Clock signal is a term used in computer science and related fields. Clock signal is usually used in synchronous circuits, playing the role oftimerTo ensure that relevant electronic components can operate synchronously.
Clock Signal isTemporal logicIt is used to determine when the status in the logical unit is updated. It is a semaphore with a fixed cycle and has nothing to do with operation.The clock signal has a fixed clock frequency, which is the reciprocal of the clock cycle.In the synchronous digital circuit of electronics, especially the signal, the clock signal is a special signal oscillation between the high and low states. The use of the signal is like a digital circuit that acts in coordination with a metronome. The digital clock signal is basically a square wave voltage, as shown in Figure 1:
Figure 1 Clock Signal
The clock signal is generated byClock generatorGenerated.It has only two levels, one is low level, the other is high level.The high level can vary according to the requirements of the circuit, for example, the high level of the TTL standard is 5V.
Although more complex arrangements are used, the most common clock signal is in the form of a fixed constant frequency square wave with a duty cycle of 50%, that is, the duration of high and low levels is the same.The synchronization of clock signal used by the circuit may become active at any rising edge, falling edge, or the clock cycle at the rising and falling edge of the dual data rate. Any clock frequency can be provided according to the use needs of the digital circuit.
Clock edge trigger
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The clock signal is usually used in the synchronous circuit, playing the role of a timer, and constituting the electronic components of the circuit.The clock edge trigger signal means that all state changes occur at the time when the clock edge arrives.Only when the synchronization signal arrives, the relevant trigger will change the output state according to the input signal, so that the relevant electronic components can operate synchronously.Whether the state quantity of the control logic unit changes is the rising edge or the falling edge of the clock signal, depending on the specific logic design.[1]
synchronization
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Synchronization isClock control systemMain constraints in.Synchronization means that the data to be written to the unit is also valid at the time when the effective signal edge occurs.Data validity means that the data volume is relatively stable (unchanged), and the value will change only when the input changes.Because the combined circuit cannot realize feedback, as long as the input quantity does not change, the output will eventually be a stable and effective quantity.
Clock generation mode
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The clock generator uses an oscillator that can provide square wave output to generate the clock.The oscillator circuit always uses feedback to oscillate the oscillator.By feedback the corresponding parameters, the oscillator can work at a specific frequency.We have many different ways to make oscillators.Figure 2 shows two famous ways.The first is to use a simple inverter with feedback components, usually crystals.
Figure 2 Clock generator
Crystal is a kind of precise electrical device, which only allows its specific frequency to pass through.It usually looks like that in Figure 3, or it can be any size and shape:
Fig. 3 Crystal
When we apply a voltage to a piezoelectric crystal, because it only allows signals with the same resonance frequency to pass through, it can produce very accurate frequencies, and the frequency depends on the size and characteristics of the crystal.This is why it is used to filter any other frequency and repair frequency signals.
In the circuit shown above, the first assumption is that the input voltage of the inverter becomes low, so the output becomes high.In this fast transition process, there are usually many harmonics with different frequencies, but only one passes through with the same frequency as the crystal.Similarly, when the input voltage of the inverter becomes higher, the output becomes lower.The continuous occurrence of this phenomenon leads tocrystal oscillator It can generate a fixed frequency square wave signal.
application
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We can use the clock to synchronize different processes of the CPU.The circuits in CPU can be divided into sequential circuits orCombinational logic circuit。Among them, the clock needs to be used on the sequential logic device.Basically, the sequential logic device can only change the cycle output through the rising or falling edge.
Clock control signal
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Clock control signal, also known as "timing signal", refers to the signal that controls the alternate switching of dual clocks through "off" and "on".Timing signal is the key to ensure that the whole digital communication system can work synchronously.yesRegenerative repeaterFor example, how to obtain the timing signal so that the equalization wave can be judged at the peak value is very important.[2]
working principle
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The timing signal is extracted from the transmitted digital signal.For some received signals, such as two-level non return to zero code and AMI code, there is no discrete timing frequency spectrum line through spectrum analysis. Therefore, the timing sine wave cannot be filtered directly with narrowband filter.The nonlinear processing circuit makes the processed signal have discrete timing frequency spectrum lines.The prefilter is a linear filter, which is used to reduce the phase jitter of timing signal in some systems.In some systems, prefilters may not be used.If there are discrete timing frequency spectral lines in the received digital signal, the non nonlinear processing circuit can also be omitted.
There are other components (continuous power spectrum and other discrete spectrum lines) in the waveform sent to the narrowband filter in addition to the timing frequency spectrum line.Narrow band filter plays a role in waveform purification, and the output waveform is close to a sine wave.Then, according to the requirements of the decision regeneration circuit for the timing pulse waveform, the sine wave is sinusoidal to the required timing pulse.For example, using a limiting circuit orSchmidt triggerThe timing square wave can be obtained, and the narrow pulse can be obtained by differential after amplitude limiting.
The purification function of narrowband filter can also be realized by phase-locked loop. The narrowband filter method is widely used because of its simple circuit and economy.The phase locked loop method can obtain timing signals with small phase jitter. With the development of integrated technology, it has also been adopted.
In baseband digital transmission, self timing mode is adopted in most occasions.If there is no discrete spectral line of fixed frequency in the digital signal, it should be generated by nonlinear processing circuit.[3]
