Programmable logic device

A simplified PAL device. The programmable elements (shown as a fuse) connect both the true and complemented inputs to the AND gates. These AND gates, also known as product terms, are ORed together to form a sum-of-products logic array.

A programmable logic device (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, that is, reconfigured.

Using a ROM as a PLD

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 2^m 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 2^2m possible Boolean functions of these m input signals. By Boolean function in this context is meant a single function that maps each of the 2^m possible combinations of the m Boolean inputs to a single Boolean output. There are 2^2m possible distinct ways to map each of 2^m inputs to a Boolean value, which explains why there are 2^2m such Boolean functions of m inputs.

Now, consider that each of the n output pins acts, independently, as a logic device that is specially selected to sample just one of the possible 2^2m such functions. At any given time, only one of the 2^m possible input values can be present on the ROM, but over time, as the input values span their full possible domain, each output pin will map out its particular function of the 2^m possible input values, from among the 2^2m possible such functions. Note that the structure of the ROM allows just n of the 2^2m possible such Boolean 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 all possible combinations of the m inputs can be made to appear at any of the n outputs, making this the most general-purpose combinational logic device available for m input pins and n output pins.

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 cannot necessarily provide safe "covers" for asynchronous logic transitions so the PROM's outputs may glitch as the inputs switch,
• they consume more power,
• they are often more expensive than programmable logic, especially if high speed is required.

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'.

Early programmable logic

In 1969, Motorola offered the XC157, a mask-programmed gate array with 12 gates and 30 uncommitted input/output pins.

In 1970, Texas Instruments developed a mask-programmable IC based on the IBM read-only associative memory or ROAM. This device, the TMS2000, was programmed by altering the metal layer during the production of the IC. The TMS2000 had up to 17 inputs and 18 outputs with 8 JK flip flop for memory. TI coined the term programmable logic array for this device.

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 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 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.

PLA

Main article: Programmable logic array

In 1970, Texas Instruments developed a mask-programmable IC based on the IBM read-only associative memory or ROAM. This device, the TMS2000, was programmed by altering the metal layer during the production of the IC. The TMS2000 had up to 17 inputs and 18 outputs with 8 JK flip flop for memory. TI coined the term programmable logic array for this device.

A programmable logic array (PLA) has a programmable AND gate array, which links to a programmable OR gate array, which can then be conditionally complemented to produce an output.

PAL

Main article: Programmable array logic

PAL devices have arrays of transistor cells arranged in a "fixed-OR, programmable-AND" plane used to implement "sum-of-products" binary logic equations for each of the outputs in terms of the inputs and either synchronous or asynchronous feedback from the outputs.

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.

GALs

Main article: Generic array logic

Lattice GAL 16V8 and 20V8

An improvement on 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 (E²) 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.

CPLDs

Main article: Complex programmable logic device

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. Some manufacturers (including Altera and Microsemi) use JTAG to program CPLD's in-circuit from .JAM files.

FPGAs

Main article: Field-programmable gate array

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). Early examples of FPGAs are the 82s100 array, and 82S105 sequencer, by Signetics, introduced in the late 1970s. The 82S100 was an array of AND terms. The 82S105 also had flip flop functions.

FPGAs use a grid of logic gates, and once stored, the data doesn't change, similar to that of an ordinary gate array. The term "field-programmable" means the device is programmed by the customer, not the manufacturer.

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).

The difference between FPGAs and CPLDs is that FPGAs are internally based on Look-up tables (LUTs) whereas CPLDs form the logic functions with sea-of-gates (e.g. sum of products). CPLDs are meant for simpler designs while FPGAs are meant for more complex designs. In general, CPLDs are a good choice for wide combinational logic applications, whereas FPGAs are more suitable for large state machines (i.e. microprocessors).

Other variants

At present, much interest exists in reconfigurable systems. 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 requires engineers to learn new methods, and that new software tools 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 let designers concentrate on adding new features to designs without having to worry about making the microprocessor work.

How PLDs retain 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
• SRAM
• EPROM or EEPROM cells
• Flash memory

Silicon antifuses 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.

PLD programming languages

Many PAL programming devices accept input in a standard file format, commonly referred to as 'JEDEC files'.They are analogous to software compilers. The languages used as source code for logic compilers are called hardware description languages, or HDLs.

PALASM, ABEL and CUPL 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.

For modern PLD programming languages, design flows, and tools, see FPGA and Reconfigurable computing.

PLD programming devices

A device programmer is used to transfer the boolean logic pattern into the programmable device. In the early days of programmable logic, every PLD manufacturer also produced a specialized device programmer for its family of logic devices. Later, universal device programmers came onto the market that supported several logic device families from different manufacturers. Today's device programmers usually can program common PLDs (mostly PAL/GAL equivalents) from all existing manufacturers. Common file formats used to store the boolean logic pattern (fuses) are JEDEC, Altera POF (programmable object file), or Xilinx BITstream.

See also

• Macrocell array
• Programmable logic array (PLA)
• Programmable array logic (PAL)
• Field-programmable gate array (FPGA)
• Complex programmable logic device (CPLD)
• Erasable programmable logic device (EPLD)
• Application-specific integrated circuit (ASIC)
• Programmable logic controller (PLC)

External links

• "PLD Tools Creating SVF, JAM, STAPL and other formats". JTAG / boundary-scan. Corelis. Dec 1, 2010. 
• "FPGAs and CPLDs". Lattice Semiconductor.