representative of a second embodiment of the present invention; Fig. 3 is a circuit diagram representative of a third embodiment of the present invention; and Fig. 4 is a circuit diagram representative of a fourth embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 of the drawings, a first embodiment of the present invention is shown. As shown, a 1-bit binary signal comparing logic circuit 100 receives two 1-bit binary signals at input terminals 9 and 11 and produces an output representative of the result of the comparison at an output terminal 8. The comparing circuit 100 includes a first, second, third, fourth and fifth n-channel metal-oxide semiconductor field-effect transistors (MOSFETs) 1, 2, 4, 6 and 7, and p-channel MOSFET 5, each of which have a source, drain and gate, and inverters 10 and 12. The first and second n-channel MOSFETs 1 and 2 each have their source electrode connected to an auxiliary input terminal 3. The third n-channel MOSFET 4 has its source electrode connected to the drain electrode of the MOSFET 1 and the drain electrode of the n-channel MOSFET 7. The fourth n-channel MOSFET 6 has its source electrode connected to the drain electrodes of the n-channel MOSFET 2 and p-channel MOSFET 5. The n-channel MOSFETs 4 and 6 have their drain electrodes connected to the output terminal 8. The input terminal 9 is connected to the input terminal of the inverter 10 and the gate electrode of the transistor 1. The output terminal of the inverter 10 is connected to the gate electrode of the p-channel transistor 5 and to the gate electrode of the n-channel transistors 2 and 7. The input terminal 11 is connected to the input terminal of the inverter 12 and to the gate electrode of the transistor 4. The output terminal of the inverter 12 is connected to the gate electrode of the transistor 6. Further, the transistor 5 has its source electrode connected to a positive power source V dd , and the transistor 7 has its source electrode connected to ground. Assume that the potential of the positive power source V dd is a ONE in terms of logical level and that the potential at the ground level is a ZERO. Then, when a ONE signal and a ZERO signal are respectively applied to the input terminals 9 and 11, the transistors 1 and 6 which receive the ONE signal at their gate electrodes and the transistor 5 which receives the ZERO signal at its gate electrode are turned on, while, at the same time, the transistors 2, 7 and 4 which receive the ZERO signal at their gate electrodes are turned off. As a result, the output terminal 8 is connected to the positive power source V dd and, therefore, a ONE appears on the output terminal 8. Conversely, when a ZERO signal and a ONE signal are respectively applied to the input-terminals 9 and 11, the transistors 2, 7 and 4 are turned on and the transistors 5, 1 and 6 are turned off. In this condition, the output terminal 8 is connected to ground via the transistors 4 and 7 with the result that a ZERO appears on the output terminal 8. When a ZERO signal is applied to both of the input terminals 9 and 11, the transistors 2, 6 and 7 are rendered conductive and the transistors 5, 1 and 4 are rendered non-conductive. On the other hand, when a ONE signal is applied to both of the input terminals 9 and 11, the transistors 1, 4 and 5 are turned on and the transistors 2, 6 and 7 are turned off. In either of such cases, therefore, a signal having the same logical level as the signal applied to the auxiliary input terminal 3 appears on the output terminal 8. With the above construction, the circuit of Fig. 1 produces a ONE on the output terminal 8 when the binary signal applied to the input terminal 9 is greater than the binary signal applied to the input terminal 11 with respect to the logical level. It produces a ZERO on the output terminal 8 when the binary signal applied to the input terminal 9 is smaller than the binary signal applied to the input terminal 11. Further, when the same binary signal is applied to the input terminals 9 and 11, the circuit produces on the output terminal 8 the logical. level of a signal which is applied to the auxiliary input terminal 3. It is to be noted that the logical level of the signal applied to the auxiliary input terminal 3 is set to ONE or ZERO depending upon which of the functions "greater than" and "greater than or equal to" is selected to express the relationship between two input binary signals, respectively. Referring to Figs 2 and 3, a second and a third embodiment of the present invention are shown, respectively. In Figs. 2 and 3, the same or similar structural elements as those shown in Fig. 1 are designated by like reference numerals. A logical 1-bit binary signal comparing circuit 200 shown in Fig. 2 is different from the circuit 100 of Fig. 1 in that it includes a p- or n-channel MOS transistor 51 in place of the p-channel MOS transistor 5, and in that the gate and the source electrode of the transistor 51 are directly connected to each other. In this embodiment the transistor 51 is used as a load. Since the transistor 51 is a load transistor, it can be replaced with a resistor 52 as shown in Fig. 3. In each of the embodiments of Figs. 2 and 3, the output changes in the same manner as in the embodiment of Fig. 1 in response to two input binary signals. In the embodiments of Figs. 1, 2 and 3, the inverters 10 and 12 are shown as being respectively connected between the input terminal 9 and the gate electrode of the transistor 2, and between the input terminal 11 and the gate electrode of the transistor 6. Alternatively, the inverters 10 and 12 may be respectively interposed between the input terminal 9 and the gate electrode of the transistor 1, and between the input terminal 11 and the gate electrode of the transistor 4. Another possible arrangement may be such that the first to five n-channel MOSFETs 1, 2, 4, 6 and 7 are each replaced with a p-channel MOSFETs and the drains and sources of the p-channel transistors are connected in the opposite manner to the case with the n-channel transistors. Such an arrangement is rather impractical, however, partly because the operation is slowed down and partly because the potential of a ZERO becomes higher than the ground level. Referring to Fig. 4, there is shown a specific construction of a n-bit binary signal comparing logic circuit where n is greater than unity and which is basically implemented by the circuit of Fig. 1. The circuit of Fig. 4 is constructed to compare an n-bit binary signal which is applied to input terminals 1₁ to 1 n and another n-bit binary signal which is applied to input terminals 2₁ to 2 n . More specifically, 1-bit comparing circuits 16₁ to 16 n are each constituted by the 1-bit binary signal comparing circuit of Fig. 1, and the auxiliary input terminals 3 and output terminals 8 of the adjacent 1-bit comparing circuits are interconnected. The logical levels of the least significant bit applied to the input terminals 1₁ and 2₁ right up the logical levels of the most significant bit applied to the input terminals 1 n and 2 n are compared at the same time on a bit basis. Assume that the bit position to be compared is the m bit (m = 2, 3, ..., n - 1). Then, the 1-bit comparing circuit 16 m for the m bit produces a ONE when the logical level of the binary signal applied to the input terminal 1 m is greater than the binary signal applied to the input terminal 2 m , a ZERO when the former is smaller than the latter, and a result of comparison associated with the bit one bit lower than m, i.e., the result of comparison associated with the m - 1 bit when they are equal to each other. Further, when all of the most significant bit to the least significant bit are the same in logical level, the output level can be controlled on the basis of the logical level of a signal which is applied to the input terminal 30 of the least significant bit. Accordingly, the result of comparison of n-bit binary signals is produced from the output terminal 80 of the comparing circuit which is asigned to the most significant bit. Again, the logical level of the signal to be applied to the auxiliary input terminal depends upon which of the functions "greater than" and "greater than or equal to" is adopted in expressing the relationship between the two n-bit binary signals. In summary, the present invention provides a binary signal comparing circuit which needs a minimum number of elements, i.e., only five n-channel MOSFETs and two gates, thereby reducing the current consumption and chip area of a semiconductor integrated circuit. Moreover, the circuit of the present invention readily implements the comparison of binary signals each having any desired number of bits by a bit slice construction.

Claims: A circuit for comparing the magnitudes of first and second binary signals, comprising: a first n-channel MOS transistor (1) having a gate electrode to which the first binary signal is applied; a second n-channel MOS transistor (2) having a source electrode connected to a source electrode of said first MOS transistor, and a gate electrode to which a third binary signal different in logical level from the first binary signal is applied; a third n-channel MOS transistor (4) having a source electrode connected to a drain electrode of said first MOS transistor, and a gate electrode to which the second binary signal is applied; a fourth n-channel MOS transistor (6) having a source electrode connected to a drain electrode of said second MOS transistor, and a gate electrode to which a fourth binary signal different in logical level from the second binary signal is applied; an output port (8) connected to a drain electrode of said third MOS transistor and a drain electrode of said fourth MOS transistor; a fifth n-channel MOS transistor (7) having a drain electrode connected to the drain electrode of said first MOS transistor and the source electrode of said third MOS transistor, a gate electrode to which the third binary signal is applied, and a source electrode maintained at ground; and an element (5; 51; 52) for supplying a supply voltage to the drain electrode of said second MOS transistor and the source electrode of said fourth transistor. A circuit as claimed in claim 1, wherein when the first and second binary signals are different in logical level from each other: said second MOS transistor (2), said third MOS transistor (4) and said fifth MOS transistor (7) are turned on, and said first MOS transistor (1) and said fourth MOS transistor (6) are turned off; or said second MOS transistor (2), said third MOS transistor (4) and said fifth MOS transistor (7) are turned off, and said first MOS transistor (1) and said fourth MOS transistor (6) are turned on; and, when the first and second binary signals are equal in logical level to each other: said second MOS transistor (2) and said fourth MOS transistor (6) are turned on; or said first MOS transistor (1) and said third MOS transistor (4) are turned on. A circuit as claimed in claim 1 or 2, wherein an auxiliary input signal (3) is applied to the source of said second MOS transistor and the source of said first MOS transistor. A circuit as claimed in claim 1, 2 or 3, wherein said supplying element comprises a p-channel MOS transistor (5). A circuit as claimed in claim 1, 2 or 3, wherein said supplying element comprises a load MOS transistor (51). A circuit as claimed in claim 1, 2 or 3, wherein said supplying element comprises a resistor (52). A circuit for comparing the magnitudes of first and second n-bit binary signals, comprising n 1-bit comparing circuits according to claim 1 wherein each 1-bit comparing circuit further comprises: an auxiliary input port (3) connected to the source electrode of said first MOS transistor and the source electrode of said second MOS transistor; and said 1-bit comparing circuits being cascaded in n consecutive stages (16₁ ... 16 n ) with said output ports (8) and said auxiliary input ports (3) of said 1-bit comparing circuits adjacent to each other being connected to each other; and each bit of said first n-bit binary signal being supplied as said first binary signal to the gate electrode of said first n-channel MOS transistor (1) of the respective one of said n 1-bit comparing circuits, while each bit of said second n-bit binary signal is supplied as said second binary signal to the gate electrode of said third n-channel MOS transistor (4) of the respective one of said n 1-bit comparing circuits.

Inventors: KANOH, TOSHIYUKI
Applicants: NEC CORPORATION
Application year: 1988
Main classification: G06F7
IPC codes: H03K19, G06F7