Edge-Trigger Pulse Generator

1. Field of the Invention

The present invention relates to a waveform generator, and more particularly to an edge-trigger pulse generator.

2. Description of Prior Art

Normally, an edge-trigger pulse generator includes a time-delay circuit, an inverter and a NAND gate or NOR gate. FIG. 1 (Prior Art) is a schematic diagram of a positive-logic edge-trigger pulse generator which includes a time-delay circuit 10, a NAND gate 12 and an inverter 14. A positive-logic edge-trigger pulse generator has the following operational features:

the output should be and remain "low" and be stable while the input is stable;

the output should be and remain "low" when the input changes from "high" to "low"; and

the output should change from "low" to "high" when the input changes from "low" to "high", and the output should return to "low" after a predetermined time delay (time-delay circuit 10).

FIG. 2a (Prior Art) is a timing diagram showing input and output waveforms of a conventional edge-trigger pulse generator. The width of an output pulse generated by a conventional edge-trigger pulse generator substantially depends on amount of delay caused by time-delay circuit 10 and the the width of the input pulse. As long as the input pulse is wide enough, a proper output will be obtained.

FIG. 2b (Prior Art) is a timing diagram of input and output pulses of the conventional edge-trigger pulse generator. However, in this case, the width of an output pulse is very small. A small pulse maybe easily filtered out by RC effect of transmission line, before the pulse is transmitted to the next circuit stage. A malfunction can occur if the input pulse is not wide enough.

FIG. 3 (Prior Art) is a schematic diagram of a conventional negative logic edge-trigger pulse generator which includes a time-delay circuit 10 and a NOR gate 16.

A negative-logic edge-trigger pulse generator should have the following operational features:

the output should remain "low" and be stable while the input is stable;

the output should remain "low" when the input changes from "low" to "high"; and

the output should change from "low" to "high" when the input changes from "high" to "low", and the output should return to "low" after a predetermined time delay (time-delay circuit 10).

FIG. 4a (Prior Art) is a timing diagram showing input and output waveforms of the conventional negative logic edge-trigger pulse generator. As long as the input pulse is wide enough, a proper output will be obtained.

FIG. 4b (Prior Art) is a timing diagram of input and output pulses of the conventional negative logic edge-trigger pulse generator. However, in this case, the width of an output pulse is very small. A small pulse maybe easily filtered out by RC effect of transmission line, before the pulse is transmitted to the next circuit stage. A malfunction can occur if the input pulse is not wide enough.

Accordingly, to overcome the problems that occur when using a conventional edge-trigger pulse generators, the primary object of the present invention is to provide an edge-trigger pulse generator that outputs a pulse having a predetermined minimum pulse width. This prevents problems from occurring in systems that utilize the edge-trigger pulse generator as a system signal generator.

A positive logic embodiment of the edge-trigger pulse generator includes a first time-delay circuit for delaying and inverting an input pulse. A second time-delay circuit broadens the width of the input pulse. A NAND gate has a first input for receiving an output from the first time-delay circuit and a second input for receiving an output from the second time-delay circuit and provides a logical NAND output. An inverter inverts the output of the NAND gate, so that the width of a pulse output from the edge-trigger pulse generator can be determined merely by the edge-trigger pulse generator while the width of the input pulse is not wider than a predetermined width.

A negative logic embodiment includes a first time-delay circuit for delaying and inverting an input pulse. A second time-delay circuit broadens the width of the input pulse. A NOR gate has a first input for receiving an output from the first time-delay circuit and a second input for receiving an output from the second time-delay circuit and providing a logical NOR output. An inverter inverts the output of the NOR gate so that the width of an pulse output from the edge-trigger pulse generator can be determined merely by the edge-trigger pulse generator while the width of the input pulse is not wider than a predetermined width.

The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings in which:

FIG. 1 (Prior Art) is a circuit diagram of a known positive-logic edge-trigger pulse generator;

FIG. 2a (Prior Art) and 2b (Prior Art) show the timing relationship between an input pulse and an output pulse of the edge-trigger pulse generator shown in FIG. 1;

FIG. 3 (Prior Art) is a circuit diagram of a known negative-logic edge-trigger pulse generator;

FIGS. 4a (Prior Art) and 4b (Prior Art) show the timing relationship between an input pulse and an output pulse of the edge-trigger pulse generator shown in FIG. 3;

FIG. 5 is a circuit, diagram of a positive-logic edge-trigger pulse generator according to the present invention;

FIGS. 6a 6b timing relationship between an input pulse and an output pulse of the edge-trigger pulse generator as shown in FIG. 5;

FIG. 7 is a circuit diagram of a negative-logic edge-trigger pulse generator according to the present invention; and

FIGS. 8a and 8b show the timing relationship between an input pulse and an output pulse of the edge-trigger pulse generator as shown in FIG. 7.

In the figures, identical reference numbers represent the like or corresponding elements.

FIG. 5 is a circuit diagram of a positive-logic edge-trigger pulse generator according to the present invention. The pulse generator includes a first time-delay circuit 20, a second time-delay circuit 30, a NAND gate 40 and an inverter 50. The first time-delay circuit 20 includes a plurality of inverters 20a to 20e and a plurality of capacitors, as shown in the drawing, each coupling a respective inverter output to ground. The second time-delay circuit 30 includes a plurality of inverters 30a, 30b, 30d, 30f and 30h, a plurality of NOR gates 30c, 30e and 30g, and a plurality of capacitors, as shown in the drawing.

FIGS. 6a and 6b illustrate the relationship between an input pulse and an output pulse in the edge-trigger pulse generator as shown in FIG. 5. Five points A, B, C, D and E labeled on FIG. 5 are selected to look at the operation of the circuit. The waveforms at the input and output of the FIG. 5 circuit, along with the waveforms at points A, B, C, D, and E are shown in FIGS. 6a and 6b.

In FIG. 6a, an input pulse is shown on the top line of the figure. This input pulse represents a "wide" input pulse. Conversely, the input pulse shown in the top line of FIG. 6b represents a "narrow" input pulse. In the case of a "wide" input pulse, as shown in FIG. 6a, at point A in the first time-delay circuit 20, the waveform is delayed for a period of time but is unchanged in shape after the input pulse is transmitted through inverters 20a and 20b. At point B in the first time-delay circuit 20, the pulse has passed through three more inverters 20c, 20d and 20e. Therefore, the waveform is delayed and inverted with respect to the input waveform. At point C in the second time-delay circuit 30, the waveform is broadened and inverted with respect to the input waveform after passing through inverters 30a and 30b and a NOR gate 30c. At point D in the second time-delay circuit 30, after passing through inverters 30d and 30f and a NOR gate 30e, the waveform is inverted again and broadened. At point E, after passing through a NOR gate 30g and an inverter 30h, the pulse width is broadened but not inverted. The output pulse is obtained by using the waveforms at B and E as inputs to NAND gate 40. The output of NAND gate 40 is inverted by an inverter 50. Comparing with the prior-art edge-trigger pulse generator, it can be inferred that the edge-trigger pulse generator of the present invention provides the same type of output as that of the prior-art, but is capable of operating on a wider input pulse.

As stated above, FIG. 6b illustrates the situation when the input pulse is "narrow". At point A in the first time-delay circuit 20, the waveform is delayed for a period of time but is unchanged in shape after the input pulse is transmitted through inverters 20a and 20b. At point B in the first time-delay circuit 20, the pulse has passed through three more inverters 20c, 20d and 20e. Therefore, the waveform is delayed and inverted with respect to the input pulse. At point C in the second time-delay circuit 30, the waveform is broadened and inverted with respect to the input pulse after passing through inverters 30a and 30b and NOR gate 30c. At point D in the second time-delay circuit 30, after passing through inverters 30d and 30f and a NOR gate 30e, the waveform is inverted and broadened. At point E, passing through a NOR gate 30g and an inverter 30h, the width of the pulse is broadened but not inverted. Finally, the output pulse is obtained by utilizing the pulses at points B and E as inputs to NAND gate 40. The output of NAND gate 40 is inverted by inverter 50. Comparing with the prior-art edge-trigger pulse generator, it can be found that the width of the output pulse is primarily controlled by the output of the second time-delay circuit 30. Because the pulse width of a pulse inputted in the-second time-delay circuit 30 is broadened while passing through the circuit, the output pulse of the edge-trigger pulse generator may remain at a predetermined width even if the width of an input pulse is narrow. The output pulse will not be filtered out during transmitting from one stage of circuit to its next stage in a system. Therefore, the edge-trigger pulse generator of the present invention can prevent the resulting malfunctioning of the system that may often occur in a system adopting the prior-art pulse generator.

The principles of the present invention can also be applied to a negative-logic edge-trigger pulse generator as shown in FIG. 7. The negative-logic edge-trigger includes a first time-delay circuit 20, a third time-delay circuit 35 and a NOR gate 60. The first time-delay circuit 20 is the same as in the FIG. 5 embodiment. However, the third time-delay circuit 35 of FIG. 7 is different from the second time delay circuit 30 of the FIG. 5 embodiment. The third time-delay circuit 35 includes a plurality of inverters, a plurality of capacitors and a plurality of NAND gates.

FIGS. 8a and 8b correspond to FIG. 7 in the same manner that FIGS. 6a and 6b correspond to FIG. 5. The circuit nodes A', B', C', D' and E', along with the input and output, shown in FIG. 7 are selected to illustrate the operation of the embodiment. The actual waveforms are shown in FIGS. 8a and 8b.

In the FIG. 7 embodiment, the output pulse becomes "high" when the input pulse goes "low". The operation of the FIG. 7 embodiment is analogous to the operation of the FIG. 5 embodiment. Therefore, a detailed description is not provided here. It should be understood from FIGS. 8a and 8b that this embodiment has the same effect as the first embodiment. That is, the edge-trigger pulse generator of the present invention can prevent the resulting malfunctioning of the system that may often occur in a system adopting the prior-art pulse generator.

In both embodiments, the width of an output pulse can be adjusted by changing the number of inverters and logical gates in the time-delay circuits.