Memory Devices

It has been my experience that students require much practice with circuit analysis to become proficient. To this end, instructors usually provide their students with lots of practice problems to work through, and provide answers for students to check their work against. While this approach makes students proficient in circuit theory, it fails to fully educate them.

Students don’t just need mathematical practice. They also need real, hands-on practice building circuits and using test equipment. So, I suggest the following alternative approach: students should build their own “practice problems” with real components, and try to predict the various logic states. This way, the digital theory “comes alive,” and students gain practical proficiency they wouldn’t gain merely by solving Boolean equations or simplifying Karnaugh maps.

Another reason for following this method of practice is to teach students scientific method: the process of testing a hypothesis (in this case, logic state predictions) by performing a real experiment. Students will also develop real troubleshooting skills as they occasionally make circuit construction errors.

Spend a few moments of time with your class to review some of the “rules” for building circuits before they begin. Discuss these issues with your students in the same Socratic manner you would normally discuss the worksheet questions, rather than simply telling them what they should and should not do. I never cease to be amazed at how poorly students grasp instructions when presented in a typical lecture (instructor monologue) format!

I highly recommend CMOS logic circuitry for at-home experiments, where students may not have access to a 5-volt regulated power supply. Modern CMOS circuitry is far more rugged with regard to static discharge than the first CMOS circuits, so fears of students harming these devices by not having a “proper” laboratory set up at home are largely unfounded.

A note to those instructors who may complain about the “wasted” time required to have students build real circuits instead of just mathematically analyzing theoretical circuits:

What is the purpose of students taking your course?

If your students will be working with real circuits, then they should learn on real circuits whenever possible. If your goal is to educate theoretical physicists, then stick with abstract analysis, by all means! But most of us plan for our students to do something in the real world with the education we give them. The “wasted” time spent building real circuits will pay huge dividends when it comes time for them to apply their knowledge to practical problems.

Furthermore, having students build their own practice problems teaches them how to perform primary research, thus empowering them to continue their electrical/electronics education autonomously.

In most sciences, realistic experiments are much more difficult and expensive to set up than electrical circuits. Nuclear physics, biology, geology, and chemistry professors would just love to be able to have their students apply advanced mathematics to real experiments posing no safety hazard and costing less than a textbook. They can’t, but you can. Exploit the convenience inherent to your science, and get those students of yours practicing their math on lots of real circuits!

Challenge students to come up with some disadvantages of digital recordings, now that they understand the difference between analog and digital data storage. While digital technology certainly enjoys some advantages over analog, it is not necessarily superior in all aspects!

The mis-use of the acronym “RAM” is another unfortunate entry in the lexicon of electronics. Your students are sure to have questions about this term, so be prepared to discuss it with them!

One of the purposes of this question is to get students to realize that “RAM” memory (solid-state, volatile memory “chips” in a computer) is not the only type of data storage device capable of randomly accessing its contents, and that the term “RAM” as it is commonly used is something of a misnomer.

ROM designations are another set of misnomers that have crept into the electronics lexicon. I mean really, how can anything be both erasable and programmable, but still be read-only?

Ask your students where they obtained their information on static versus dynamic RAM technologies. Extra credit for consulting datasheets!

Discuss with your students some of the different applications of Flash memory technology. Talk about how those applications fit the capabilities (and weaknesses) of Flash technology well.

Analogies are often helpful to communicate the concepts of “address” and “data” to new students. I like to use the example of post office boxes (lots of addressed boxes, each containing different items) when I explain address and data.

Discuss with your students why there are not 4000 (exactly) addresses in a “4k” memory chip.

Be sure to ask your students how they calculated 13 bits for the address. Of course, there is the trial-and-error method of trying different powers of two, but there is a much more elegant solution involving logarithms to find the requisite number of bits.

Be sure to spend some time discussing the common mistake referenced in the follow-up question. This is something I’ve seen more than once, and it reveals a fundamental gap in understanding on the part of the mistaken student. What students are prone to do is try to memorize the sequence of connections rather than really understand why memory array expansion works, which leads to errors such as this.

Note that the answer to “what might happen” depends on whether the first operation is a read, or a write.

Be sure to spend some time discussing the follow-up question. Once again, I’ve noticed students are prone to memorizing the connection pattern rather than take the time to figure out why the chips are connected as they are.

Although no schematic is given here in the answer, I expect my students to be able to draw one on their own.

Explain to your students that address multiplexing is not technically limited to application in dynamic RAM chips only, but that it is usually applied there because of the high address density afforded by dynamic RAM technology. Most static RAMS, by contrast, are not dense enough to require address lines serve double-duty!

Once again, there is little I can reveal in the answer without giving everything away. There are enough resources available for students to learn about checksum on their own, that you should not have to supply additional information.

The EBCDIC-to-ASCII code converter concept is not hypothetical! I actually designed and helped build such a circuit to allow standard personal computers to “talk” to an obsolete CNC machine tool control computer which didn’t understand ASCII data, only EBCDIC. A look-up table implemented in a UVEPROM served as a neat way to implement this function, without a lot of complex circuitry.

This question is multi-faceted. Students must consider volatility and ease of updating (the data), as well as simply applying the concept of a look-up table to a car’s engine control computer, in order to intelligently answer this question.

Discuss with your students why someone would choose to implement these functions in a look-up table instead of using combinational logic or a microprocessor/microcontroller. What advantage(s) might be realized with the look-up table approach?