Boolean), integer or real representation of the circuit operation. At behavioural level the design is represented in terms of programming statements which makes no use of logic gates. The language allows a system to be described at many different levels from the lowest level of logic gates (called structural) through to behavioural level. This language was developed by the USA Department of Defense and is now a world-wide standard for describing general digital hardware.
VHDL is an acronym for VHSIC Hardware Description Language where VHSIC (Very High Scale Integrated Circuits) is the next level of integration above VLSI. The most commonly accepted behavioural language is that standardised by the IEEE (standard 1076) in 1987 called VHDL. This is a high-level programming language that is textual in nature, describes behaviour and maps to hardware. For designs above 10000 gates an alternative design entry technique of behavioural specification is invariably employed. John Crowe, Barrie Hayes-Gill, in Introduction to Digital Electronics, 1998 11.5 VHDLĪs systems become more complex the use of schematic capture programs to specify the design becomes unmanageable. In this way, complex designs can be created and validated for use multiple times in a structured manner by multiple designers. Complex designs also commonly implement a hierarchical design approach in which symbols identifying the input and output connections of blocks of circuitry are connected together in a schematic, and other schematics contain the detailed circuits for the symbols used. A design description that was initially technology independent would therefore be of greater use as the same description could be used to target a range of implementation technologies. In this view, it is not possible to identify particular logic gates, and navigating through the design to identify particular logic gates and interconnect wires would be a time-consuming task. An example of a complex logic schematic is shown in Figure 6.4. Additionally, the schematic is created for a particular implementation technology and is therefore technology dependent. However, consider a design with hundreds or thousands of logic gates and interconnect wiring: the task of creating, debugging, modifying, and maintaining the schematic becomes immense. For a small circuit of this form, creating a circuit schematic (initially on paper, then within an EDA tool) is a straightforward task.