It is impossible to discuss PLD technology without mentioning some of the companies involved in its development. However, it is not the purpose of this article to list all manufacturers of PLDs. Inclusion or omission of a particular company from this article is intended as neither a recommendation nor a criticism.
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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. A PAL may be programmed only once.
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.
A similar device called a PEEL (programmable electrically erasable logic) was introduced by the Integrated Circuit Technology (ICT) corporation.
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 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. In most larger FPGAs the configuration is volatile, and must be re-loaded into the device whenever power is applied or different functionality is required.
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.
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.
Silicon antifuses are the storage elements used in the PAL, the first type of PLD. These are connections that are made by applying a voltage across a modified area of silicon inside the chip. They are called antifuses because they work in the opposite way to normal fuses, which begin life as connections until they are broken by an electric current.
SRAM, or static RAM, is a volatile type of memory, meaning that its contents are lost each time the power is switched off. SRAM-based PLDs therefore have to be programmed every time the circuit is switched on. This is usually done automatically by another part of the circuit.
Flash memory is non-volatile, retaining its contents even when the power is switched off. It can be erased and reprogrammed as required. This makes it useful for PLD memory.
An EPROM cell is a MOS (metal-oxide-semiconductor) transistor that can be switched on by trapping an electric charge permanently on its gate electrode. This is done by a PAL programmer. The charge remains for many years and can only be removed by exposing the chip to strong ultraviolet light in a device called an EPROM eraser.
wikipedia.org dumped 2003-03-17 with terodump