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Electronic Design

Create A More Precise One-Shot

The most common way to obtain a one-shot pulse is to use a monostable chip, a resistor, and a capacitor. The resulting monostable pulse width, however, is determined by resistor and capacitor values that vary with the ambient environment. Consequently, the pulse width is unstable and imprecise.

A more accurate approach uses a flip-flop, a shift register, and two gates (Fig. 1a). Before the one-shot pulse, the output of the NOR gate is 0. Consequently, the data input of the D-type flip-flop is equivalent to the trigger. When a trigger pulse is present, the flip-flop initiates the one-shot pulse, and the n-stage shift register controls the pulse width, tw, which is a multiple of the clock's period (Fig. 1b).

The precision of the one-shot pulse is determined by the clock period, which is inversely proportional to its frequency. For the circuit to work properly, the width of the trigger pulse, twt, should be greater than one clock period.

The OR gate masks the trigger's effect when the circuit is generating the desired pulse. The net result is a circuit that functions as a non-retriggerable multivibrator.

When the pulse needs to be only one-clock-period wide, the circuit can be simplified. All that's required are two D-type flip-flops and an AND gate (Fig. 2). However, despite its simplicity, this circuit generates a more stable and precise one-shot pulse than a multivibrator.

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