Memory Devices

Digital Circuits

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  • Question 1

    Don’t just sit there! Build something!!

    Learning to analyze digital circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

    You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

    1. Draw the schematic diagram for the digital circuit to be analyzed.
    2. Carefully build this circuit on a breadboard or other convenient medium.
    3. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
    4. Analyze the circuit, determining all output logic states for given input conditions.
    5. Carefully measure those logic states, to verify the accuracy of your analysis.
    6. If there are any errors, carefully check your circuit’s construction against the diagram, then carefully re-analyze the circuit and re-measure.

    Always be sure that the power supply voltage levels are within specification for the logic circuits you plan to use. If TTL, the power supply must be a 5-volt regulated supply, adjusted to a value as close to 5.0 volts DC as possible.

    One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

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  • Question 2

    When Digital Audio Tape (DAT) was first introduced to the American public, it was touted as delivering superior sound quality. Most importantly, this high quality of sound was not supposed to degrade over time like standard (analog) audio cassette tape recordings.

    The magnetic media from which DAT was manufactured was basically the same stuff used to make analog audio tape. Explain why the encoding of audio data digitally on the same media would provide superior resistance to degradation over analog recordings even though the recording media was the same. Also, explain how this is significant to modern digital data storage technologies such as those used to store photographic images and numerical data.

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  • Question 3

    Define the following terms, as they relate to digital memory devices:


    In particular, explain why “RAM” is a misleading term.

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  • Question 4

    Determine whether the following recording devices are random access or sequential access, and discuss the advantage(s) of one type of access over the other:

    DVD (disk)
    Audio tape cassette
    CD-ROM (disk)
    ROM memory chip
    Vinyl phonograph record
    Video tape cassette
    Magnetic “hard” drive
    Magnetic bubble memory
    Paper tape (a long strip of tape with holes punched in it)
    RAM memory chip
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  • Question 5

    Define the following acronyms:


    Be prepared to explain a few things about each of these memory technologies: how they work, what applications they might be found in, advantages and disadvantages of each.

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  • Question 6

    Explain the difference between static RAM (“SRAM”) and dynamic RAM (“DRAM”) memory technologies. Which type of memory technology provides faster access of data, and why? Which type of memory technology provides the greatest storage density, and why?

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  • Question 7

    Flash memory is a nonvolatile memory technology, offering greater density than either SRAM or DRAM, and faster erasure than standard EPROMs. At first, it would seem Flash memory outperforms all other memory types, but it doesn’t. What are some of the disadvantages of Flash memory, and what kind of applications is it best suited for?

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  • Question 8

    Two very important concepts to understand when working with digital memory devices are address and data. Define each of these terms in your own words.

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  • Question 9

    A ROM memory chip is rated at 4k × 8 bits. What, exactly, does this designation mean? How many addresses are there inside this memory chip? How many bits of storage are there, total, in this memory chip? How many address bits are there, and how many data bits are there?

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  • Question 10

    Suppose you need to store a text message in digital memory, consisting of 7500 ASCII characters. What is the most logical memory organization (addresses × data lines) to do this? How many address bits would be needed to store these 7500 characters?

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  • Question 11

    Suppose you need a memory array with 1k × 8 organization, but all you have on hand are 1k × 4 memory chips. Show how you could connect two of them to form the desired array:

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  • Question 12

    Suppose you need a memory array with 2k × 4 organization, but all you have on hand are 1k × 4 memory chips. Show how you could connect two of them to form the desired array:

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  • Question 13

    Suppose you need a memory array with 4k × 4 organization, but all you have on hand are 1k × 4 memory chips. Explain how you could build a memory array of this size using multiple 1k × 4 chips.

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  • Question 14

    Dynamic RAM chips often contain more addresses than they have address lines to select them with. For example, the MCM516100 DRAM chip has an organization of 16M × 1, yet it only has twelve address lines.

    Explain how it is possible to select one out of 16 million unique addresses while using only twelve address lines. Hint: the technique is known as address multiplexing. Be sure to refer to one or more dynamic RAM datasheets when doing your research!

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  • Question 15

    After a ROM memory has been programmed with data, it is good to verify that the data now stored is okay, and not corrupted with any errors. A popular method of doing this is to calculate a checksum on the stored data, and compare that against the checksum for the original data. If the checksum numbers are identical, chances are there are no corruptions in the stored data.

    Explain exactly what checksum is, and how it works as an error-detection strategy.

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  • Question 16

    An important use for read-only semiconductor memories is as look-up tables. Describe what a “look-up table” is, and what one might be used for.

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  • Question 17

    Suppose an automobile manufacturer was designing a new car engine design, and they needed a memory chip to store look-up tables for the engine’s control computer, holding data such as optimum fuel/air ratios for different engine loads which the computer would then consult to maintain best performance, or economy, or emissions. What type of memory chip would you recommend for the task, and why? Choose from the following list:

    Static RAM (SRAM)
    Mask ROM
    Dynamic RAM (DRAM)
    Magnetic core
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  • Question 18

    Research datasheets for the 74LS184 and 74LS185 integrated circuits, and then explain how read-only memory technology is used to perform the BCD/binary conversion functions.

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