The concept of modularity enables parallelization of the design process. Locality: The concept of locality in system ensures that the internal details of each Answer: VLSI Design flow consists of several steps which are carried out in linear . VLSI Design Methodologies EEB (Winter ): Lecture # 4Mani of Structured Design Techniques Hierarchy Regularity Modularity Locality; . architecture structure of accumulator is component reg — definition of. Digital VLSI Circuits. 1. Introduction to ASIC Design a. Design Strategies: Hierarchy, Regularity, Modularity & Locality b. Chip Design Options: Gate Array, Field.

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In the following example, inputs In1 and In2 are at specified locations on the Metal1 layer while moodularity output, Out, is located as specified on the Metal2 layer: Individual modules are then implemented with leaf cells.

Connections to Sub-Cells When making connections to any sub-module connections may only be made at defined ports.

Modularity in design means that the various functional blocks which make up the larger system must have well-defined functions and interfaces.

A good rule to use is to ensure that taps must be 1. The input and output pins are located on the upper and lower boundaries of the cell. Your work with magic will not require explicit keep out masks, but you will be required to observe implied keep out areas as appropriate. If a regulraity interconnect density can be achieved in the routing channel, the standard cell rows can be placed closer to each other, resulting in a smaller chip area.

Hierarchy Rules for Layout

If you don’t obey hierarchy rules, a few things may not work but in general you’ll just get a messy, difficult to debug, difficult to explain system. The availability of these routing channels simplifies the interconnections, even using one metal layer only. This is translated into the increase in the design cycle time, which is the time period from the start of the chip development until the mask-tape delivery time. The current leading-edge technologies such as low bit-rate video and cellular communications already provide the end-users a certain amount of processing power and portability.

In fact magic can cope with diffusions closer than 1. Although top-down design flow provides an excellent design process control, in reality, there is no truly unidirectional top-down design flow. For this reason it is best to avoid this style in order to provide cells which are portable between different layout tools.


Such changes may require significant modification of the original requirements. Standard-cell based designs may consist of several such macro-blocks, each corresponding to a specific unit of the system architecture such as ALU, control logic, etc. Both top-down and bottom-up approaches gegularity to be combined. However, the development cost of such a design style is becoming prohibitively high.

In the figure below magic satisfactorily joins one pair of diffusions while the other causes a design rule error: The interconnection patterns to realize basic logic gates can be stored in a library, which can then be used to customize rows of uncommitted transistors according to the netlist. The third evolution starts with a behavioral module description. The failure to properly verify a design in its early phases typically causes significant and expensive re-design at a later stage, which ultimately increases the time-to-market.

Black Box or Abstract View The following figure shows the ports defined earlier together with explicit Metal1 and Metal2 keep out areas which ensure that no unwanted interaction takes place.

For instance, if a chip designer defined an architecture without close estimation of the corresponding chip area, then it is very likely that the resulting chip layout exceeds the area limit of the available technology. Sophisticated computer-aided design CAD tools and methodologies are developed and applied in order to manage the rapidly increasing design complexity. By defining well-characterized interfaces for each module in the system, we effectively ensure that the internals of each module become unimportant to the exterior modules.

The programming of the interconnects is implemented by programming of RAM cells whose output terminals are connected to the gates of MOS pass transistors.

The control terminals of multiplexers are not shown explicitly in Fig. The design process, at various levels, is usually evolutionary in nature. Ports By convention, ports in magic are indicated by non-point labels on a particular layer. Below are two abstract layouts for NAND gates, illustrating some more complex features: As a result, their design conncept is considered much higher than that of memory regulrity, although advanced memory chips contain some sophisticated logic functions.

At lower levels of the physical hierarchy, the internal mask.

Hierarchy Rules for Layout

The most rigorous full custom design can be the design of a memory cell, be it static or dynamic. The strategy is to avoid replacing a complex system design with a complexity of sub-modules. Generally speaking, logic chips such as microprocessor chips and digital signal processing DSP chips contain not only large arrays of memory SRAM cells, but also many different functional units. The physical design and layout of logic cells ensure that when cells are placed into rows, their heights are matched and neighboring cells can be abutted side-by-side, which provides natural connections for power and ground lines in each row.


Regularity Regularity controls the manner loality which sub-modules are chosen. The concept of locality also ensures that connections are mostly between neighboring modules, avoiding long-distance connections as much as possible.

The keep out modhlarity for each layer should extend for one half of one design rule distance beyond the edge of the cell. In this case we create a single design for this module but we use several instances of this design in different parts of the system. The CLB is configured such that many different logic functions can be realized by programming its array.

Design of VLSI Systems – Chapter 1

Exceptions to this include the design of high-volume products such as memory chips, high- performance microprocessors and FPGA masters. The signal delay, noise margins, and power consumption of each cell should be also optimized with proper sizing of transistors using circuit simulation. With the use of multiple interconnect layers, the routing can be achieved over the active cell areas; thus, the routing channels can be removed as in Sea-of-Gates SOG chips.

Next, the placement and routing step assigns individual logic cells to FPGA sites CLBs and determines the routing patterns among the cells in accordance with the netlist.

It also allows the use of generic modules in various designs – the well-defined functionality and signal reguarity allow plug-and-play design. This physical view describes the external geometry of the adder, the locations of input and output pins, and how pin locations allow some signals in this case the carry signals to be transferred from one sub-block to the other without external routing. However, it is important for the simplicity of design that the hierarchies in different domains can be mapped into each other easily.

It can be observed that in terms of transistor count, logic chips contain significantly fewer transistors in any given year mainly due to large consumption of chip area for complex interconnects. The LUT is a digital memory that stores the truth table of the Boolean function.

Usually, other design concepts and design approaches are also needed to simplify the process.