Transition Detection Circuit

This invention relates to a transition detection circuit for detecting a change or transition of the logic level of a signal in a digital circuit.

Conventional transition detection circuits are exemplified by those shown in FIGS. 1 and 2. The transition detection circuit shown in FIG. 1 comprises four serially connected inverters 1, 2, 3, 4, N-channel MOS transistors 40, 50, 70, 80, and a resistor 60. The outputs of the first and fourth stage inverters 1 and 4 are respectively input to the gates of the N-channel MOS transistors 40, 50, while an input signal A itself and the output of the third inverter 3 are respectively input to the gates of the N-channel MOS transistors 70, 80. A NAND gate comprising the N-channel MOS transistors 70, 80 detects a transition of the logic level of the input signal A from the low level (L) to the high level (H), and another NAND gate comprising the N-channel MOS transistors 40, 50 detects a transition from the high level (H) to the low level (L).

The transition detection circuit as described above, however, has problems in that many elements and connections are required and hence the pattern area needed to form the circuit is extensive.

The transition detection circuit shown in FIG. 2 comprises three inverters 5, 6, 7, N-channel MOS transistors 40, 50, and a resistor 60. The threshold value V.sub.TH1 of the inverter 5 and the threshold value V.sub.TH2 of the inverter 6 are set at different values from each other. The output of the inverter 5 and the inverted output of the inverter 6 are respectively input to the gates of the N-channel MOS transistors 40, 50 which constitute a NAND gate. By virtue of the difference of the threshold values V.sub.TH1 and V.sub.TH2, the transition of the logic level of the input signal A can be detected.

This transition detection circuit can be fabricated with less circuit connections. But it has drawbacks in that adjustment of the threshold values V.sub.TH1, V.sub.TH2 is difficult. Moreover, as the rate of change of the input signal increases, the pulse width of a coincidence detection signal becomes narrower which makes it difficult to achieve a stable detection of the transition.

It is an object of the present invention to provide a transition detection circuit capable of stably detecting the transition, with lesser circuit connections and relatively simple construction.

According to the invention, there is provided a transition detection circuit comprising:

an inverter circuit for inverting an input signal and outputting an inverted input signal;

an S-R flip-flop circuit having two input terminals and two output terminals, receiving said input signal at one of said input terminals and said inverted input signal at the other of said input terminals, and having a time difference between the transitions of the signals at said two output terminals; and

a coincidence detection circuit receiving the output signals from said output terminals of said flip-flop circuit and generating a coincidence detection signal when said output signals are both at the same and specific logic level;

whereby said coincidence detection signal serves to represent a transition of logic level of said input signal.

FIG. 1 is a circuit diagram of a prior art transition detection circuit;

FIG. 2 is a circuit diagram of a prior art transition detection circuit;

FIG. 3 is a circuit diagram of a transition detection circuit according to a first embodiment of the present invention;

FIG. 4 is a timing chart illustrating the operation of the circuit shown in FIG. 3;

FIG. 5 is a circuit diagram of a transition detection circuit according to a second embodiment of the present invention;

FIG. 6 is a timing chart illustrating the operation of the circuit shown in FIG. 5;

FIG. 7 is a circuit diagram of a transition detection circuit according to a third embodiment of the present invention; and

FIG. 8 is a circuit diagram of a transition detection circuit according to a fourth embodiment of the present invention.

The present invention will now be described with reference to the embodiments specifically shown in the respective figures.

The transition detection circuit according to the first embodiment of the present invention comprises, as shown in FIG. 3, an inverter 11, a logic gate such as an S-R flip-flop 6 formed of NAND gates 21, 31, and a coincidence detection circuit in the form of a two-input NAND gate formed of N-channel MOS transistors 41, 51 and a resistor 61. The S-R flip-flop is supplied at its set input terminal with an input signal A, and is supplied at its reset input terminal with an inverted input signal A which has been derived by inverting the input signal A by the inverter 11. The gates of the N-channel MOS transistors 41, 51 are supplied with the output signals B.sub.1, B.sub.2 of the S-R flip-flop, respectively.

The operation of the circuit will be described with reference to FIG. 4. As the logic level of the input signal A transits from a high level to the low level, the output signal B.sub.1 of the NAND gate 31 rises with some delay from a low level to the high level. The output signal B.sub.1 as well as the inverted input signal A is input to the NAND gate 21, causing the output signal B.sub.2 of the NAND gate 21 to change from a high level to the low level. As particularly shown in FIG. 4, owing to the delay in operation of the NAND circuit 21, the change of the output signal B.sub.2 lags by a certain time behind that of the output signal B.sub.1. As a result, logic signals are both at a high level for a certain time interval. Thus, the N-channel MOS transistors 41, 51 are concurrently rendered conductive, thereby outputting a pulsative detection signal which is at a "0" level when a transition is detected.

Contrary to the above, as the logic level of the input signal A transits from a low level to the high level, the output signal B.sub.2 rises, and in turn the output signal B.sub.1 changes from a high level to the low level with a delay time in operation of the NAND circuit 31. Therefore, as shown in FIG. 4, a time difference between the transitions of the output signals B.sub.1, B.sub.2 appears so that output signals B.sub.1, B.sub.2 are both at a high level for a certain time interval. Thus, the N-channel MOS transistors 41, 51 are concurrently rendered conductive to thereby output a pulsative detection signal.

As seen from the above description of the embodiment, it will be understood that the transitions in both directions of an input signal can be detected with a simple circuit arrangement.

Referring further to FIGS. 5 and 6, a transition detection circuit according to the second embodiment of the present invention will be discussed. In this embodiment, an S-R flip-flop 7 is formed of NOR gates 22, 32, while a two-input NOR gate 17 serving as a coincidence detection circuit is formed of P-channel MOS transistors 42, 52. As shown in FIG. 6, in response to the change of the input signal A from a high level to the low level, the inverted input signal A is also changed from a low level to the high level, causing the output signal B.sub.2 of the S-R flip-flop to fall. The fall of the output signal B.sub.2 makes the output signal B.sub.1 change from a low level to the high level. However, by virtue of the delay in operation of the NOR circuit 32, the change of the output signal B.sub.1 lags by a certain time so that a time difference of the transitions between the output signals B.sub.1, B.sub.2 appears so that both P-channel MOS transistors 42, 52 are concurrently rendered conductive for a certain time interval. Thus, a detection signal C is generated which takes a "1" level when a transition is detected. Similarly to the above, in the case of the change of the input signal A from a low level to the high level, a detection signal C can be generated because of the presence of the delay in operation of the NOR circuit 22, thus making it possible to detect the transition in a opposite direction of the input signal A.

As particularly shown in FIGS. 4 and 6, the pulse width of the detection signal C is determined by the time delay in operation of either the NAND circuits 21, 31, or the NOR circuits 22, 32. Thus, the pulse width is kept constant irrespective of the rising/falling rate or transition rate of the input signal A. The operation of the transition detection circuit of the first or second embodiment is therefore stable.

Next, a still further transition detection circuit according to the third embodiment of the present invention will be described with reference to FIG. 7. The transition detection circuit of this embodiment is for detecting the transition caused in any one of a plurality of input signals A.sub.1, A.sub.2, . . . The circuit comprises inverters 13a, 13b, . . . , S-R flip-flops 23, 33, . . . , and a coincidence detection circuit 18. The inverters 13a, 13b, . . . are provided in association with respective input signals A.sub.1, A.sub.2, . . . for producing inverted input signals A.sub.1, A.sub.2, . . . . The S-R flip-flops 23, 33, . . . are formed of NAND gates similarly as those shown in FIG. 3. The coincidence detection circuit 18 is constructed of a wired-OR circuit. The wired-OR circuit or coincidence detection circuit 18 comprises series circuits provided in association with the respective flip-flops 23, 33, . . . . Each of the series circuits has serially connected switching elements which receive at the respective control input terminals thereof the outputs from the associated flip-flops 23, 33, . . . . The switching elements, in this embodiment, are N-channel MOS transistors 43, 53, 73, 83, . . . . The series circuits are connected in parallel with each other, that is their outputs are commonly coupled. With this circuit configuration, a pulse is generated on a detection terminal C when a transition occurs in any one of the input signals A.sub.1, A.sub.2, . . . .

The transition detection circuit shown in FIG. 8 is generally similar to that shown in FIG. 7, except as noted below. In FIG. 8, S-R flip-flops 24, 34, . . . are of the same construction as the flip-flops 7 in FIG. 5, that is, the flip-flops 24, 34, . . . are formed of NOR gates. In addition, a coincidence detection circuit 19 is formed of P-channel MOS transistors 44, 54, 74, 84, . . . .

The coincidence circuits described above have been described as being formed of NAND or NOR gates, however, it is not limited thereto, but any other type of coincidence detection circuits can be used as far as an output signal is generated therefrom when the two output signals of the flip-flop assume the same and specific logic level. Moreover, if the inherent delay in operation of the NAND circuits 21, 31, or the NOR circuits 22, 32 is not long enough, the output terminals of the NAND or NOR circuits may be grounded through appropriate capacitors to increase the delay.

As seen from the above, according to the present invention, the transition detection circuit can be embodied with a simple construction and it requires only a small pattern area. Furthermore, since the pulse width of the coincidence detection signal is determined independent of the rate of change of the input signals, the circuit operation is stable. In addition, even if plural input signals for the circuit are employed, the transition detection circuit can be realized with a simple construction.