Science Fair Project Encyclopedia
Programmable logic device
A programmable logic device or PLD is an electronic component used to build 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.
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.
Using a ROM as a PLD
Before PLDs were invented, read-only memory (ROM) chips were used to create arbitrary combinatorial logic functions of a number of inputs. Consider a ROM with m inputs (the address lines) and n outputs (the data lines). When used as a memory, the ROM contains 2m words of n bits each. Now imagine that the inputs are driven not by an m-bit address, but by m independent logic signals. Theoretically, there are 2m possible Boolean functions of these m signals, but the structure of the ROM allows just n of these functions to be produced at the output pins. The ROM therefore becomes equivalent to n separate logic circuits, each of which generates a chosen function of the m inputs.
The advantage of using a ROM in this way is that any conceivable function of the m inputs can be made to appear at any of the n outputs, making this the most general-purpose combinatorial logic device available. Also, PROMs (programmable ROMs), EPROMs (ultraviolet-erasable PROMs) and EEPROMs (electrically erasable PROMs) are available that can be programmed using a standard PROM programmer without requiring specialised hardware or software. However, there are several disadvantages: they are usually much slower than dedicated logic circuits, they consume more power, and because only a small fraction of their capacity is used in any one application, they make an inefficient use of space. Also, they cannot easily be used for sequential logic, because they contain no flip-flops.
Common EPROMs, for example the 2716, are still sometimes used in this way by hobby circuit designers, who often have some laying around. This use is sometimes called a 'poor man's PAL'.
Early programmable logic
Main article: Programmable array logic.
The first programmable logic devices for the commercial market were introduced by Monolithic Memories, Inc.(MMI) under the name of PAL (Programmable array logic), although IBM produced similar devices for internal use in the mid-1970s.
After MMI succeeded with the 20-pin PAL parts, AMD introduced the 24-pin 22V10 PAL with additional features. After buying out MMI (1987?), AMD spun off a consolidated operation as Vantis, and that business was acquired by Lattice Semiconductor in 1999.
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 International CMOS Technology (ICT) corporation.
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).
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). An early example of an FPGA is the 82s100 by Signetics introduced in the late 1970s.
FPGAs use a grid of logic gates, similar to that of an ordinary gate array, but the programming is done by the customer, not by the manufacturer. The term "field-programmable" may be obscure to some, but "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.
Other types of PLDs
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.
How PLDs remember their configuration
A PLD is a combination of a logic device and a memory device. The memory is used to store the pattern that was given to the chip during programming. Most of the methods for storing data in an integrated circuit have been adapted for use in PLDs. These include:
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.
PLD programming languages
As mentioned in the "PAL" section above, JEDEC files are usually too complex to create by hand, so a computer program is used to generate them. This program is called a logic compiler, and is analogous to a software compiler. The languages used as source code for logic compilers are called hardware description languages, or HDLs. ABEL is frequently used for low-complexity devices, while Verilog and VHDL are popular higher-level description languages for more complex devices.
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