Memory and Programmable Logic

1. List basic types of programmable logic devices.

Read only memory Programmable logic Array Programmable Array Logic

2. Explain ROM

A read only memory (ROM) is a device that includes both the decoder and the OR gates within a single IC package. It consists of n input lines and m output lines. Each bit combination of the input variables is called an address. Each bit combination that comes out of the output lines is called a word. The number of distinct addresses possible with n input variables is 2n.

3. Define address and word:

In a ROM, each bit combination of the input variable is called on address. Each bit combination that comes out of the output lines is called a word.

4. State the types of ROM

1. .  Masked ROM.
2. . Programmable Read only Memory
3. . Erasable Programmable Read only memory.
4. . Electrically Erasable Programmable Read only Memory.

5. What is programmable logic array? How it differs from ROM?

In some cases the number of don’t care ditionscon is excessive, it is more economical to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept; however it does not provide full decoding of the variables and does not generates all the min terms as in the ROM.

6. Explain PROM.

PROM (Programmable Read Only Memory)

It allows user to store data or program. PROMs use the fuses with material like nichrome and polycrystalline. The user can blow these fuses by passing around 20 to 50 mA of current for the period 5 to 20μs.The blowing of fuses is called programming of ROM. The PROMs are one time programmable. Once programmed, the information is stored permanent.

7. Explain EPROM.

EPROM (Erasable Programmable Read Only Memory)

EPROM use MOS circuitry. They store 1’s andas a0’spacket of charge in a buried layer of the IC chip. We can erase the stored data in the EPROMs by exposing the chip to ultraviolet light via its quartz window for 15 to 20 minutes. It is not possible to erase selective information. The chip can be reprogrammed.

8. Explain EEPROM.

EEPROM (Electrically Erasable Programmable Read Only Memory) EEPROM also use MOS circuitry. Data is stored as charge or no charge on an insulated layer or an insulated floating gate in the device.

EEPROM allows selective erasing at the register level rather than erasing all the information since the information can be changed by using electrical signals.

9. What is RAM?

Random Access Memory-Read and write operations can be carried out.

10. What is programmable logic array? How it differs from ROM?

In some cases the number of don’t care conditions is excessive, it is more economical to type of LSI component called a PLA. A PLA is similar to a ROM in concept; however it does not provide full decoding of the variables and does not generates all the minterms as in the ROM.

11. What is mask – programmable?

With a mask programmable PLA, the user must submit a PLA program table to the manufacturer.

12. What is field programmable logic array?

The second type of PLA is called a field programmable logic array. The user by means of certain recommended procedures can program the EPLA.

13. List the major differences between PLA and PAL

Both AND and OR arrays are programmable and Complex Costlier than PAL PAL, AND arrays are programmable OR arrays are fixed Cheaper and Simpler.

14. Define PLD.

Programmable Logic Devices consist of a large array of AND gates and OR gates that can programmed to achieve specific logic functions.

15. Give the classification of PLDs.

PLDs are classified as PROM (Programmable Read Only Memory), Programmable Logic Array(PLA), Programmable Array Logic (PAL), and Generic Array Logic(GAL)

16. Define PROM.

PROM – Programmable Read Only Memory. It consists of a set of fixed AND gates connected to a decoder and a programmable OR array.

17. Define PLA

PLA is Programmable Logic Array (PLA). The PLA is a PLD that consists of a programmable AND array and a programmable OR array.

18. Define PAL

PAL is Programmable Array Logic. PAL consists of a programmable AND array and a fixed OR array with output logic.

19. Why was PAL developed?

It is a PLD that was developed to overcome certain disadvantages of PLA, such as longer delays due to additional fusible links that result from using two programmable arrays and more circuit complexity.

20. Why the input variables to a PAL are buffered

The numerous AND gate inputs that are available or its complement can be connected to buffer the input variables to PALs in order to prevent loading.

21. What does PAL 10L8 specify?

PAL – Programmable Logic Array 10 – Ten inputs L – Active LOW Ouput 8 – Eight Outputs

22. Give the comparison between PROM and PLA.

PROM

And array is fixed and OR array is programmable.

Cheaper and simple to use.

PLA

Both AND and OR arrays are Programmable.

Costliest and complex than PROMS.

GLOSSARY TERMS

1.     FPGA-Field-Programmable Gate Arrays -use read/write memory cells to control the state of each connection.

2.     Memory unit– A memory unit is a collection of cells capable of storing a large quantity of binary information.

3.     Write operation- The process of storing new information into memory is referred to as amemory “write” operation.

4.     Read operation- The process of transferring the stored information out of memory is referredto as a memory “read” operation.

5.     ROM– Read only Memory-ROM can perform only the read operation.

6.     RAM-Random Access Memory-Read and write operations can be carried out.

7.     PLD– A PLD is an integrated circuit with internal logic gates connected through electronicpaths that behave similarly to fuses.

8.     Programmable array logic (PAL) – PAL is Programmable Array Logic, consists of aprogrammable AND array and a fixed OR array with output logic.

9.     Programmable logic array (PLA) – PLA is Programmable Logic Array (PLA) consists of aprogrammable AND array and a programmable OR array.

10.  ASIC (Application specific integrated circuits)– Chips designed for a particular, limited product or applications.