Very-large-scale integration (VLSI)



Very-large-scale integration (VLSI) is the process of creating integrated circuits by combining thousands of transistor-based circuits into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device. The term is no longer as common as it once was, as chips have increased in complexity into the hundreds of millions of transistors.
The first semiconductor chips held one transistor each. Subsequent advances added more and more transistors, and, as a consequence, more individual functions or systems were integrated over time. The first integrated circuits held only a few devices, perhaps as many as ten diodes, transistors, resistors and capacitors, making it possible to fabricate one or more logic gates on a single device. Now known retrospectively as "small-scale integration" (SSI), improvements in technique led to devices with hundreds of logic gates, known as large-scale integration (LSI), i.e. systems with at least a thousand logic gates. Current technology has moved far past this mark and today's microprocessors have many millions of gates and hundreds of millions of individual transistors.




Programmable logic device
• A programmable logic device or PLD is an electronic component used to build reconfigurable digital circuits.
• Unlike a logic gate, which has a fixed function, a PLD has an undefined function at the time of manufacture. Before the PLD can be used in a circuit it must be programmed (i. e. reconfigured).

Programmable Array Logic
The term Programmable Array Logic (PAL) is used to describe a family of programmable logic device semiconductors used to implement logic functions in digital circuits introduced by Monolithic Memories, Inc. (MMI) in March 1978.
PAL devices consisted of a small PROM (programmable read-only memory) core and additional output logic used to implement particular desired logic functions with few components.
Using specialized machines, PAL devices were "field-programmable". Each PAL device was "one-time programmable" (OTP), meaning that it could not be updated and reused after its initial programming. (MMI also offered a similar family called HAL, or "hard array logic", which were like PAL devices except that they were mask-programmed at the factory.)


Programmable logic array
A programmable logic array (PLA) is a programmable device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which page link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output. This layout allows for a large number of logic functions to be synthesized in the sum of products (and sometimes product of sums) canonical forms.
One application of a PLA is to implement the control over a data path. It defines various states in an instruction set, and produces the next state (by conditional branching). [eg. if the machine is in state 2, and will go to state 4 if the instruction contains an immediate field; then the PLA should define the actions of the control in state 2, will set the next state to be 4 if the instruction contains an immediate field, and will define the actions of the control in state 4]. Programmable Logic Arrays should correspond to a state diagram for the system.
Other commonly used programmable logic devices are PAL, CPLD and FPGA.

PALs and PLAs
• The programmable array logic (PAL) is a logic device with fixed OR array
• and a programmable AND array. It is user ‘friendly à PAL’: easier to
• program but not as flexible as PLA.
• The programmable logic array (PLA) is a logic device with programmable AND and OR array.

What is difference between Programmable Array Logic (PAL) and Programmable Logic Array (PLA)?
For PLAs
• Of the two organizations the PLA is the most flexible
• Both AND and OR arrays are programmable
• One PLA can implement a huge range of logic functions
• But many pins; large package, higher cost
For PALs :
• AND array is programmable, OR array is fixed at fabrication
• Each device is cheaper than a PLA
• less flexible


Complex programmable logic device

A complex programmable logic device (CPLD) is a programmable logic device with complexity between that of PALs and FPGAs, and architectural features of both. The building block of a CPLD is the macro cell, which contains logic implementing disjunctive normal form expressions and more specialized logic operations.

Features in common with PALs:

• Non-volatile configuration memory. Unlike many FPGAs, an external configuration ROM isn't required, and the CPLD can function immediately on system start-up.
• For many legacy CPLD devices, routing constrains most logic blocks to have input and output signals connected to external pins, reducing opportunities for internal state storage and deeply layered logic. This is usually not a factor for larger CPLDs and newer CPLD product families.

Features in common with FPGAs:


• Large number of gates available. CPLDs typically have the equivalent of thousands to tens of thousands of logic gates, allowing implementation of moderately complicated data processing devices. PALs typically have a few hundred gate equivalents at most, while FPGAs typically range from tens of thousands to several million.
• Some provisions for logic more flexible than sum-of-product expressions, including complicated feedback paths between macro cells, and specialized logic for implementing various commonly-used functions, such as integer arithmetic.
The most noticeable difference between a large CPLD and a small FPGA is the presence of on-chip non-volatile memory in the CPLD. This distinction is rapidly becoming less relevant, as several of the latest FPGA products also offer models with embedded configuration memory.
The characteristic of non-volatility makes the CPLD the device of choice in modern digital designs to perform boot loader.

 
Field-programmable gate array
A field-programmable gate array (FPGA) is a semiconductor device that can be configured by the customer or designer after manufacturing—hence the name "field-programmable". To program an FPGA you specify how you want the chip to work with a logic circuit diagram or a source code in a hardware description language (HDL). FPGAs can be used to implement any logical function that an application-specific integrated circuit (ASIC) could perform, but the ability to update the functionality after shipping offers advantages for many applications.
FPGAs contain programmable logic components called "logic blocks", and a hierarchy of reconfigurable interconnects that allow the blocks to be "wired together"—somewhat like a one-chip programmable breadboard. Logic blocks can be configured to perform complex combinational functions, or merely simple logic gates like AND and XOR. In most FPGAs, the logic blocks also include memory elements, which may be simple flip-flops or more complete blocks of memory.
For any given semiconductor process, FPGAs are usually slower than their fixed ASIC counterparts. They also draw more power, and generally achieve less functionality using a given amount of circuit complexity. But their advantages include a shorter time to market, ability to re-program in the field to fix bugs, and lower non-recurring engineering costs. Vendors can also take a middle road by developing their hardware on ordinary FPGAs, but manufacture their final version so it can no longer be modified after the design has been committed.