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Programmable Logic Technology
10 questions By Tony R. Kuphaldt
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Question 7 of 10
A common term used to describe the internal workings of a programmable logic device is a macrocell. What, exactly, is a macrocell?
Reveal answerA macrocell is a collection of logic gates and a flip-flop, lumped together in one unit. PLDs usually have many macrocells, which may be interconnected to form a variety of synchronous logic functions.
Notes:Ask your students to show you where they found their information, and if they were able to determine how many macrocells are in a typical PLD.
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Question 8 of 10
Most microcomputers can only perform one task (operation) at a time. They achieve the illusion of “multi-tasking” by alternately devoting time to one of several tasks in a rapid fashion - a sort of multiplexed computation. Most programmable logic devices, on the other hand, are easily able to perform multiple logic operations in a truly simultaneous manner. Explain how this is possible, whereas a microprocessor can only do one thing at a time.
Reveal answerThe secret is in the programming: programmable logic devices are literally “wired” by the programs you write for them, with thousands of logic elements available to be connected in almost any way you desire. Microprocessors, on the other hand, have fixed wiring that responds to sequences of steps, the program merely specifying those sequence of those steps.
Notes:Understanding the distinction between microcontrollers and programmable logic devices can be difficult, especially if one has limited experience with both (as most students do). The purpose of this question is to shed some more light on this often misunderstood subject, while simultaneously highlighting an important feature of programmable logic: true simultaneity.
The fundamental principle I want students to see from these analogies is that microcontrollers and microprocessors are re-programmed by changing a sequence of fixed operations, while programmable logic systems are re-programmed by changing associations between fixed elements.
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Question 9 of 10
Verilog and VHDL are two popular examples of a hardware description language, used when working with programmable logic. Explain the purpose of such a “language.” What does it mean for a technician or engineer to “speak” this language, and how is it “spoken” to an actual programmable chip?
Reveal answerA hardware description language (HDL) is a textual convention for specifying the interconnections of a programmable logic device. Text files are written by a human programmer, then “compiled” into a form that the programmable logic device can directly accept and use.
Notes:If time permits, you may want to compare and contrast fully-featured languages such as Verilog and VHDL with more primitive hardware description languages such as ABEL. In either case, though, files written in an HDL are intended to describe the interconnections of available logic elements inside a programmable logic device.
