Digital Circuit Systems

Programmable Logic Devices Viva Questions

Programmable Logic Devices Viva Questions

Programmable Logic Devices Viva Questions, Viva Questions on Programmable Logic Devices, Digital Electronics Viva Questions, Short Questions answers on Programmable Logic Devices, PLD Viva Questions, ROM Viva Questions, PROM Viva Questions, Read-Only Memory Viva Questions, Programmable Logic Devices Viva Questions, Programmable Logic Devices Viva Questions

Logic Gates Viva Questions

Short Questions and Answers

Q.1. What is a PLD?

Ans. A PLD (programmable logic device) is an IC that contains a large number of gates. Flip flops and registers that are interconnected on the chip. Many of the connections, however, are fusible links that can be broken.

Q.2. What is the principal advantage of a PLD?

Ans. The principal advantage of PLDs is that in many applications, they can replace a number of other circuits.

Q.3. What is programming?

Ans. The fuse blowing process is called programming.

Q.4. What are the advantages of PLDs over fixed-function ICs?

Ans. The advantages of PLDs over fixed-function ICs are:

  1. Low development cost
  2. Less space requirement
  3. Less power requirement
  4. High reliability
  5. Easy circuit testing
  6. Easy design modification
  7. High design security
  8. Less design time

Q.5. What is a ROM?

Ans. A ROM (Read Only Memory) is essentially a memory device in which permanent information is stored. Data can only be read from it.

Q.6. What are the technologies used for the fabrication of ROMs?

Ans. ROMs and PROMs can be fabricated using bipolar or MOS technology but EPROMs and EEPROMs are possible only with MOS technology.

Q.7. How is the memory size specified?

Ans. The memory size is specified as M x N bits where M is the number of locations and N is the number of bits in each location.

Q.8. Is there no provision of entering information in the read only memory? If no what can be read from the memory and if yes why it is called as read only memory.

Ans. There is a provision of entering information in ROM. This process is known as programming. In case of programmable ROMs, the ROM is removed from the circuit and is programmed using a PROM programmer. In the case of non-programmable ROM, the information is entered as a part of the fabrication process itself. Because of the requirement of programming, it is known as read-only memory. 

Q.9. A memory has a 16-bit address bus. How many locations are there in this?

Ans. The number of memory locations in a memory with 16-bit address bus = 216 = 65,536 = 64 K.

Q.10. What for is the letter ‘K’ used in memories?

Ans. Digital systems operate on binary numbers and 210 = 1024 is represented by 1K.

Q.11. What happens to the information stored in memory location after it has been read?

Ans. The reading operation is non-destructive, which means the stored information remains intact after it has been read and can be read any number of times.

Q.12. Explain the programming of ROM.

Ans. A ROM is programmed at the time of manufacturing. The information to be entered is supplied by the user. The contents of this are fixed at the time of its fabrication and these can never be changed. That means it cannot be erased.

Q.13. Is the ROM a volatile memory? Explain.

Ans. Programming of ROM involves making the required interconnections at the time of fabrication and therefore, its contents are unaffected even when the power is off. Thus it is a non-volatile memory.

Q.14. What is a PROM?

Ans. A PROM is a ROM that can be programmed.

Q.15. What does a 32 x 8 ROM contain?

Ans. A 32 x 8 ROM contains 32 words (addresses) of 8 bits each.

Q.16. Which decoder is contained in a 32 x 8 ROM?

Ans. A 32 x 8 ROM contains a 5 x 32 decoder.

Q.17. How many OR gates are there in a 32 x 8 ROM and how many inputs does each OR gate of a 32 x 8 ROM have?

Ans. A 32 x 8 ROM contains 8 OR gates and each OR gate has 32 inputs.

Q.18. What should be the size of a ROM to produce the square of a 3-bit input?

Ans. The size of the ROM to produce the square of a 3-bit input is 8 x 6.

Q.19. What is the size of the decoder in a 32 x 4 ROM?

Ans. A 32 x 4 ROM contains a 5 x 32 decoder.

Q.20. What is the size of the decoder in a 8 x 4 ROM?

Ans. An 8 x 4 ROM contains a 3 x 8 decoder.

Q.21. What are the types of ROMs?

Ans. The various types of ROMs are:

  • The mask programmed ROMs (MROMs)
  • Programmable read only memories (PROMS)
  • Erasable programmable read only memories (EPROMs)
  • Electrically erasable programmable read only memories (EEPROMs)

Q.22. What is an MROM?

Ans. An MROM is a ROM that has its storage locations written into (programmed) by the manufacturer during the last fabrication/process of the customer’s specifications. A major disadvantage of MROM is that it cannot be reprogrammed in the event of a design change requiring modification of storage.  

Q.23. What is a PROM?

Ans. A PROM is a field programmable ROM. It is not programmed during the manufacturing processes but is custom programmed by the user. Once programmed, the date cannot be altered. PROMs are manufactured with fusible links.

Q.24. Is the PROM volatile or non-volatile?

Ans. It is non-volatile similar to the ROM.

Q.25. What are the technologies used for the fabrication of ROMs?

Ans. ROMs and PROMs can be fabricated using bipolar or MOS technologies, but EPROMs and EEPROMs are possible only with MOS technology.

Q.26. How can a ROM device be considered a combinational circuit?

Ans. A ROM device has M locations and N bits are stored at each location. Each location has its unique address. Therefore, if the signals corresponding to the input variables are applied at the address input pins of ROM, then the contents stored at that location are available at output pins. Thus, it operates as a combinational circuit.

Q.27. Is it possible to locate any ROM location at random?

Ans. Any ROM location can be selected for reading (or accessed) at random by applying the corresponding input variables at the address input pins.

Q.28. Is it possible to design multiple output circuits using ROM?

Ans. Multiple output circuits can be designed using ROM by selecting a ROM which has the number of bits at each location at least equal to the number of outputs desired.

Q.29. What is an EPROM?

Ans. An EPROM is a ROM whose contents can be erased and reprogrammed enabling the device to be used repeatedly. Its contents can be erased by exposing it to ultraviolet light. Selective erasure is not possible.

Q.30. Differentiate between PROM and EPROM.

Ans. A PROM can be programmed only once whereas an EPROM can be programmed any number of times.

Q.31. In an EPROM chip, is it possible to erase the contents of only some of the locations? If not, why?

Ans. No. When the chip is exposed to UV radiation, all the contents get erased simultaneously. It is not possible to erase some locations leaving the contents of the remaining locations intact.

Q.32. Some information is stored in EPROM which is required to be modified. How will you do it?

Ans. First, whatever is stored should be erased by exposing the EPROMs to UV radiation, and then it is programmed to store the new information.

Q.33. What is an EEPROM?

Ans. An EEPROM or EAROM is a ROM whose contents can be electrically erased and reprogrammed enabling the device to be used repeatedly. Selective erasure is possible.

Q.34. What are the two major disadvantages of EPROM?

Ans. The two major disadvantages of EPROM are as follows:

  • They have to be removed from their sockets in order to be erased and reprogrammed.
  • The erasure removes the complete memory contents. This necessitates complete reprogramming even when one memory word has to be changed.

Q.35. What is a combinational PLD?

Ans. A combinational PLD is an IC with programmable gates divided into an AND array and an OR array to provide an AND-OR sum of products implementation.

Q.36. What are the major types of combinational PLDs?

Ans. There are three major types of combinational PLDs and they differ in the placement of the programmable connection in the AND-OR array. The various PLDs used are PALs (programmable array logic), PLAs (programmable logic arrays), and PROMs (programmable read-only memories).

Q.37. What is a PROM?

And. A PROM is a combinational PLD with a fixed AND array and a programmable OR array.

Q.38. What is a PAL?

Ans. A PAL is a combinational PLD with a programmable AND array and a fixed OR array.

Q.39. What is a PLA?

Ans. A PLA is a combinational PLD with both programmable AND, and OR arrays.

Q.40. What is FPLA?

Ans. FPLA is a field programmable logic array. It can be programmed by the user by means of certain recommended procedures.

Q.41. What is the programming table of a PLA?

Ans. The programming table of a PLA is a table specifying the fuse map of a PLA.

Q.42. How is the size of a PLA specified?

Ans. The size of a PLA is specified by the number of inputs, the number of product terms and the number of outputs.

Q.43. What is a fuse map?

Ans. A fuse map is a map that shows which fuses to blow.

Q.44. What is the basic architecture of a PLA?

Ans. A PLA consists of an array of programmable AND, and OR gates. The number of inputs to every AND gate is twice the number of input variables possible for a chip, and the number of inputs to every OR gate is equal to the number of AND gates. The outputs of the OR gates give the output of the realized logic functions.

Q.45. How is the capacity of a PLA specified?

Ans. The capacity of a PLA is specified as the number of inputs, product terms, and outputs.

Q.46. Are PLAs and FPLAs volatile or non-volatile?

Ans. PLAs and FPLAs are non-volatile.

Q.47. Are erasable and programmable PLAs available?

Ans. No. Erasable and programmable PLAs are not available.

Q.48. Is it possible to share the product terms between different outputs in a PLA? If yes, how?

Ans. Yes. Since each OR gate may be connected to all the product terms, the output of the AND gate with product terms required for different output functions can be connected to corresponding OR gates.

Q.49. Is it possible to share the product terms between different outputs in a PAL?  Justify your answer.

Ans. No. In a PAL the OR gates are non-programmable. Every AND gate can supply input to only one OR gate. Therefore, it is not possible to share the product terms between different outputs.

Q.50. What are the advantages and disadvantages of using a PROM as a PLD?

Ans. The advantages of using a PROM as a PLD are as follows:

  • Ease of design since no simplification or minimization of logic function is required.
  • Design can be changed, modified rapidly.
  • It is usually faster than discrete SSI/MSI circuit. 
  • Cost is reduced.

The disadvantages of using ROM-based circuits as PLDs are as follows:

  • Non-utilization of complete circuit.
  • Increase of power requirement and enormous increase in size with increase in the number of input variables making it impractical.

Q.51. Compare the three combinational PLDs – PROM, PLA, and PAL.

Ans. All the three combinational PLDs – PROM, PLA, and PAL are used for synthesizing multiple-output functions. All these basically comprise an AND array followed by an OR array. The programming flexibility for the user in each case is summarized below.

PROMPLAPAL
1. AND array is fixed and OR array is programmable. I. Both AND and OR arrays are programmable. 1. OR array is fixed and AND array is programmable.
2. Cheaper and simple to use.2. Costliest and more complex than PALs and PROMs. 2. Cheaper and simpler.
3. All minterms are decoded.3. AND array can be programmed to get desired numerals. 3. AND array can be programmed to get desired minterms.
4. Only Boolean functions in the standard SOP form can be implemented using PROM.4. Any Boolean function in the SOP form can be implemented using PLA.4. Any Boolean function in the SOP form can be implemented using PAL.

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