Facebook
Google
GitHub
LinkedinDigital Circuits
Programmable Logic Technology
10 questions By Tony R. Kuphaldt
-
Question 4 of 10
Microcontrollers are single-chip microcomputers, containing a microprocessor core, memory, I/O control, and other associated components necessary to make the system self-contained. Simply put, a microcontroller follows sequential instructions that someone enters into its memory.
Programmable logic devices, however, are fundamentally different from microcontrollers both in how they are programmed and how they function after being programmed. Explain what some of these differences are.
Reveal answerUnlike microcontrollers, programmable logic devices are not (necessarily) sequential devices: the latter act as a collection of logic gates and other “primitive” logic elements to directly implement certain logic functions.
Notes:Discuss with your students how programmable logic devices are more primitive and direct devices than microcontrollers, which are more abstract by comparison. Perhaps the easiest distinction to understand is in terms of gate connections. In a microcontroller, the connections between its constituent gates are fixed; only the software (bits stored in memory) ever change. In a programmable logic device, it is as though you are directly forging connections between its constituent gates (as many or as few as needed), creating a hard-wired circuit by specifying connections in a “hardware description language” (HDL).
-
Question 5 of 10
The simplest types of programmable logic ICs are called PLDs (Programmable Logic Devices), PALs (Programmable Array Logic), PLAs (Programmable Logic Array), and GALs (Generic Array Logic). While each acronym represents a slightly different internal design architecture, these devices share a common feature of using inverters, AND gates, and OR gates to implement any desired combinational logic function.
Explain how it is possible to generate any arbitrary logic function with just these gate types (inverter, AND, OR), without any others. What principle or convention of Boolean algebra is used by these devices to do this?
Reveal answerWith a sufficient number of AND, OR, and inverter gates, any SOP or POS expression may be generated.
Notes:This question requires students to review the principles of how SOP and POS expressions relate to truth tables, and in doing so explain how any arbitrary truth table may be fulfilled.
-
Question 6 of 10
Some programmable logic devices (and PROM memory devices as well) use tiny fuses which are intentionally “blown” in specific patterns to represent the desired program. Programming a device by blowing tiny fuses inside of it carries certain advantages and disadvantages - describe what some of these are.
Reveal answerCertainly, the stored program will be nonvolatile, but it will also be read-only. This is why fuse-programmed devices are sometimes called ÖTP”. (I’ll let you research what that acronym means.)
Notes:It is interesting to mention that some programmable devices (Texas Instruments’ TIBPAL series, for example) are built with a “security fuse” inside which prevents anyone from reverse-engineering a programmed chip!
