Programmable Array Logic

The term Programmable Array Logic (PAL) is used to describe a family of programmable logic device semiconductors used to implement logic functions in digital circuits introduced by Monolithic Memories, Inc. (MMI) in March 1978.

PAL devices consisted of a small PROM (programmable read-only memory) core and additional output logic used to implement particular desired logic functions with few components.

Using specialized machines, PAL devices were "field-programmable". Each PAL device was "one-time programmable" (OTP), meaning that it could not be updated and reused after its initial programming. (MMI also offered a similar family called HAL, or "hard array logic", which were like PAL devices except that they were mask-programmed at the factory.)

Early history

Before PALs were introduced, designers of digital logic circuits would use small-scale integration (SSI) components, such as those in the 7400 series TTL (transistor-transistor logic) family; the 7400 family included a variety of logic building blocks, such as gates (NOT, NAND, NOR, AND, OR), multiplexers (MUXes) and demultiplexers (DEMUXes), flip flops (D-type, JK, etc.) and others. One PAL device would typically replace dozens of such "discrete" logic packages, so the SSI business went into decline as the PAL business took off. PALs were used advantageously in many products, such as minicomputers, as documented in Tracy Kidder's best-selling book "The Soul of a New Machine."

PALs were not the first commercial programmable logic devices; Signetics had been selling its field programmable logic array (FPLA) since 1975. These devices were completely unfamiliar to most circuit designers and were perceived to be too difficult to use. The FPLA had a relatively slow maximum operating speed (due to having both programmable-AND and programmable-OR arrays), was expensive, and had a poor reputation for testability. Another factor limiting the acceptance of the FPLA was the large package, a 600-mil (0.6", or 15.24 mm) wide 28-pin dual in-line package (DIP).

The project to create the PAL device was managed by John Birkner and the actual PAL circuit was designed by H. T. Chua. In a previous job, Mr. Birkner had developed a 16-bit processor using 80 standard logic devices. His experience with standard logic led him to believe that user programmable devices would be more attractive to users if the devices were designed to replace standard logic. This meant that the package sizes had to be more typical of the existing devices, and the speeds had to be improved. The PAL met these requirements and was a huge success and were "second sourced" by National Semiconductor, Texas Instruments, and Advanced Micro Devices.

Process technologies

Early PALs were 20-pin DIP components fabricated in silicon using bipolar transistor technology with one-time programmable (OTP) titanium-tungsten programming fuses. Later devices were manufactured by Lattice Semiconductor and Advanced Micro Devices using CMOS technology.

The original 20 and 24-pin PALs were described by MMI as medium-scale integration (MSI) devices.

PAL architecture

The PAL architecture consists of two main components: a logic plane and output logic macrocells.

Programmable logic plane

The programmable logic plane is a programmable read-only memory (PROM) array that allows the signals present on the devices pins (or the logical complements of those signals) to be routed to an output logic macrocell.

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.

Output logic

The early 20-pin PALs had 10 inputs and 8 outputs. The outputs were active low and could be registered or combinational. Members of the PAL family were available with various output structures called "output logic macrocells" or OLMCs. Prior to the introduction of the "V" (for "variable") series, the types of OLMCs available in each PAL were fixed at the time of manufacture. (The PAL16L8 had 8 combinational outputs and the PAL16R8 had 8 registered outputs. The PAL16R6 had 6 registered and 2 combinational while the PAL16R4 had 4 of each.) Each output could have up to 8 product terms (effectively AND gates), however the combinational outputs used one of the terms to control a bidirectional output buffer. There were other combinations that had fewer outputs with more product term per output and were available with active high outputs. The 16X8 family or registered devices had an XOR gate before the register. There were also similar 24-pin versions of these PALs.

This fixed output structure often frustrated designers attempting to optimize the utility of PAL devices because output structures of different types were often required by their applications. (For example, one could not get 5 registered outputs with 3 active high combinational outputs.) So, in June 1983 AMD introduced the 22V10, a 24 pin device with 10 output logic macrocells. Each macrocell could be configured by the user to be combinational or registered, active high or active low. The number of product term allocated to an output varied from 8 to 16. This one device could replace all of the 24 pin fixed function PAL devices. Members of the PAL "V" ("variable") series included the PAL16V8, PAL20V8 and PAL22V10.

Programming PALs

PALs were programmed electrically using binary patterns (as JEDEC ASCII/hexadecimal files) and a special electronic programming system available from either the manufacturer or a third-party, such as DATA/IO. In addition to single-unit device programmers, device feeders and gang programmers were often used when more than just a few PALs needed to be programmed. (For large volumes, electrical programming costs could be eliminated by having the manufacturer fabricate a custom metal mask used to program the customers' patterns at the time of manufacture; MMI used the term "hard array logic" (HAL) to refer to devices programmed in this way.)

Programming languages

Though some engineers programmed PAL devices by manually editing files containing the binary fuse pattern data, most opted to design their logic using a hardware description language (HDL) such as Data I/O's ABEL, Logical Devices' CUPL, or MMI's PALASM. These were computer-assisted design (CAD) (now referred to as "design automation") programs which translated (or "compiled") the designers' logic equations into binary fuse map files used to program (and often test) each device.

PALASM

The PALASM (from "PAL assembler") language was used to express boolean equations for the outputs pins in a text file which was then converted to the 'fuse map' file for the programming system using a vendor-supplied program; later the option of translation from schematics became common, and later still, 'fuse maps' could be 'synthesized' from an HDL (hardware description language,) such as Verilog.

The PALASM compiler was written by MMI in FORTRAN IV on an IBM 370/168. MMI made the source code available to users at no cost. By 1983, MMI customers ran versions on the DEC PDP-11, Data General NOVA, Hewlett-Packard HP2100, MDS800 and others.

ABEL

Data I/O Corporation released ABEL.

CUPL

Logical Devices, Inc. released the Universal Compiler for Programmable Logic (CUPL), which ran under MSDOS on the IBM PC.

Device programmers

Popular device programmers included Data I/O Corporation's Model 60A Logic Programmer and Model 2900.

Successors

After MMI succeeded with the 20-pin PAL parts introduced circa 1978, AMD introduced the 24-pin 22V10 PAL with additional features. After buying out MMI (circa 1987), AMD spun off a consolidated operation as Vantis, and that business was acquired by Lattice Semiconductor in 1999.

Altera introduced the EP300 (first CMOS PAL) in 1983 and later moved into the FPGA business.

Lattice Semiconductor introduced the generic array logic (GAL) family in 1985, with functional equivalents of the "V" series PALs that used reprogrammable logic planes based on EEPROM (electrically eraseable programmable read-only memory) technology. National Semiconductor was a "second source" of GAL parts. AMD introduced a similar family called PALCE.

ICT (International CMOS Technology) introduced the PEEL 18CV8 in 1986. The 20-pin CMOS EEPROM part could be used in place of any of the registered-output bipolar PALs and used much less power.

Larger-scale programmable logic devices were introduced by Atmel, Lattice Semiconductor, and others. These devices extended the PAL architecture by including multiple logic planes and/or burying logic macrocells within the logic plane(s). The term "complex programmable logic device" (CPLD) was introduced to differentiate these devices from their PAL and GAL predecessors, which were then sometimes referred to as "simple programmable logic devices" or SPLDs.

Another large programmable logic device is the "field-programmable gate array" or FPGA. This term is often used to describe devices currently made by Altera and Xilinx.