Before PLDs were invented, read-only memory (ROM) chips were used to create arbitrary combinational 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 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 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:
Since most ROMs do not have input or output registers, they cannot be used stand-alone for sequential logic. An external TTL register was often used for sequential designs such as state machines. Common EPROMs, for example the 2716, are still sometimes used in this way by hobby circuit designers, who often have some lying around. This use is sometimes called a 'poor man's PAL'.
In 1973 National Semiconductor introduced a mask-programmable PLA device (DM7575) with 14 inputs and 8 outputs with no memory registers. This was more popular than the TI part but cost of making the metal mask limited its use. The device is significant because it was the basis for the field programmable logic array produced by Signetics in 1975, the 82S100. (Intersil actually beat Signetics to market but poor yield doomed their part.)
In 1971, General Electric Company (GE) was developing a programmable logic device based on the new PROM technology. This experimental device improved on IBM's ROAM by allowing multilevel logic. Intel had just introduced the floating-gate UV erasable PROM so the researcher at GE incorporated that technology. The GE device was the first erasable PLD ever developed, predating the Altera EPLD by over a decade. GE obtained several early patents on programmable logic devices.
In 1974 GE entered into an agreement with Monolithic Memories to develop a mask- programmable logic device incorporating the GE innovations. The device was named the 'Programmable Associative Logic Array' or PALA. The MMI 5760 was completed in 1976 and could implement multilevel or sequential circuits of over 100 gates. The device was supported by a GE design environment where Boolean equations would be converted to mask patterns for configuring the device. The part was never brought to market.
MMI introduced a breakthrough device in 1978, the Programmable Array Logic or PAL. The architecture was simpler than that of Signetics FPLA because it omitted the programmable OR array. This made the parts faster, smaller and cheaper. They were available in 20 pin 300 mil DIP packages while the FPLAs came in 28 pin 600 mil packages. The PAL Handbook demystified the design process. The PALASM design software (PAL Assembler) converted the engineers' Boolean equations into the fuse pattern required to program the part. The PAL devices were soon second-sourced by National Semiconductor, Texas Instruments and AMD.
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.
There are also PLA's : Programmable Logic Array.
An innovation of the PAL was the generic array logic device, or GAL, invented by Lattice Semiconductor in 1985. 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, or by using the in-circuit programming technique on supporting chips.
Lattice GALs combine CMOS and electrically erasable (E^2) floating gate technology for a high-speed, low-power logic device.
A similar device called a PEEL (programmable electrically erasable logic) was introduced by the International CMOS 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 Semiconductor 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 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" means the array is done outside the factory, or "in the field."
FPGAs are usually programmed after being soldered down to the circuit board, in a manner similar to that of 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. Configuration is typically stored in a configuration PROM or EEPROM. EEPROM versions may be in-system programmable (typically via JTAG).
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 or experience.
PLDs are being sold now that contain a microprocessor with a fixed function (the so-called core) surrounded by programmable logic. These devices let designers concentrate on adding new features to designs 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.
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.
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.
As of 2005, most CPLDs are electrically programmable and erasable, and non-volatile. This is because they are too small to justify the inconvenience of programming internal SRAM cells every time they start up, and EPROM cells are more expensive due to their ceramic package with a quartz window.
PALASM and ABEL are frequently used for low-complexity devices, while Verilog and VHDL are popular higher-level description languages for more complex devices. The more limited ABEL is often used for historical reasons, but for new designs VHDL is more popular, even for low-complexity designs.