VLSI DESIGN FLOW AND STICK DIAGRAMS

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The VLSI Design Flow

The VLSI IC circuits design flow is shown in the figure below. The various level of design are numbered and the gray coloured blocks show processes in the design flow.

Specifications comes first, they describe abstractly the functionality, interface, and the architecture of the digital IC circuit to be designed.

Behavioral description is then created to analyze the design in terms of functionality, performance, compliance to given standards, and other specifications. RTL description is done using HDLs. This RTL description is simulated to test functionality. RTL description is then converted to a gate-level netlist using logic synthesis tools. A gate-level netlist is a description of the circuit in terms of gates and connections between them, which are made in such a way that they meet the timing, power and area specifications.

You must have studied FPGA chip architecture of XILINX as it forms the basis of the course on VHDL.

Finally a physical layout is made, which will be verified and then sent to fabrication. This physical layout is also called floor-planning in VLSI industry.

Stick diagram(Simplified Form of Layout)

A "stick" diagram is a simplified layout form which does contain the information related to each of the process steps, but does not contain the actual size of the individual features. Instead the features are represented by simple lines hence the name "stick" diagram.

The stick diagram can easily be drawn by hand and is a handy intermediate form between the circuit diagram and the physical layout since it can easily be modified and corrected. It can therefore be used to anticipate and avoid possible problems when laying out the circuit.

Example: The circuit diagram and stick diagram of a gated R/S flipflop:

Circuit diagram (left) and the corresponding "stick" diagram (right) of a gated R/S flipflop(GIVEN IN ATTACHED FILE)

Shown are the six transistors which can be identified by the red squares which indicate the presence of a gate oxide. The diffused areas are show in green and the aluminum metal is shown in blue. Contact between the aluminum metal and the diffused regions is made by a via hole as indicated by a black circle. The color code used is listed in the table below: (Remember the following table and use colour pencils in Exams also)

Description	Color/shape
Diffusion	Green lines
Gate oxide	Red squares
Via hole	Black circles
Aluminum metal	Blue lines