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Memory Devices
18 questions By Tony R. Kuphaldt
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Question 16 of 18
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.
Reveal answerA look-up table is a set of data programmed into a memory device, used to map a function of some kind: for each unique input (address), there is an output (data) that means something to the system in which it is installed.
An example of a look-up table is an EBCDIC-to-ASCII code converter, where an EBCDIC code input to the address lines of a ROM chip “looks up” the equivalent ASCII character value from memory, and outputs it as the result through the ROM chip’s data lines.
Notes: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.
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Question 17 of 18
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
- PROM
- Dynamic RAM (DRAM)
- EPROM
- Magnetic core
Reveal answerEPROM would probably be the best choice. I’ll let you discuss this with your classmates and with your instructor, though!
Notes: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.
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Question 18 of 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.
Reveal answerThese integrated circuits are really just read-only memory chips programmed with look-up tables for converting BCD to binary (74LS184) and binary to BCD (74LS185).
Notes: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?
