Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

1.6 PROGRAMMABLE LOGIC DEVICES

Advances in VLSI technology made possible the design of special chips, which can be configured by a user to implement different logic circuits. These chips, known as programmable logic devices (PLDs), have a very general structure and contain programmable switches, which allow the user to configure the internal circuitry to perform the desired function. The programmer (end user) has simply to change the configuration of these switches. This is usually done by writing a program in a hardware description language (HDL) such as VHDL or Verilog and “downloading” it into the chip. Most types of PLDs are reprogrammable for a fixed number of times (generally, a very high number). This makes PLDs excellent for use in prototyping of ASICs and standard chips. A designer can program a PLD to perform a particular function and then make changes and reprogram it for retesting on the same chip. Also, there is a great cost savings in using a device that is reprogrammable for prototyping purposes. The main disadvantage of PLDs is that they may not be the best performing. The performance of a functionally equivalent ASIC or standard chip is likely to be better. This is because all functions have to be realized from existing blocks of logic inside the PLD. The most popular types of PLDs include:

• Simple programmable logic devices (SPLDs)
• Programmable array logic (PAL)
• Programmable logic array (PLA)
• Generic array logic (GAL)
• Complex programmable logic devices (CPLDs) ...