Structured approaches to design

The VLSI design process must address the fields of definition, realisation and verification of complex systems. The most important aspect is the management of the complexity in all three fields. The paper discusses the use of structures to aid in these objectives, and describes a particular process sequence. The field of VLSI design is characterised by somewhat obscure terminology. The key to understanding the operations involved in VLSI is seen as a clear definition of these terms in this context, and also their inter-relationships. The terms addressed here are: structure; function; behaviour; hierarchy; abstraction. The design process is widely regarded as being essentially a top-down process, from a precise definition of required function, progressing through a hierarchy of functional levels to the full implementation of the system. A practical situation is discussed, where a process of refinement is applied at each level and the use of structured design enables a degree of bottom-up definition to be incorporated into the design flow. The use of structures in a design introduces the need for design partition. Partition implies communication between the parts. Communication is seen as a vital aspect of system behaviour. Partition frequently is applied to the design team also, and this requires communications between designers, a further important link in the design process. The detailed interplay between definitions and descriptions, both up and down the hierarchy and also between the different design abstractions is seen as the essential activity of the design process where a structural approach is applied. The features of the design system necessary to support this activity are discussed. The status of test in the design process can never be placed too high. The significance of structured design on the ease and validation of test is therefore also considered.