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Friday, 29 November 2019

Difference between Static Ram and Dynamic Ram

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A static RAM essentially contains an array of flip-flops, one for each stored bit. Data written into a flip-flop remains stored as long as a d.c. power is maintained. The memory capacity of a static RAM varies from 64 bits to 1 Mega bit.

Static RAM cell: The logic diagram of a static RAM cell is shown in Fig. 4.5. The cell (or a group of cells) is selected by HIGH values on the ROW and COLUMN lines. The input data bit (1 or 0) is written into the cell by setting the flip-flop for a 1 and resetting the flip-flop for a 0 when the READ/ WRITE’ line is LOW (i.e., write). When the READ/ WRITE’ line is HIGH, the flip-flop is unaffected. It means that the stored bit (data) is gated to the data out line.

The flip-flop in static memory cell can be constructed using Bipolar Junction Transistor (BJT) and MOSFETs that are shown in Fig respectively.
(a) Using Bipolar Transistor
(b) Using MOS Transistor
In a bipolar static RAM cell shown in Fig (a), two BJTs Q1 and Q2, are cross-coupled to form a flip-flop. Here, each transistor has three emitters, namely Row Select input, Column Select input, and Write input. To select the cell, both the row and column select lines must be held HIGH. When selected, a data bit can be stored in the cell (Write operation) or the content of the cell can be read (Read operation). If either row or column select line is LOW, then the memory cell is disabled.
In a MOS stalk RAM cell shown in Fig, Q1 and Q2 act like switches while Q3 and Q4 acts as active load resistors. The transistor Q1 conducts and Q2 is cut off or vice versa. As a static RAM uses a flip-flop as the basic memory cell, it consists of thousands of flip-flops.


The Dynamic Random Access Memory (DRAM) is the lowest cost, highest density random access memory available. Nowadays, computers use DRAM for main memory storage with the memory sizes ranging from 16 to 256 Mega bytes.

Data are stored as charge on every capacitor, which must be recharged or refreshed thousands of times every second in order to retain the stored charge. These memory devices make use of an integrated MOS capacitor as basic memory cell instead of a flip-flop. The disadvantage is that the MOS capacitor cannot hold the stored charge over an extended period of time and it has to be refreshed every few milliseconds. This requires more circuitry and complicates the design problem. Static RAMs are simpler than dynamic RAMs.

A typical dynamic RAM cell consisting of a single MOSFET and a capacitor is shown in Figure. A dynamic RAM consists of an array of such memory cells. In this type of cell, the transistor acts as a switch. The memory cell also requires MOSFETs for READ and WRITE, gating to operated the cell. Data input is connected for storage by a WRITE control signal.

The dynamic RAM offers reduced power consumption and huge storage capacity in a single memory chip.


Advantages of DRAM over SRAM :

• DRAMs, due to their simple cell structure have 4 times the density of SRAMs. This permits 4 times the memory of SRAMs on a board of the same size.

• The cost of DRAMs for each bit of storage is nearly one-fifth that of SRAMs.

• DRAMs have lower power consumption as compared to SRAMs. So, smaller and cheaper power supplied can be used for DRAMs and also the cost of the overall system can be reduced.

• Because of their high capacity and low power consumption, DRAMs are used in the main internal memory of most personal computers.

Disadvantages of DRAM over SRAM :

• DRAMs are slower in speed and more complex as compared to SRAMs.

• DRAMs require refreshing operation after regular intervals, whereas SRAMs do not require any refreshing.

• DRAMs can not be used where only a small amount of memory, typically less than 64kB and high speed is required.

Difference between Static Ram and Dynamic Ram in tabular form:

Static RAM

Dynamic RAM

1. Stored data is retained as long as power is ON.
1. Stored data gets lost and refreshing is needed.
2. Stored data do not change with time.
2. Stored data changes with time.
3. Consumes more power.
3. Consumes less power.
4. Expensive.
4. Economical.
5. Construction is complex.
5. Construction is simple.
6. Low packing density.
6. High packing density.
7. No refreshing is required and hence the operation is easy.
7. Refreshing is needed with additional memory circuitry and hence complicates the operation.
8. No maintenance is required.
8. Maintenance is required.

Thursday, 28 November 2019

ROM Working, Types and Application

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Working Principle of ROM

Though the current ROMs do not make use of diode matrix but it will help in understanding the working of ROM. The principle on which ROM is based can be explained with the help of Figure which shows storage in a diode ROM. A storage cell is capable of storing a 1 or a 0. A ROM can be formed from a matrix of diodes and such memories are called diode matrix ROM.
Storage in a Diode ROM
A storage cell in the diode matrix is located at each intersection of a row line and column line. For storing a 0 the diode is not connected and for storing a 1 the diode is connected at the intersection of row line and column line as shown in Figure. These coupling binary cells are normally fixed at the time of manufacture and cannot be altered later. These ROMs are either made as per users requirement or are available in standard form for special purposes such as binary-to-gray code conversion for determining the squares of numbers etc.

For explaining the working of diode matrix ROM let us consider a memory which can store eight words each of 4 bits and any one of these words can be read whenever required. The connections for such a ROM are shown in Figure. The words to be stored from location 0 to 7 are given in Table - 1.

For addressing or accessing eight locations we need eight lines or we may use a three line to eight line decoder and we will have only three address lines. This decoder is made inside the ROM and with the three input address line we can access any of the eight locations by assigning proper input address signal. The address signal required for accessing any location from 0 - 7 is shown in Table - 2 and is called look up table for ROM.

Table – 1, Location or Address and Words to be Stored

Location or Address
Word to be stored

Table – 2, Binary Address or Input Signal and Stored Word

Address A2 A1 A0
Stored word D2 D1 D0

In Figure, we have shown the use of three to eight line decoder. We may extend the capacity of such memories. In general, a ROM with capacity to store in words for n bits is known as m x n ROM and is represented as shown in Figure.

Working : Let us see how the circuit of ROM (Figure) works. The address of location which we wish to read is given as input signal to the decoder say we wish to read the contents of 5th location so we give input 101 to the decoder and decoder will activate the line corresponding to 5th row. As no diode is connected between 5th row line and 1, 2, 3 and 4th column line so all the outputs will be 0s, i.e., the output word will be 0000 and this is what we expect. Now let us examine what happens when 011 is the address signal, this signal will make the decoder to activate line corresponding to 3rd row line. As no diode is connected between this row line and first column line the output D3 will be a 0, at the junction of row line 3 and column line 2, a diode is connected, so D2 will be a 1. Similarly D1 will be a 0 and D0 will be a 1. Hence the word at the output D3 D2 D1 D0 will be 0101 as we should have.

Types of ROM in Digital Electronics

Depending upon the methodology of programming, crusing and reprogramming information into ROMs, these are classified as

1. Mask-Programmed ROM (MROM)
2. Programmable ROM (PROM)
3. Erasable Programmable ROM (EPROM)
4. Electrically Erasable Programmable ROM (EEPROM)
5. Flash ROM.

• MROMs are permanently programmed by the manufacturer during the fabrication process by using a custom designed mask as per the system design specification. These are non-programmable ROMs.

• In case of PROMs, the programming is done by the customer with the help of a special gadget called PROM programmer. These are one time programmable ROMs.

• An EPROM can be erased and reprogrammed as many times as desired. Once programmed, it is non-volatile, i.e., it holds the stored data indefinitely.

• The stored information can however be erased by exposing the chip to ultra-violet (UV) radiation through a transparent window on the top of the chip meant for the purpose. These EPROMs are referred to as UVEPROMs.

• EEPROMs, also called EAPROMs (Electrically Alterable PROMs) can be erased and programmed by the application of controlled electric pulses the IC. EEPROM is a rugged, low power low density semiconductor device and it occupies less space. The cost is higher as compared to EPROMs.

• The flash memory combines the low cost and high density features of EPROM and in-circuit electrical erasability of EEPROM without compromising on the high speed access of both.

Differences between EPROM and UVEPROM

S. No.
Erasure and programming is done with electrical pulses.
Erasure and programming is done with ultra violet light.
Erasure and reprogramming is possible when the EEPROM is still in the circuit.
Erasure and reprogramming should be done by taking out the UVEPROM chip from the circuit.
Easy to construct
Difficult to construct.
The voltage on the floating gate permits the storage.
The photocurrent from the insulated gate structure permits the storage.
Speed of operation is more.
Speed of operation is less.
Shorter time of erasure.
Longer time of erasure.
Ability to erase and write individual bytes
Ability to erase and write memory array.
Low density
High density.
Suitable for field and remote control applications.
Suitable for experimental projects, product development and college labs.

Applications of ROM in Digital Electronics

Some of the applications of ROM in Digital Electronics are

1. ROMs are used for a variety of tasks within a digital system. They can be used as a direct substitute for any random logic of AND, OR and NOR gates.

2. ROMs are used to store bootstrap program that loads operating system program available in secondary memory and language interpreters in personal and business computers and to store, monitor or control programs in microcomputer and microprocessor based systems like electronic games, electronic cash registers, electronic scales and microcomputer controlled automobile fuel injection.

3. A very significant application of MOS ROM is for character generation. This includes display control for moving billboards and LED arrays.

4. They can also be used for code conversion, e.g., ASCII to EBCDIC conversion.

5. A ROM and a DAC can be used to generate sine waves, saw-tooth waves, triangular waves and square waves.

6. A ROM can be used to implement any or a set of logic expressions and is therefore used in the design of combinational circuits.

Tuesday, 26 November 2019

RAM and ROM in Digital Electronics

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Random Access Memory (RAM in Digital Electronics) is a type of memory device where the data can be accessed randomly. The term usually refers to random access read / write memory RAMs are basically sequential circuits (flip-flops). When the power is switches off, the data stored in a RAM is lost. Hence, RAMs are also called volatile memories. The name RAM comes from the fact that data can be stored into and retrieved from these cells at random.

RAM is the main memory of a computer. The speed of a computer CPU may be hundred of mega hertz. For example, Intel Pentium - IV chip operates at speed of 1500 MHz. However, this speed is limited by various factors, a major one among them being the speed of the RAM used in the computer. RAMs are read-write-erasable memories.
Read Only Memory (ROM in Digital Electronics) is a type of memory where data are permanently stored and can only be read, not written. ROM enclose permanently and semi-permanently accumulated data which can be read from the memory other than either cannot be changed at all or cannot be changed without specific equipment. A ROM stores data that are used repeatedly in system applications, such as tables, conversions, or programmed instructions for systems initialization and operation. ROMs retain stored data when the power is off and are therefore called non-volatile memories.




1. Data storage power is switched off.
Temporary, Vanishes when not affect data stored
Permanent, Power failure does
2. Data entry
Data can be entered fast. No special program-writers are required.
Special program-writers are required. So. data entry is normally slow. But this is being rectified.
3. Data read out time it is entered.
Data is read-out at the same entered.
Data is read-out only after it is
4. Repeatability of read out
Once the data is erased, it cannot be recovered.
Data is not erased; hence, it can be read out any number of times.
5. Erasability operation.
Easily erasable, very fast operation
Not easily erasable, slower
6. Packing density
Currently large
Currently low
7. Technology used
Bipolar, NMOS. CMOS (currently CMOS, mostly)
Bipolar, NMOS, CMOS (currently CMOS, mostly)
8. Static/dynamic operations
static and dynamic operations exist.
Only static ROMs exist.
9. Sense-amplifiers in many cases.
Sense amplifiers are required are required.
Normally, no sense amplifiers
10. Data-entry circuit
Separate address decoders are used for X and Y addressing.
Decoders are used for data entry (X-address), and multiplexers (Y-address) are used for data output.
11. Use of memory
Data entered in any computer is first stored in RAM. If the data is to be stored permanently it will be saved in the hard disk.
This memory stores data of a permanent nature such as operating-system commands, look-up tables, etc., which are not stored in hard disks.

Sunday, 24 November 2019

Semiconductor Memory in Digital Electronics

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Numerous developments in semiconductor technology has emerged into large numbers ot LSI and MSI memory devices, called memory chips. Besides being faster and compatible with CPU they are economical also( semiconductor memory is organized as a rectangular array (preferably square array) of storage cells that are integrated on a silicon wafer and are available in DIP packages)tue to this organization any memory cell can be accessed randomly, thus all the semiconductor memories are called Random Access Memory (RAM). The basic meitiory cell may be a flip-flop, or a charge storing element like capacitor that is said to hold logic 1 when charged and logic 0 when discharged. The type of transistors e.g. bipolar, or MOS, or CMOS, used to form a memory cell dictates the storage capacity and speed of a memory chip.

Basic Memory Unit :

A simple block diagram of a memory unit can be drawn as shown in Figure. It contains data bus, address bus and control signals. A bus is a group of parallel conductors whose job is to carry binary information. Each of the conductors carries 1-bit information. The number of bits that a memory data bus carries simultaneously is called memory bus width. Usually, but not necessarily, the memory word length and memory bus widths are same.
Block Diagram of Memory Unit
The m-bit data bus is used to transfer data to and from the memory. The n-bit address bus carries the address of memory locations. An n-bit address bus can access upto 2n storage cells i.e., storage capacity is 2n bits as each cell can store 1-bit. The Read (RD) & Write (WR) signals specify the operation to be performed. As shown the two signals are active low i.e., they are activated when logic '0'. At any time either of the two signals can be activated. In some memory chips read & write controls are available at one signal line. Chip select signal (CS) (sometimes labeled as enable EN also) is used to enable/disable the chip in multiple memory system. It is also a logic low signal. There may be multiple enable signals in a single chip that must be activated simultaneously to enable the chip.

Friday, 22 November 2019

Types of Memories in Digital Electronics

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Memories are classified as

(i) Registers, Main memory and Secondary memory.
(ii) Sequential Access Memory and Random Access Memory.
(iii) Static and Dynamic Memory.
(iv) Volatile and Non-Volatile Memory.
(v) Magnetic and Semiconductor Memory.

(i) Registers, Main Memory and Secondary Memory

Though memories are scattered throughout the computer, those based on the location and usage are called Registers, Main memory and Secondary memory. Registers are available within the CPU to store data temporarily during arithmetic and logical operations like addition, subtraction, AND, OR, etc. They have very low access time, as they are available inside the CPU. Main memories of a computer, usually of semiconductor type, are available external to the CPU to store program and data during the execution of a program. In the main memory, each memory location is identified by an unique address and is accessed for read/write operation in a lesser speed than registers. As the storage capacity of main memory is inadequate, secondary or auxiliary memories are added to enhance storage capabilities. This secondary memory operates at a lesser speed when compared to registers and main memory. Normally, secondary memories are of magnetic memory type (Magnetic tape, Magnetic drum, Floppy disk and Hard disk) that are used to store large quantities of data.

(ii) Sequential Access Memory and Random Access Memory

Based on the method of access, memory devices can be classified as Sequential Access and Random Access Memories (RAM). The access time of a sequential memory varies depending on the location to be accessed. An example of sequential access memory is the magnetic tape memory.
On the other hand, a random access memory is one in which any location can be accessed in a random manner and thus has equal access time for all memory locations.
An example of random access memory is the semiconductor RAM.

(iii) Static and Dynamic Memory:

In static memory, the content does not change with time, in dynamic memory, its content changes with time. Dynamic memory cells use the capacitance of a transistor as the storage device. Only one transistor is needed to store one bit of information. The capacitor must be refreshed periodically without being discharged in order to prevent loss of information. Static memory devices require no refreshing and hold data as long as d.c. power is applied. Examples for static memory are register and MOS cell; semiconductor dynamic RAM and circulating registers using Charge-coupled Devices (CCD) are examples of dynamic memory.

(iv) Volatile and Non-Volatile Memory :

Volatile memory loses its stored data when power to the memory circuit is removed; a non-volatile memory retains stored data permanently even after the power supply is turned OFF. Magnetic Core Memory and Read Only Memory (ROM) are examples of non-volatile memory devices.

(v) Magnetic and Semiconductor Memory:

These memories are classified based on the material used for construction. The magnetic memories are constructed using magnetic material, e.g. magnetic tape, floppy and compact disks. Magnetic recording is the process of storing data magnetically on the surface of a tape, disk or drum. Magnetic tape is a storage medium using the surface of a magnetic tape to hold data. Magnetic disk is a storage medium using the surfaces of a disk to hold magnetically stored data. Magnetic drum is a storage medium using the surface of a rotating magnetic drum to hold data.

Semiconductor memories are constructed out of semiconductor material using LSI and VLSI technologies. The examples of this type are Random Access Memory (RAM) and Read Only Memory (ROM).