Schmitt trigger gates may also be used to restore the shape and integrity of clock signals before they are applied to gates in different parts of the circuit. The usual way to achieve this is to feed the clock signal via a special clock buffer gate, which will have the necessary low output impedance and a large fan out factor. To avoid this, the clock output must have a low enough impedance to rapidly charge and discharge any natural capacitance in the circuit. 5.1.7 Two Phase Clock Waveforms Circuit Capacitanceīecause the clock must feed many gates, the small capacitance of each of these gates will add, to become an appreciable capacitance, which loads the clock output tending to slow the rise and fall time of the clock signal. The waveform should be kept as close as possible to a perfect square wave shape.įig. Also, by maintaining fast rise and fall times, ringing on the waveform can become a problem. Whatever circuit is used to generate a clock signal, it is important that its output has sufficient fan-out capability to drive the necessary number of ICs requiring a clock input, and that the clock signal is not degraded in amplitude, speed of its rise and fall times or accuracy of its frequency. Distributing Clock Signalsįor more demanding applications there are very many specialised clock oscillator ICs available that are typically optimised for a particular range of applications, such as computer hardware, wireless communications, automotive or medical applications etc. If positive going clock pulses are required, the outputs from the NAND gates may be inverted using Schmitt inverters, which will also help to sharpen the rise and fall times of the clock waveforms. Typical output waveforms are illustrated in Fig. The NAND gate producing Φ01 therefore creates a logic 0 pulse whenever CK and Q are at logic 1, and the NAND gate producing Φ02 creates a logic 0 pulse whenever CK and Q are at logic 1. Each of the NAND gates will produce a logic 0 output whenever both its inputs are at logic 1. 5.1.6 illustrates the operation of Fig 5.1.5. In general form the Mealy circuit can be represented with its block schematic as shown in Fig.3.40.Fig. The false outputs can be eliminated by allowing input to change only at the active transition of the clock (in our example HIGH-to-LOW). Due to this, if the input variations are not synchronized with the clock, the derived output will also not be synchronized with the clock and we get false output (as it is a synchronous sequential circuits). However, they can affect the output of the circuit. 3.39, we can easily realize that, changes in the input within the clock pulses can not affect the state of the flip-flop. 3.39, the output of the circuit is derived from the combination of present state of flip-flops and input (s) of the circuit. When the output of the sequential circuit depends on both the present state of flip-flop(s) and on the input(s), the sequential circuit is referred to as Mealy Circuit. it varies in synchronism with the clock input. In the Moore Circuit, as output depends only on present state of flip-flops, it appears only after the clock pulse is applied, i.e.
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