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Digital Logic Design UNIT 5 1. Explain about memory.  The ability to store information is an important requirement in a digital system.  The information usually consists of instructions in...

Digital Logic Design UNIT 5 1. Explain about memory.  The ability to store information is an important requirement in a digital system.  The information usually consists of instructions in binary form, data to be processed, intermediate and final results, etc. Circuits and/or systems designed specifically for storage are referred to as memory.  In small applications, flip-flop or register is used as memory.  In a larger system like microprocessor, semiconductor memory chips may be used for storing information’s.  But really large amounts of data are generally stored by using magnetic memory, such as magnetic tapes, hard disks and floppy disks.  The magnetic memories are placed externally with the digital system 2. Explain about basic term in memory. a) Memory read: Memory read operation transfers the data stored in a specific memory location to its output line. For doing read operation,  Apply the address of the desired memory location to the address lines.  Activate the Read control line. b) Memory write: The memory write operation transfers the data placed the input line to the specific location of the memory. For doing write operation,  Apply the address of the desired memory location to the address lines.  Activate the Write control line  Apply the data to be stored in the memory to the data input line. c) Access time: Memory access time is the time required to store the data in a selected memory location or the time required to read the data from a selected memory location. d) Memory capacity:  Memory capacity is the maximum amount of Information that can be retained in the memory.  The memory capacity is expressed in Kilobytes (KB) Megabytes (MB), Gigabytes (GB), and Terabytes  (TB) etc. e) Address lines:  Address lines are the physical lines, used to send the address of the memory location where to access the data.  Address lines are used to point the location of the memory.  Address lines are unidirectional.  At a time the address lines can point only one location. f) Word length:  Word length is the number of bits of the word processed by a program or operating system at a time.  So in a 16 it CPU, the word length is 16 bits. 3. Explain about rom and its types.  A "read only memory" is a semiconductor memory device used for storing permanent information.  This memory is an important part of many digital systems because of its low cost, high speed, more flexibility and data non-volatility.  The ROM memories are manufactured by either bipolar technology or MOS technology.  The bipolar devices are faster and have higher capability. The MOS devices require less silicon area and consume less power.  The process of entering information into a memory is referred to as programming.  According to the programming process, the ROMs are classified as follows. (i) Mask programmable read only memory (ROM)  In these memories, the data pattern must be programmed during the fabrication time.  Once it is programmed, the data pattern can never be changed or erased. (ii) Programmable read only memory (PROMs)  It is electrically programmable.  After fabrication, the required data pattern is programmed by the user, by using PROM programmer.  It is only one time programmable, reprogramming is not possible in PROMs. (iii) Erasable Programmable ROM (EPROM)  The EPROMs can be programmed by the user and it can be erased and reprogrammed as and when desired.  Once programmed, the EPROM is a non volatile memory that will hold its stored data indefinitely.  The process for programming an EPROM involves the application of special voltage levels to the  appropriate chip inputs for a specified amount of time.  Once an EPROM can be programmed, it can be erased by exposing it to ultraviolet light.  The ultraviolet light e made to pass through a quartz window on the chip package.  The complete erasing takes 20 to 30 minutes of exposure to ultraviolet rays. (v) Electrically Erasable PROM (EEPROM)  The main advantage of EEPROM over the EPROM is its ability to electrical erase and reprogram individual words in the memory.  Erasing is done by applying high voltage pulses. The erasing is done with a negligible small time of 10milliseconds. (vi) Flash ROM  In flash memory, information is stored in memory cells.  These cells have floating gate transistors that can capture electrons for an extended period.  Depending on where voltage is applied, these cells can perform reading, writing and erasing tasks.  In this memory, data can be written to it and it can be erased although only in blocks, but data can be read on an Individual cell basis. 4. Explain about ram and its types  RAM is a read/write memory.  It should be possible to write into or read from, any memory location with the same speed.  In this memory, the data stored at any location can be changed during the operation of the system.  The various Types of RAMs are given below. (i)Static RAM  In this memory, a digital flip-flop formed by sever Wansistors is used for storing data bit.  This is less dense more expensive but faster and does not require memory refresh.  Static RAMs are fabricated by using either bipolar transistors or MOS transistors. ii) Dynamic RAM  This memory uses memory cells consisting of capacitor and transistors for storing data bit.  This is more dense and cheap.  So it is used for main memory in computers.  Dynamic RAMs are fabricated only by using MOS transistors. iii) SDRAM   This is a Synchronous Dynamic RAM.  In this memory a clock line is added for enabling it to operate in synchronism with computer's memory bus clock.. iv) DDR RAM  This is a Double Data Rate SDRAM.  This memory transfers twice the memory data on each clock cycle.  The data is transferred on both the leading and falling edges of the clock pulse. 5. Explain about memory accessing The operation of reading the stored data from the memory location or writing data to the memory location is called memory accessing. There are 4 types of memory access methods. i) Sequential access In this method, the memory is accessed in a specific linear sequential manner. ii) Random access In this method, any location of the memory can be accessed randomly. iii) Direct access In this method, individual blocks of memory have a unique address based on physical location. iv) Associate access In this method, a word is accessed rather than its address. 6. Explain about Memory Processing  Memory processing is the practice of taking action on data entirely in computer memory (e.g: RAM).  There are three main processes that categorize how memory works.  These processes are encoding, storage and retrieval (or recall).  Encoding is defined as the initial learning of information.  Storage refers for maintaining information overtime.  Retrieval is the ability to access information when we need it. 7. Explain about memory hierarchy.  The computer memory can be divided into 5 major hierarchies that are based on use as well as speed.  A processor can easily move from any one level to some other one on the basis of its requirements.  These five hierarchies in a system's memory are register, cache memory, main memory, magnetic disk and magnetic tape.   Register are small high speed memory units located in the CPU.  They are used to store the most frequently used data and instructions.  Cache memory is a small fast memory unit located close to the CPU.  It stores frequently used data and instructions that have been recently accessed from the main memory.  Main memory is the primary memory of the computer system.  It has a larger storage capacity than cache memory but it is slower.  Magnetic disks work at high speed which are placed inside the computer.  Magnetic tape is generally used for the backup of data. 8. Explain about memory management.  Memory management is the process of controlling and coordinating a computer's main memory.  It ensures that blocks of memory space are properly managed and allocated.  So the operating system (OS), applications and other running processes have the memory they need to carry out their operations. Memory management is usually divided into the following three areas.  Hardware memory management  Operating system memory management  Application memory management The memory management enhancing system stability and preventing crashes. It allows multiple applications to run simultaneously without causing conflicts. 9. Explain about difference between flash ram and nv ram.  Flash memory is an electronic nonvolatile computer memory that can be electrically erased and reprogrammed multiple times.  It has faster access time.  It can store large amounts of data.  It uses less amount of electricity.  The two types of flash memories are NAND flash memory and NOR flash memory.  NV RAM is a Non-Volatile Random Access Memory.  It retains stored data even if the power is switched OFF.  NV RAM uses a variety of technologies to store data.   One common type of NV RAM is flash memory, which uses floating gate transistors to store electrical charges that represent data.  Another type is ferroelectric RAM (FeRAM).  which uses ferroelectric material to store information using the polarization of its atoms  It is used to store the basic input/output system (BIOS) settings of a computer. 10. Explain about pen drive.  Pen drive or USB flash drive is a "plug and play" portable storage device that uses flash memory and is light weight enough to attach to a key chain.  Pen drive can be used in place of a compact disc.  The pen drive can store important files and data backups, carry favorite settings or applications.  The pen drive now comes in capacities ranging between 8 gigabytes (GB) and 1 terabyte (TB) depending on manufacturer.  Flash memory is a specific type of EEPROM used in pen drives.  Compared with floppy disks or CDs, they are smaller, faster, have significantly more capacity and are durable due to the lack of moving parts.  A pen drive consists of a small printed circuit board.  Carrying the circuit elements and a USB connector, insulated electrically and protected inside a plastic or rubberized case. 11. Explain about Sd card.  SD card or secure digital card is a type of memory card used with digital cameras, cell phones music players etc.  SD card can save files, like photos and videos.  We can't install apps on the SD card.  It is a type of removable storage device used to store and transfer digital data.  SD card can transfer data at high rate with low battery consumption.  SD card has a series of electronic components called NAND chips.  SD cards use small scale processor and NAND flash memory chips.  These electronic components allow for data management, ensuring the card can speedily read and  write data.  Many micro SD cards have a standard SD adapter. 12. Explain about solid state hard disk.  SSD is a semiconductor based storage device.  It is a new generation of storage device used in computers.  SSD store data permanently inside an integrated circuit, typically using NAND flash memory.  SSDs don't have any moving parts.  SSD is used in computers.  It replace traditional hard disk drives (HDD) in computers and perform the same basic functions as a hard drive. 13. Explain about different between fixed logic and programmable logic.  Fixed logic devices such as logic gates, multiplexers or flip-flops performs a given logic function at the time of manufacture.  But programmable logic devices, can be configured by the user for their requirements.  The arrangement of building blocks and their interconnections are permanent in fixed logic devices.  It cannot be altered after the device is manufactured.  In fixed logic devices, the time required to go from design to prototype, to a manufacturing run can take several months depending on the complexity of the device.  But in programmable logic devices designers use inexpensive software tools to quickly develop, simulate and test their designs.  During the design phase, customers can change the circuitry as often as they want until the design operates to their satisfaction.  Programmable logic device is an integrated circuit with Internal logic gates that are connected by electronics fuses.   Programming signifies blowing of fuses. 14. Explain about programmable logic devices.  Programmable logic device is also a different type of IC, programmable by users for their specific applications.  The PLD also has some more advantages than ordinary IC's and ASIC'S.  It is a LSI chip.  It contains a regular structure.  It allows the designer to customize it for any specific applications.  A programmable logic device (PLD) usually consists of programmable array of logic gates and inter- connections with array of inputs and outputs.  PLD is classified into three types. They are,  Programmable read only memory (PROM)  Programmable logic array (PLA), and  Programmable array logic (PAL).  An user can design any logic circuit by using 'fuse blowing process in accordance with customer's instructions This process is called programming.  It shows an array of AND gates, and an array of OR gates that can be inter-connected to generate four outputs.  The two inputs of A and B combined with NOT gates, produce additionally two more inputs.  Hence 4 input lines are formed.  Each of the product lines is connected to one of the four inputs of each OR gate through a fusible link. That means Y,= AB+AB+AB+AB  = A (B+B)+A(B+B) =A+A=1  Each of the four outputs Y,, Y, Y, and Y, can be programmed to be any function of A and B by selectively blowing the appropriate fuses. Y=0+AB+AB+0=AB+AB  Once all the outputs can be programmed, the device will permanently generate the selected output functions.  The input AND-gate array used in PROM is fixed and cannot be altered, while the output OR gate array is fusible linked, and thus programmable.  The PLA is much more versatile than the PROM or the PAL, since both its AND gate array and its OR- gate array are fusible-linked and thus programmable.  The already explained PLD is a PROM PLD, and it contains fixed AND array and programmable output OR array. The advantages of PLDs over fixed - function ICs are (i) Reduction in board space requirements (ii) Reduction in power requirements (iii) Compact circuitry (vi) Higher switching speed (v) Design security The advantages of PLDs over ASICs are (i) Higher densities (ii) Lower production cost (iii) Design security (iv) Reduced space requirement (v) Reduced power requirement. 15. Explain about programmable logic array.  Programmable logic array is one type of programmable logic device.  It is suitable for implementing logic functions in sum of product form.  The number of AND and OR gates and their inputs is fixed for a given PLA chip.  The AND gates provides the product terms, and the OR gates logically sum these product terms and thereby generate a sum of product expressions.  A PLA combines the characteristics of programmable read only memory (PROM) and the  programmable array logic (PAL).  The AND array and also the OR array are programmable, connected with fusible links.  It is a most versatile PLD.  The given PLA has 3 input variables (ABC) and 3 output variables.  8 AND gates are required to decode the 8 possible input states.  The PLA has some disadvantages, because it has two sets of fuses.  PLAs are often referred to as field programmable gate array (FPGA).