VLSI Design In Verilog

Verilog

Very-large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device. Before the introduction of VLSI technology most ICs had a limited set of functions they could perform. An electronic circuit might consist of a CPU, ROM, RAM and other glue logic. VLSI lets IC designers add all of these into one chip.
Gone are the days when huge computers made of vacuum tubes sat humming in entire dedicated rooms and could do about 360 multiplications of 10 digit numbers in a second. Though they were heralded as the fastest computing machines of that time, they surely don’t stand a chance when compared to the modern day machines. Modern day computers are getting smaller, faster, and cheaper and more power efficient every progressing second. But what drove this change? The whole domain of computing ushered into a new dawn of electronic miniaturization with the advent of semiconductor transistor by Bardeen (1947-48) and then the Bipolar Transistor by Shockley (1949) in the Bell Laboratory.
History & EvolutionThe development of microelectronics spans a time which is even lesser than the average life expectancy of a human, and yet it has seen as many as four generations. Early 60’s saw the low density fabrication processes classified under Small Scale Integration (SSI) in which transistor count was limited to about 10. This rapidly gave way to Medium Scale Integration in the late 60’s when around 100 transistors could be placed on a single chip.
It was the time when the cost of research began to decline and private firms started entering the competition in contrast to the earlier years where the main burden was borne by the military. Transistor-Transistor logic (TTL) offering higher integration densities outlasted other IC families like ECL and became the basis of the first integrated circuit revolution. It was the production of this family that gave impetus to semiconductor giants like Texas Instruments, Fairchild and National Semiconductors. Early seventies marked the growth of transistor count to about 1000 per chip called the Large Scale Integration. By mid eighties, the transistor count on a single chip had already exceeded 1000 and hence came the age of Very Large Scale Integration orVLSI. Though many improvements have been made and the transistor count is still rising, further names of generations like ULSI are generally avoided. It was during this time when TTL lost the battle to MOS family owing to the same problems that had pushed vacuum tubes into negligence, power dissipation and the limit it imposed on the number of gates that could be placed on a single die.
The second age of Integrated Circuits revolution started with the introduction of the first microprocessor, the 4004 by Intel in 1972 and the 8080 in 1974. Today many companies like Texas Instruments, Infineon, Alliance Semiconductors, Cadence, Synopsys, Celox Networks, Cisco, Micron Tech, National Semiconductors, ST Microelectronics, Qualcomm, Lucent, Mentor Graphics, Analog Devices, Intel, Philips, Motorola and many other firms have been established and are dedicated to the various fields in "VLSI" like Programmable Logic Devices, Hardware Descriptive Languages, Design tools, Embedded Systems etc.

The VLSI System Design Process

Hierarchy

The first significant characteristic of VLSI and all other design is a heavy reliance on hierarchical description. The major reason for using hierarchical description is to hide the vast amount of detail in a design. By reducing the distracting detail to a single object that is lower in the hierarchy, one can greatly simplify many CAD operations. For example, simulation, verification, design-rule checking, and layout constraints can all benefit from hierarchical representation, which makes them much more computationally tractable.

The opposite of top-down hierarchical organization is, of course, bottom-up. This style of design can be used when the details of a circuit are already known and must be properly implemented. For example, suppose that a 4K memory chip is to be designed, and further suppose that the design for a single bit of that memory is already done. Since the size of this bit of memory is the most important factor in the chip, all other circuitry must be designed to accommodate this cell. Therefore the design composition must proceed in a bottom-up manner, starting with the single bit of memory (see Fig. 1.14). In this example there are six levels of hierarchy starting at the single bit, aggregating a row of eight bits; stacking four of those vertically; stacking eight at the next higher level; and so on. The highest level of the hierarchy shows four arrays, each containing 32 × 32 bits. Memory-driving circuitry with the correct spacing is then placed around the bits.

Branching Factor

branching factors that are larger than nine probably will yield designs with too much detail at any level of hierarchy. Either situation will cause designers to be unable to perceive functionality effectively.

Layout Overlap

Verilog code Basics:

Hardware Description Languages, or HDLs, are languages used to design hardware with. As the name implies, an HDL can also be used to describe the functionality of hardware as well as its implementation.

The first HDL was ISP, invented by C. Gordon Bell and Alan Newell at Carnegie Mellon University and described in their book Computer Structures in 1972. This language was also the first to use the term register transfer level. This came from the use of ISP in describing the behavior of the PDP-8 computer as a set of registers and logical functions describing the transfer of data from source register to destination register.

Module:

In this example, there are four modules: system, comp_1, comp_2, and sub_3. System is the parent of comp_1 and comp_2, and comp_2 is the parent of sub_3. comp_1 and comp_2 are the children of system, and sub_3 is the child of comp_2.

Modules in a hierarchy have both a type and a name. Module types are defined in Verilog. There can be many module instances of the same type of module in a single hierarchy. To be unique, each different instance at the same level must have a unique name.

A module is defined like this: module <module_name> (<portlist>); . . // module components . endmodule module <module_name_1> (<portlist>); . . <module_name_2> <instance_name> (<portlist>); . . endmodule Bottom-Up Design The traditional method of electronic design is bottom-up. Each design is performed at the gate-level using the standard gates (refer to the Digital Section for more details). With the increasing complexity of new designs this approach is nearly impossible to maintain. New systems consist of ASIC or microprocessors with a complexity of thousands of transistors. These traditional bottom-up designs have to give way to new structural, hierarchical design methods. Without these new practices it would be impossible to handle the new complexity. Top-Down Design The desired design-style of all designers is the top-down one. A real top-down design allows early testing, easy change of different technologies, a structured system design and offers many other advantages. But it is very difficult to follow a pure top-down design. Due to this fact most designs are a mix of both methods, implementing some key elements of both design styles.