Sequential logic

A switch mechanism (English: sequential circuit) processes different input values ​​( Boolean variables ) according to the specified switch matrix to an output value. In contrast to switching networks, in which, by definition, there are no feedback is fed back to a switching device at least one of the outputs at least one of the inputs, making the circuit a stored character is obtained ( a memory).

A switching device is "synchronous" if the input and feedback clock signals are synchronized by, otherwise it is called " asynchronous".

Basics

Asynchronous sequential circuit

To link values ​​corresponding to a preset input to an output value, there are many processes. The link by a switching network is a forgetful process because at any given time, the output value is always dependent on the applied only at this time input. But in order to solve problems that are not only dependent on a snapshot is required circuits with memory, that is a circuit that combines the input values ​​at a certain time before this time with the resulting values.

The asynchronous sequential circuit is the basic form of these circuits. It represents the circuit realization of a Boolean automata is and can be, as shown in the picture, regarded as interconnection of two switching networks, which the Boolean functions f and g realize. The function f takes the input values ​​x and the state values ​​of u, which exist at a time t, and links them to a new state vector ud. The vector u represents the time-delayed, ie stored in the memory of previous result of the function f represents the time delay ( engl. delay) is shown separately from the switching power f by the gray box. Simultaneously, the values ​​x and u by the function g connected to the output vector y at time t. The gray box is replaced with synchronous switching works by flip-flops.

Synchronous sequential circuit

In reality, each of the switching network generates its result after a certain time, which depends on its construction. Thus, in order to link at a given moment applied with the proper input values ​​between stored values ​​of the input signals and the feedback outputs are often synchronized with the aid of flip-flops and a clock signal. This signal pulses at a fixed frequency. Always after the regular occurrence of a signal sequence in the same unit of time has passed. The distances of these sequences are chosen so large that in the meantime all switching networks in switch to complete their calculations, ie all gate delays involved, are possibly also in succession ( ie, if relevant intermediate results ), expired. Then is, for example, at time 3, the result y at the output, which has come through the combination of the values ​​x and u, g of the plant at time 2, about. For the same values ​​of x and u at time 2 arises at time 3 and the new, flip-flops, however, retained value ud one to f. The value of u at time 2 was obtained by clocking the flip-flops from the value of ud at time 1, which in turn x and u resulted from the values ​​that plants at time 0 to f.

Realization

Derailleurs are realized by logic circuits that are built with individual transistors or manufactured on wafers. In the early days of computer technology, vacuum tubes were used instead of transistors. In digital technology, synchronous sequential circuits are often implemented with shift registers. Such a shift register is itself a synchronous sequential circuit dar. In circuits with processors derailleurs are often implemented as program and the memory of previous inputs is designed as a ring structure.

Applications

Almost any device for electronic data processing ( computer ) chips used in its derailleurs. And in the data transmission switching mechanisms are important, for example, each serial interface based on a switching device. In particular, the receiver side must collect all the individual bits in succession, so that after eight cycles, a complete byte is available.

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