Basic principles of sequential design

A structural style architecture can be used to model sequential circuits. The storage elements used in a sequential circuit are edge-triggered latches - flip flops - or level-triggered latches - transparent latches. These can be instantiated from libraries provided by the synthesis tool or technology vendor. This can limit the flexibility of an implementation as not all vendor technologies will support all types of storage elements. Elements such as flip flops with asynchronous preset and clear inputs, for example, may be omitted from a library. 

By using a structural design style and explicitly declaring the elements (component instantiation) a number of b efits can be obtained over inferred elements in a behavioural style. In particular the designer has full control of the implementation as  

Inferring storage elements is a mor complex process than instantiating them for three reasons. 

1 To ensure that a storage element will be synthesized the sequential circuit s template must be correctly constructed. 

2 The designer will not know the type of storage elements that will be used until the design has been synthesized. 

---