Sequential logic element

Sequential logic elements, that is, flip-flops and latches, can be inferred by writing statements within an always statement using styles described in Chapter 2. It is best not to synthesize a memory as a two-dimensional array of flip-flops because this is an inefficient way to implement a memory. The best way to create a memory is to instantiate a predefined memory block using a module instantiation statement. 

The next state logic and the output logic are purely combinational while the present state consists of sequential memoiy elements (flip-flops). Each active clock transition causes a change of state from the present state to the next state. 

This style is not appropriate for the modelling of sequential logic. Instead, and as its name suggests, it is best applied in the modelling of data driven circuit elements such as an arithmetic logic unit (ALU). 

How does the designer know what the synthesizer will produce This chapter should make dear the structure of inferred sequential logic circuits. However, the actual choice of storage element made by the synthesizer is one aspect of the design that is most often very difficult to predetermine. Although a library may have a range of different types av able, indud-ing JK and SR types, the standard choice is usually a variant of tihe D-t5q e. Any additional logic to implement a different type is then constructed on the front end of this. 

Sequential constructs enable the designer to describe the behaviour of the systems without specifying exactly the logic and interconnect required to achieve it. This is in contrast to the concurrent constructs, which go further down the path of describing discrete logic elements or modules with a clearer (if not fully defined) imderstanding of the physical circuit itself. 

The tree diagram is used for systematically breaking down a subject into its basic elements. It shows the logical and sequential links between the subjects and the component elements. The core issue, problem or goal is broken into subcategories which are further broken into the component elements and if necessary sub-elements. 

A combinational logic circuit contains logic gates but does not contain storage elements. A symbolic representation of a multiple-input multiple-output combinational logic circuit is shown in Fig. 1.54(a). The outputs of a combinational logic circuit depend only on the present inputs to the circuit. A sequential... 

Scan A design technique where sequential elements (latches, or flipflops) are chained together in a mode that allows data shifting into and out of the latches. In general, scan allows easy access to the logic between latches and greatly reduces the test generation time and effort. 

Phase 1—Sequential Loop Check. This assures that each device within a loop is "logically correct." By simulating signals from transmitters to receivers and then to final control elements and all auxiliaries (alarms, interlocks, other components within the loops), the commissioning or maintenance team should verify the following ... 

SUTURE does not attempt to generate a correct circuit directly, but rather builds it up incrementally by beginning with a skeletal design that will have timing violations and race conditions. The circuit is modifled in a series of local transformations to correct these problems. Only small primitive elements (i.e., logic gates, flip-flops, and latches) are used to assemble the circuit initially so that sequential transformations can later be more easily applied to reduce the overall size of the logic. The four principal steps of the synthesis method are outlined below. 

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