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Programmable logic device English name: programmable logic device or PLD. PLD is produced as a general-purpose integrated circuit, and its logic function is determined by the user programming the device. The integration of general PLD is very high, enough to meet the needs of designing general digital systems.



In this way, designers can program themselves to "integrate" a digital system on a PLD without having to ask chip manufacturers to design and make special integrated circuit chips.

The difference between PLD and general digital chips is that the internal digital circuits of PLD can be planned and decided after leaving the factory. Some types of PLD also allow changes and changes after the planning decision, and general digital chips have already decided on their internals before leaving the factory. The circuit cannot be changed again after leaving the factory. In fact, the general analog chips and mixing chips are also the same. It is impossible to adjust the internal circuit after leaving the factory.

Development History

1970s: PROM (Programmable Read only Memory), PLA (Programmable Logic Array)

Late 1970s: AMD introduced Programmable Array Logic (PAL)

1980s: Lattice Company launched GAL (Generic Array Logic)

Mid 1980s: Xilinx Company introduced Field Programmable Gate Array FPGA (Field Programmable GateArray). Altera has launched an erasable programmable logic device EPLD (Erase Programmable LogicDevice), which has high integration, flexible design, and can be programmed repeatedly

In the early 1990s: Lattice also introduced the in-system programmable concept ISP and its in-system programmable large-scale integrated device ispLSI)

Xilinx, Altera, and Lattice are now the main manufacturers. The FPGA chip produced can reach tens of millions of gates, the speed can reach 550MHz, and 65nm or higher lithography technology is used.


Logic devices can be divided into two categories-fixed logic devices and programmable logic devices. As the name implies, the circuits in fixed logic devices are permanent, they perform a function or set of functions-once manufactured, they cannot be changed. On the other hand, a programmable logic device (PLD) is a standard finished part that can provide customers with a wide range of logic capabilities, characteristics, speed and voltage characteristics-and such devices can be changed at any time to complete many different Function.

For fixed logic devices, depending on the complexity of the device, the time required from design, prototype to final production can vary from months to more than a year. Moreover, if the device does not work properly, or if the application requirements change, then a completely new design must be developed. The preliminary work of designing and verifying fixed logic requires a lot of "non-recurring engineering costs", or NRE. NRE represents all the costs that customers need to invest before the fixed logic device is finally manufactured from the chip manufacturer. These costs include engineering resources, expensive software design tools, expensive lithography mask sets used to manufacture different metal layers of the chip, and initial The production cost of the prototype device. These NRE costs may range from hundreds of thousands of dollars to millions of dollars.

For programmable logic devices, designers can use low-cost software tools to quickly develop, simulate, and test their designs. Then, you can quickly program the design into the device and immediately test the design in the actual running circuit. The PLD device used in the prototype is exactly the same as the PLD used in the formal production of the final equipment (such as a network router, ADSL modem, DVD player, or car navigation system). This eliminates the cost of NRE and the final design is completed faster than when using custom fixed logic devices.

Another key advantage of using PLD is that customers can modify the circuit as needed during the design phase until they are satisfied with the design work. This is because PLD is based on rewritable memory technology-to change the design, simply reprogram the device. Once the design is completed, the customer can immediately put into production, just use the final software design file to simply program the required number of PLDs.

The two main types of programmable logic devices are field programmable gate arrays (FPGA) and complex programmable logic devices (CPLD). Among these two types of programmable logic devices, FPGA provides the highest logic density, the most abundant features and the highest performance. The latest FPGA devices, such as some devices in the Xilinx Virtex series, can provide eight million "system gates" (relative logic density). These advanced devices also provide features such as built-in hard-wired processors (such as IBM Power PC), mass storage, and clock management systems, and support a variety of the latest ultra-fast device-to-device Signal technology. FPGAs are used in a wide range of applications, from data processing and storage, to instrumentation, telecommunications, and digital signal processing.

Compared to this, PLD provides much less logic resources-up to about 10,000 gates. However, PLD provides very good predictability, so it is ideal for critical control applications. And the power consumption required by Xilinx CoolRunner series PLD devices is extremely low.


PLDs include programmable read only memory (PROM), erasable programmable memory (EPROM), programmable logic array (PLA for short), programmable array logic (PAL for short), and general array logic (GAL for short). Their structural characteristics and functions are listed in the table.

The overall structure of PLA is similar to that of PROM, and is also composed of AND gate array, OR gate array and output buffer; its AND gate array is programmable. When generating the same combinational logic function, using PLA saves the number of cells in the AND gate array and OR gate array than using PROM.

Some PAL devices are register output structures, so using PAL can not only constitute a combinational logic circuit, but also a sequential logic circuit. GAL's output macro logic unit has different working modes and allows selection through programming. These working modes include various output structures of PAL. GAL is more versatile. The programming work of PAL and GAL is more complicated, and it needs to use special development tools (including programmers and programming languages). These development tools are very convenient to use.


Fixed logic devices and PLDs have their own advantages. For example, fixed logic designs are often more suitable for high-volume applications because they can be produced in large quantities more economically. For some applications that require extremely high performance, fixed logic may also be the best choice.

However, programmable logic devices provide some important advantages over fixed logic devices, including: PLD provides customers with greater flexibility in the design process, because for PLD, the design repeatedly only needs to simply change the programming file That’s it, and the results of the design changes can be seen immediately in the working device.

PLD does not require a long lead time to produce prototypes or formal products-PLD devices are already on the distributor’s shelf and can be shipped at any time. PLD does not require customers to pay high NRE costs and purchase expensive mask sets-PLD suppliers have already paid for these costs when designing their programmable devices, and these costs can be shared through the PLD product line for many years of life.

PLD allows customers to order only the quantity they need when they need it, so that customers can control their inventory. Customers who use fixed logic devices often face excessive inventory that needs to be scrapped, and when demand for their products soars, they may suffer from insufficient supply of devices and have to face the reality of production delays.

PLD can be reprogrammed even after the equipment is shipped to the customer. In fact, with programmable logic devices, some equipment manufacturers are trying to add new functions or upgrades for products already installed in the field. To achieve this, you only need to upload new programming files to the PLD via the Internet to create new hardware logic in the system.


Over the past few years, programmable logic vendors have made tremendous technological progress, so that PLD is regarded by many designers as the logical choice of logic solutions. One of the important reasons for achieving this is that PLD suppliers like Xilinx are "fabless manufacturers" and do not directly own chip manufacturing plants. Xilinx outsources chip manufacturing to major businesses such as IBM Microelectronics and UMC Is the partner that makes the chip. This strategy allows Xilinx to focus on designing new product structures, software tools, and IP cores, while also utilizing the most advanced semiconductor manufacturing technology. Advanced process technology has helped PLD in a series of key areas: faster performance, integration of more functions, reduction of power consumption and cost, etc. Xilinx uses an advanced 0.13um low-K copper metal process to produce programmable logic devices, which is also one of the best processes in the industry.

For example, only a few years ago, the largest FPGA devices were only tens of thousands of system gates, operating at 40 MHz. In the past, FPGAs were relatively expensive. The most advanced FPGA devices at that time cost about $150. However, FPGAs with the most advanced features can provide millions of gates of logic capacity, operate at 300 MHz, cost as low as less than $10, and also provide higher levels of integrated features such as processors and memory.

Equally important, PLDs are supported by more and more intellectual property (IP) core libraries-users can use these pre-defined and pre-tested software modules to quickly implement system functions within the PLD. The IP core includes everything from complex digital signal processing algorithms and memory controllers to bus interfaces and mature software microprocessors. This type of IP core saves customers a lot of time and costs-otherwise, users may need several months to realize these functions, and will further delay the time to market the product.

Other related

Programming language

Regarding the "PAL" mentioned earlier, it is too complicated to manually generate JEDEC files, so most of them are converted to computer programs (also called: computer programs) to generate. This kind of program (program) is called " Logic compiler, which is similar to the software compiler used in program development and writing, and the original code (also called: source code) before compilation must also use a specific programming language (also called: programming language, Programming language), this is called hardware description language (hardware description language), referred to as: HDL.

Moreover, HDL is not only one, but there are many, such as ABEL, AHDL, Confluence, CUPL, HDCal, JHDL, Lava, Lola, MyHDL, PALASM, RHDL, etc., but the most well-known and most commonly used are VHDL and Verilog.


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