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.