![]() ![]() Also when clock signals need to be sent from one system to another over an external wired or wireless link it is common to use one of the several ECL or LVDS logic families with their differential outputs to minimise interference, and there are many application specific ICs (ASICS) using these technologies for high frequency clock distribution. Miniaturisation brought about by surface mount technology can help minimise these problems. clock signals arriving at different parts of the circuit at slightly different times, due to small changes in the phase of some of the distributed clock signals. ![]() Problems such as this will increase the likelihood of ‘skew’ errors, i.e. Where the clock signal has to be distributed around large circuits, there is a greater chance of introducing noise, and possible ‘cross-talk’ where data in one conductor is radiated into another nearby conductor. 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. (Network Time Protocol) A distributed algorithm, developed by Dr.David Mills, that synchronizes a systems clock with a reference source of time. (Network Interface Controller) The hardware that connects a computer to a network. To avoid this, the clock output must have a low enough impedance to rapidly charge and discharge any natural capacitance in the circuit. The network hardware that routes packets throughout the network. 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.
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