Programmable logic device
A programmable logic device or PLD is a component used to build digital logic circuits. Unlike a logic gate, which has a fixed function, a PLD's function is not determined at the time of manufacture. Before the PLD can perform in a circuit it must be programmed.
Early PALs
The first programmable logic devices were produced the Advanced Micro Devices (AMD) corporation. The devices were called PALs, for programmable array logic. The programmable array contains logic gates, themselves fixed in function, with programmable interconnections between them. The array has a number of inputs and outputs, and can create any Boolean function of a selection of the inputs at any of its outputs. A single PAL could replace a circuit containing a large number of fixed logic gates.
The PLD business split from AMD under the name Vantis, and was acquired by Lattice Semiconductor in 1999.
In a PAL the logic gates are arranged as a sum-of-products array. In Boolean terms, this means a number of AND gates whose outputs feed into a large OR gate that drives one output. By selecting which inputs drive each AND gate, and which AND gates drive the OR gate, any Boolean function can be created. It can be shown that any Boolean function can be reduced to a sum of products, and can therefore be created by a sufficiently large PAL.
A PAL is programmed by fitting it into a machine called a PAL programmer. PAL programmers are usually general-purpose machines that can program all types of PLD from all manufacturers. Once programmed, a PAL's function is fixed.
The PAL programmer must be supplied with a description of the PAL's desired configuration. This is usually in the form of a computer text file with a standard format defined by the Joint Electron Device Engineering Council (JEDEC). JEDEC files can be hand-typed by the design engineer or, more commonly, produced by a computer program similar to the language compilers used by software engineers.
PALs are small-scale logic devices with no more than a few hundred logic gates.
GALs
An innovation of the PAL was the generic array logic device, or GAL, invented by Lattice Semiconductor Inc. This device has the same logical properties as the PAL but can be erased and reprogrammed. The GAL is very useful in the prototyping stage of a design, when any bugs in the logic can be corrected by reprogramming. GALs are programmed and reprogrammed using a PAL programmer.
A similar device called a PEEL (programmable electrically erasable logic) was introduced by the Integrated Circuit Technology (ICT) corporation.
CPLDs
PALs and GALs are available only in small sizes, equivalent to a few hundred logic gates. For bigger logic circuits, complex PLDs or CPLDs can be used. These contain the equivalent of several PALs linked by programmable interconnections, all in one integrated circuit. CPLDs can replace thousands, or even hundreds of thousands, of logic gates.
Some CPLDs are programmed using a PAL programmer, but this method becomes inconvenient for devices with hundreds of pins. A second method of programming is to solder the device to its printed circuit board, then feed it with a serial data stream from a personal computer. The CPLD contains a circuit that decodes the data stream and configures the CPLD to perform its specified logic function.
Each manufacturer has a proprietary name for this programming system. For example, Lattice calls it "in-system programming". However, these proprietary systems are beginning to give way to a standard from the Joint Test Action Group (JTAG).
FPGAs
While PALs were busy developing into GALs and CPLDs (all discussed above), a separate stream of development was happening. This type of device is based on gate-array technology and is called the field-programmable gate array (FPGA). Gate arrays are non-programmable devices that can be manufactured more cheaply than other types of IC, because they contain a standard grid of logic gates whose interconnections are specified by the customer. When a customer orders a new type of chip, the manufacturer does not have to design it from scratch, but can just take a standard gate array and modify it to the customer's requirement.
FPGAs use a similar grid of logic gates, but the programming is done by the customer, not by the manufacturer. The term "field-programmable" may be obscure to some, but the "field" is just an engineering term for the world outside the factory where customers live.
FPGAs are usually programmed after being soldered down to the circuit board, in the same way as larger CPLDs.
FPGAs and CPLDs are often equally good choices for a particular task. Sometimes the decision is more an economic one than a technical one, or may depend on the engineer's personal preference and history. Each manufacturer champions his own system, and competition between them keeps the different types of technology roughly in step with each other.
Other Types of PLD
There is much interest in reconfigurable systems at present. These are microprocessor circuits that contain some fixed functions and other functions that can be altered by code running on the processor. Designing self-altering systems will require engineers to learn new methods, and will probably require new software tools to be developed.
PLDs are being sold now that contain a microprocessor with a fixed function (the so-called core) surrounded by programmable logic. These devices allow the designer to concentrate on adding new features to his design without having to worry about making the microprocessor work.