Integrating Type Digital Voltmeter

•    The voltmeter measures the true average value of the input voltage over a fixed measuring period.

•    In contrast the ramp type DVM samples the voltage at the end of the measuring period.

•    This voltmeter employs an integration technique which uses a voltage to frequency conversion.

•    The voltage to frequency (VIF) converter functions as a feedback control system which governs the rate of pulse generation in proportion to the magnitude of input voltage.

•    Actually when we employ the voltage to frequency conversion techniques, a train of pulses, whose frequency depends upon the voltage being measured, is generated.

•    Then the number of pulses appearing in a definite interval of time is counted.

•    Since the frequency of these pulses is a function of unknown voltage, the number of pulses counted in that period of time is an indication of the input (unknown) voltage.

•    The heart of this technique is the operational amplifier acting as an Integrator.

•    Output voltage of integrator E = -Ei / RC*t

•    Thus if a constant input voltage E is applied, an output voltage E is produced which rises at a uniform rate and has a polarity opposite to that input voltage.

•     In other words, it is clear from the above relationship that for a constant input voltage the integrator produces a ramp output voltage of opposite polarity.

The basic block diagram of a typical integrating type of DVM is shown in

•    The unknown voltage is applied to the input of the integrator, and the output voltage starts to rise.

•    The slope of output voltage is determined by the value of input voltage

•     This voltage is fed a level detector, and when voltage reaches a certain reference level, the detector sends a pulse to the pulse generator gate.

•    The level detector is a device similar to a voltage comparator. The output voltage from integrator is compared with the fixed voltage of an internal reference source, and, when voltage reaches that level, the detector produces an output pulse.

•    It is evident that greater then value of input voltage the sharper will be the slope of output voltage and quicker the output voltage will reach its reference level.

•    The output pulse of the level detector opens the pulse level gate, permitting pulses from a fixed frequency clock oscillator to pass through pulse generator.

•    The generator is a device such as a Schmitt trigger that produces an output pulse of fixed amplitude and width for every pulse it receives.

•    This output pulse, whose polarity is opposite to that of and has greater amplitude, is fedback of the input of the integrator.

•    Thus no more pulses from the clock oscillator can pass through to trigger the pulse generator.

•    When the output voltage pulse from the pulse generator has passed, is restored to its original value and starts its rise again.

•    When it reaches the level of reference voltage again, the pulse generator gate is opened.

•    The pulse generator is trigger by a pulse from the clock generator and the entire cycle is repeated again.

•    Thus, the waveform of is a saw tooth wave whose rise time is dependent upon the value of output voltage and the fail time is determined by the width of the output pulse from the pulse generator.

•    Thus the frequency of the saw tooth wave is a function of the value of the voltage being measured.

•    Since one pulse from the pulse generator is produced for each cycle of the saw tooth wave, the number of pulses produced in a given time interval and hence the frequency of saw tooth wave is an indication of the voltage being measured.

Ramp Type Digital Voltmeter

•    The operating principle of a ramp type digital voltmeter is to measure the time that a linear ramp voltage takes to change from level of input voltage to zero voltage (or vice versa).

•    This time interval is measured with an electronic time interval counter and the count is displayed as a number of digits on electronic indicating tubes of the output readout of the voltmeter.

•    The conversion of a voltage value of a time interval is shown in the timing diagram of Fig.

•    At the start of measurement a ramp voltage is initiated.

•    A negative going ramp is shown in Fig. but a positive going ramp may also be used.

•    The ramp voltage value is continuously compared with the voltage being measured (unknown voltage).

•    At the instant the value of ramp voltage is equal to that of unknown voltage.

•     The ramp voltage continues to decrease till it reaches ground level (zero voltage).

•    At this instant another comparator called ground comparator generates. a pulse and closes the gate.

•    The time elapsed between opening and closing of the gate is t as indicated in Fig.

•    During this time interval pulses from a clock pulse generator pass through the gate and are counted and displayed.

•    The decimal number as indicated by the readout is a measure of the value of input voltage.

•    The sample rate multivibrator determines the rate at which the measurement cycles are initiated.

•    The sample rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage.

•    At the same time it sends a pulse to the counters which set all of them to 0.

•    This momentarily removes the digital display of the readout.

Rectifier Type Instruments

•    Rectifier type instruments are used for measurement of ac. voltages and currents by employing a rectifier element, which converts a.c. to a unidirectional d.c. and then using a meter responsive to d.c. to indicate the value of rectified a.c.

•    The indicating instrument is PMMC instrument, which uses a d’Arsonval movement.

•    This method is very attractive since PMMC instruments have a higher sensitivity than the electrodynamometer or the moving iron instruments. The arrangement which employs a full wave

(Fig) voltmeter using full wave rectifier

Digital Voltmeter

•    A digital voltmeter (DVM) displays the value of a.c. or d.c. voltage being measured directly as discrete numerals in the decimal number system.

•     Numerical readout of DVMs is advantageous since it eliminates observational errors committed by operators.

•    The errors on account of parallax and approximations are entirely eliminated.

•    The use of digital voltmeters increases tile speed with which readings can be taken.

•    A digital voltmeter is a versatile and accurate voltmeter, which has many laboratory applications.

•    On account of developments in the integrated circuit (IC) technology, it has been possible to reduce the size, power requirements and cost of digital voltmeters.

•    In fact, for the same accuracy, a digital voltmeter now is less costly than its analog counterpart.

•    The decrease in size of DVMs on account of use of ICs, the portability of the instruments has increased.

Types of DVMs

The increasing popularity of DVMs has brought forth a wide number of types employing different circuits. The various types of DVMs in general use are

(i)    Ramp type DVM


(ii)    Integrating type DVM

(iii)    Potentiometric type DVM

(iv)    Successive approximation type DVM

(v)    Continuous balance type DVM

Electrodynamometer (Electrodynamics) Type Instruments

•    The necessity for the a.c. calibration of moving iron instruments as well as other types of instruments, which cannot be correctly calibrated, requires the use of a transfer type of instrument.

•    A transfer instrument is one that may be calibrated with a d.c. Source and then used without modification to measure a.c.

•    This requires the transfer type instrument to have same accuracy for both d.c. and a.c., which the electrodynamometer instruments have.

•    These standards are precision resistors and the Weston standard cell (which is a d.c. cell).

•     It is obvious, therefore, that it would be impossible to calibrate an a.c. instrument directly against the fundamental standards.

•    The calibration of an a.c. instrument may be performed as follows.

•     The transfer instrument is first calibrated on d.c.

•    This calibration is then transferred to the a.c. instrument on alternating current, using operating conditions under which the latter operates properly.

•    Electrodynamics instruments are capable of service as transfer instruments.

•    Indeed, their principal use as ammeters and voltmeters in laboratory and measurement work is for the transfer calibration of working instruments and as standards for calibration of other instruments as their accuracy is very high.

•    Electrodynamometer types of instruments are used as a.c. voltmeters and ammeters both in the range of power frequencies and lower part of the audio power frequency range. They are used as wattmeters, voltmeters and with some modification as power factor meters and frequency meters.

Operating Principle

•    It would have a torque in one direction during one half of the cycle and an equal effect in the opposite direction during the other half of the cycle.

•    If the frequency were very low, the pointer would swing back and forth around the zero point.

•     However, for an ordinary meter, the inertia is so great that on power frequencies the pointer does not go very far in either direction but merely stays (vibrates slightly) around zero.

•     If, however, we were to reverse the direction of the flux each time the current through the movable coil reverses, a unidirectional torque would be produced for both positive and negative halves of the cycle.

•     In electrodynamometer instruments the field can be made to reverse simultaneously with the current in the movable coil if the field (fixed) coil is connected in series with the movable coil.


Construction

Fixed Coils

•    The field is produced by a fixed coil.
•     This coil is divided into two sections to give a more uniform field near the centre and to allow passage of the instrument shaft.

Moving Coil

•    A single element instrument has one moving coil.
•    The moving coil is wound either as a self-sustaining coil or else on a non-metallic former.
•    A metallic former cannot be used as eddy current would be induced in it by the alternating field.
•    Light but rigid construction is used for the moving coil.
•     It should be noted that both fixed and moving coils are air cored.

Control

•    The controlling torque is provided by two control springs.
•    These springs act as leads to the moving coil.

Moving System

•    The moving coil is mounted on an aluminum spindle.
•    The moving system also carries the counter weights and truss type pointer.
•    Sometimes a suspension may be used in case a high sensitivity is desired.

Damping

•    Air friction damping is employed for these instruments and is provided by a pair of aluminum vanes, attached to the spindle at the bottom.
•    These vanes move in sector shaped chambers.
•    Eddy current damping cannot be used in these instruments as the operating field is very weak (on account of the fact that the coils are air cored) and any introduction of a permanent magnet required for eddy current damping would distort the operating magnetic field of the instrument.

Shielding

•    The field produced by the fixed coils is somewhat weaker than in other types of instruments
•    It is nearly 0.005 to 0.006 Wb/m
•    In D.C. Measurements even the earth magnetic field may affect the readings.
•    Thus it is necessary to shield an electrodynamometer type instrument from the effect of stray magnetic fields.
•    Air cored electrodynamometer type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy.
•    This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication.

Cases and Scales

•    Laboratory standard instruments are usually contained in highly polished wooden cases.
•    These cases are so constructed as to remain dimensionally stable over long periods of time.
•    The glass is coated with some conducting material to completely remove the electrostatic effects.
•    Adjustable leveling screws support the case.
•    A spirit level is also provided to ensure proper leveling.
•    The scales are hand drawn, using machine sub-dividing equipment.
•    Diagonal lines for fine sub-division are usually drawn for main markings on the scale.
•    Most of the high-precision instruments have a 300 mr scale with 100, 120 or 150 divisions.



Torque Equation

Let,
    i1 = instantaneous value of current in the fixed coils: A.

    i2   = instantaneous value of current in the moving coil: A.

    L1 = self-inductance of fixed coils: H.

    L2 = self-inductance of moving coils H,

    M = mutual inductance between fixed and moving coils:

Flux linkages of coil 1, ψ1 = L1 i1 + Mi2
 
Flux linkages f coil 2, ψ2  = L2 i2 + Mi1
 
Electrical input energy     = e1i1dt+e2i2dt

Errors in Electrodynamometer Instruments

i)    Frequency error

ii)    Eddy current error

iii)    External magnetic field

iv)    Temperature changes

Advantages
i)    These instruments can be used on both a.c & d.c

ii)    Accurate rms value

Disadvantages

(i) They have a low torque/weight ratio and hence have a low sensitivity.

(ii) Low torque/weight ratio gives increased frictional losses.

(iii) They are more expensive than either the PMMC or the moving iron type instruments.

(iv) These instruments are sensitive to overloads and mechanical impacts. Therefore, they must be handled with great care.

(v) The operating current of these instruments is large owing to the fact that they have weak magnetic field. The flux density is about 0.006 Wb/m as against 0.1 to 0.5 Wb/m in PMCC instruments

(vi) They have a non-uniform scale.

INTRODUCTION TO ELECTRICAL AND ELECTRONIC INSTRUMENTS

•     Basically an electrical indicating instrument is divided into two types. They are 
•       i) Analog instruments              ii) Digital Instruments.
•     Analog instruments are nothing but its output is the deflection of pointer, which is proportional to its input.
•     Digital Instruments are its output is in decimal form.
•     Analog ammeters and voltmeters are classed together as there are no fundamental differences in their operating principles.
•     The action of all ammeters and voltmeters, with the exception of electrostatic type of instruments, depends upon a deflecting torque produced by an electric current.
•     In an ammeter this torque is produced by a current to be measured or by a definite fraction of it.
•     In a voltmeter a current produces this torque, which is proportional to the voltage to be measured.
•     Thus all analog voltmeters and ammeters are essentially current measuring devices.

The essential requirements of a measuring instrument are

(i)    That its introduction into the circuit, where measurements are to be made, does not alter the circuit conditions;

(ii)     The power consumed by them for their operation is small.

Ammeters are connected in series in the circuit whose current is to be measured. The power loss in an ammeter is I2Ra where I is the current to be measured and R is the resistance of ammeter. Therefore, ammeters should have a low electrical resistance so that they cause a small voltage drop and consequently absorb small power.

Voltmeters are connected in parallel with the circuit whose voltage is to be measured. The power loss in voltmeters is V where V is the voltage U) be measured and R is the resistance of voltmeter. The voltmeters should have a high electrical resistance, in order that the current drawn by them is small and consequently the power consumed is small.

Types of instruments

 The main types of instruments used as ammeters and voltmeters are

(i)    Permanent magnet moving coil (PMMC)   

(ii)     Moving iron

(iii)     Electro-dynamometer   

(iv)     Hot wire

(v)     Thermocouple   

(vi)     Induction

(vii)     Electrostatic

(viii)     Rectifier.

Classification of Moving Iron Instruments

Moving iron instruments are of two types

(i) Attraction type.

(ii) Repulsion type.




 Attraction type.

 

•     The coil is flat and has a narrow slot like opening.

•     The moving iron is a flat disc or a sector eccentrically mounted.

•     When the current flows through the coil, a magnetic field is produced and the moving iron moves from the weaker field outside the coil to the stronger field inside it or in other words the moving iron is attracted in.

•     The controlling torque is provide by springs hut gravity control can be used for panel type of instruments which are vertically mounted.

•     Damping is provided by air friction with the help of a light aluminium piston (attached to the moving system) which move in a fixed chamber closed at one end as shown in Fig. or with the help of a vane (attached to the moving system) which moves in a fixed sector shaped chamber a shown.

Repulsion Type

      In the repulsion type, there are two vanes inside the coil one fixed and other movable. These are similarly magnetized when the current flows through the coil and there is a force of repulsion between the two vanes resulting in the movement of the moving vane. Two different designs are in common use

(I) Radial Vane Type

•     In this type, the vanes are radial strips of iron.

•     The strips are placed within the coil as shown in Fig.

•     The fixed vane is attached to the coil and the movable one to the spindle of the instrument.

                    (a)    Radial vane type.                         (b) Co-axial vane type

(ii) Co-axial Vane Type

•     In this type of instrument, the fixed and moving vanes are sections of co axial cylinders as shown in Fig.
•     The controlling torque is provided by springs. Gravity control can also he used in vertically mounted instruments.
•     The damping torque is produced by air friction as in attraction type instruments.
•     The operating magnetic field in moving iron instruments is very weak and therefore eddy current damping is not used in them as introduction of a permanent magnet required for eddy current damping would destroy the operating magnetic field.
•     It is clear that whatever may be the direction of the current in the coil of the instrument, the iron vanes are so magnetized that there is always a force of attraction in the attraction type and repulsion in the repulsion type of instruments.
•     Thus moving iron instruments are unpolarised instruments i.e., they are independent of the direction in which the current passes.
•      Therefore, these instruments can be used on both ac. and D.C.

Torque Equation of Moving Iron Instrument:

         Considering the energy relations when there is a small increment in current supplied to the instrument may derive an expression for the torque moving iron instrument. When this happens there will be a small deflection dθ a mechanical work will be done. Let Td be the deflecting torque.

Mechanical work done = Td. dθ

Alongside there will be a change in the energy stored in the magnetic field owing to change in inductance.
Suppose the initial current is I, the instrument inductance L and the deflection θ. If the current is increased by di then the deflection changes by dθ and the inductance by dL. In order to affect an increment the current there must be an increase in the applied voltage given by

Comparison between Attraction and Repulsion Types of Instruments

•    In general it may be said that attraction-type instruments possess the same advantages, and are subject to the limitations, described for the repulsion type.

•    An attraction type instrument will usually have a lower inductance than the corresponding repulsion type instrument, and voltmeters will therefore be accurate over a wider range of frequency and there is a greater possibility of using shunts with ammeters.

•    On the other hand, repulsion instruments are more suitable for economical production in manufacture, and a nearly uniform scale is more easily obtained; they are, therefore, much more common than the attraction type.

Errors in Moving Iron Instruments

There are two types of errors, which occur in moving iron instruments — errors which occur with both a.c. and D.C. and the other which occur only with ac. only.

Errors with both D.C. and A.C

i)    Hysteresis Error

ii)    Temperature error

iii)    Stray magnetic field

Errors with only A.C

 Frequency errors

Advantages & Disadvantages

1) Universal use
(2) Less Friction Errors
(3) Cheapness
(4) Robustness
(5) Accuracy
(6) Scale
(7) Errors
(8) Waveform errors.

Permanent Magnet Moving Coil Instrument (PMMC)

The permanent magnet moving coil instrument is the most accurate type for D.C. Measurements. The working principle of these instruments is the same as that of the d’Arsonval type of galvanometers, the difference being that a direct reading instrument is provided with a pointer and a scale

(Fig) Permanent magnet moving coil instrument

Construction of PMMC Instruments

•     The constructional features of this instrument are shown in Fig.
•     The moving coil is wound with many turns of enameled or silk covered copper wire.
•     The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings.
•     The coils move freely in the field of a permanent magnet.
•     Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping.
•     Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt.
•     The coil itself, therefore, provides electro magnetic damping.

Magnet Systems

•     Old style magnet system consisted of relatively long U shaped permanent magnets having soft iron pole pieces.
•     Owing to development of materials like Alcomax and Alnico, which have a high co-ercive force, it is possible to use smaller magnet lengths and high field intensities.
•     The flux densities used in PMIMC instruments vary from 0.1 Wb/m to 1 Wb/m.

Control

•     When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.
•     These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension as shown.
•     This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

Damping

•     Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.

Pointer and Scale

•     The pointer is carried by the spindle and moves over a graduated scale.
•     The pointer is of lightweight construction and, apart from those used in some inexpensive instruments has the section over the scale twisted to form a fine blade.

•      This helps to reduce parallax errors in the reading of the scale. When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.
•     These springs also serve to lead current in and out of the coil.

Torque Equation.

The torque equation of a moving coil instrument is given by

As the deflection is directly proportional to the current passing through the meter (K and G being constants) we get a uniform (linear) scale for the instrument.

 Errors in PMMC Instruments

The main sources of errors in moving coil instruments are due to

•     Weakening of permanent magnets due to ageing at temperature effects.
•     Weakening of springs due to ageing and temperature effects.
•     Change of resistance of the moving coil with temperature.

Advantages and Disadvantages of PMMC Instruments

The main advantages of PMMC instruments are

•     The scale is uniformly divided.
•     The power consumption is very low
•     The torque-weight ratio is high which gives a high accuracy. The accuracy is of the order of generally 2 percent of full-scale deflection.
•     Using different values for shunts and multipliers may use a single instrument for many different current and voltage ranges.
•     Since the operating forces are large on account of large flux densities, which may be as high as 0.5 Wb/m, the errors due to stray magnetic fields are small.
•     Self-shielding magnets make the core magnet mechanism particularly useful in aircraft and aerospace applications.

The chief disadvantages are

•     These instruments are useful only for D.C. The torque reverses if the current reverses. If the instrument is connected to a.c., the pointer cannot follow the rapid reversals and the deflection corresponds to mean torque, which is zero. Hence these instruments cannot be used for a.c.
•     The cost of these instruments is higher than that of moving iron instruments.

BOUNDARY CONDITION FOR PERFECT DIELECTRICS

Consider the interface or the boundary between two dielectrics with different properties, L H  }1    }o}r1  DQG }2    }o}r2  are the permittivities of the two media 1 and 2. Assume that there are no free charges at the interface between the two media. If we construct a pillbox with infinitesimally small thickness and area.

if D->1  and D->2 are the electric flux densities or the displacement densities in the media 1and 2 respectively, then since there are no charges enclosed, it follows from Gauss’s law that  the  surface  integral of D  over  the  pill box surface  is zero.  Thus,  D->1.n1^ûV    D->2 .n^2ûV    ZKHUH ûs is the pillbox surface (top and bottom) and n^1  and n^2  are the unit outward normals respectively. Thus, (D->1  - D->2).n^1  =0. Since n^1  is the unit normal to the  interface,  D->1  -  D->2  =  0,  where  D->1  and  D->2  are  the  normal  components  of  the electric  flux densities  in the respective media. Thus D->1  = D->2.  The normal components of  displacement  densities  are  equal  at  the  boundary  between  two  dielectric  media.  In other  words,  the  normal  component  of  the  electric  flux  density  across  the  charge  free interface  between  the  two  media  is  continuous,  meaning  that  the  number  of  lines  of displacement flux entering one face is the same as the number leaving the other face.

The above relation is useful in evaluation of the flux density and hence field intensity- components in one medium due to those in the adjacent medium.

Short Answers In MC 1702 – MICRO PROCESSOR AND ITS APPLICATIONS

1.  Define Microprocessor?

Microprocessor is a multipurpose, programmable, clock-driven, register based electronic  device  that  reads  binary  instructions  from  a  storage  device  called memory,  accepts  binary  data  as  input  and  processes  data  according  to  those instructions, and provides as output.




2.  What is Hardware and Software?

The physical components of the system i.e. computer are called Hardware. Group of programs is called software.

3.  Why the microprocessor is viewed as a programmable Device?

Microprocessor is programmable because it can be instructed to perform given tasks within its capability. Microprocessor is designed to understand and execute many binary instructions.

4.  What is Central processing Unit ( CPU ) ? And Write the use of  it.

CPU is a heart of the computer. Central processing Unit controls the operation of the computer. In  a microcomputer the CPU is a microprocessor. The CPU fetches binary coded instructions from memory, decodes the instructions into a series of simple actions and carries out these actions in a sequence of steps.

5.  What is a chip?

A chip is also called an integrated circuit. Generally it is a small, thin piece of silicon onto which the transistors making up the microprocessor have been etched. A chip might be as large as an inch on a side and can contain tens of millions  of  transistors.  Simpler  processors  might  consist  of  a  few  thousand transistors etched onto a chip just a few millimeters square.

6.  What is System Bus?

The System bus is a communication path between the microprocessor and peripherals. It is nothing but a group of wires to carry bits.

7.    What is Address Bus?

The address bus consists of 16, 20, 24 or 32 parallel signal lines. On these lines the CPU sends out the address of the memory location that is to be written to or read from.   The number of address lines determines the number of memory locations that the CPU can address. If the CPU has N address lines, then it can directly address 2N  memory locations. Simply, we can say that Address Bus is
used to carry the address.


8.  What is Data Bus?

The data bus consists of 8, 16, or 32 parallel signal lines. The data bus lines are bidirectional. This means that the CPU can read data in from memory or from a port on these lines, or it can send data out to memory or to a port on these lines. Simply we can say that data bus is used to carry the data.

9.    What is Assembly Language?

A  medium  of  communication  with  a  computer  in  which  programs  are written in mnemonics. Binary instructions are given abbreviated names called mnemonics, which form the assembly language for a given processor.

10. What is Machine Language?

The binary medium of communication with a computer through a designed set of instructions specific to each computer.

11.  What is Bit-Slice processor?

For some Applications , general purpose CPUs such as the 8080 and 6800 are  not  fast  enough  or  do  not  have  suitable  instruction  sets.  For  these applications ,several manufacturers produce devices which can be used to build the custom CPU. This family includes 4 bit ALUs, multiplexers, sequencers and other parts needed for custom building a CPU. The term slice comes from the fact that these parts can be connected in parallel to work with 8 bit words, 16- bit words, or 32 bit words.

12.  What is microcontroller?

Microcontroller is a   Device that includes microprocessor, memory and
I/O signal lines on a single chip, fabricated using VLSI technology.


13.  List the  main applications of 8 bit microprocessors?

8  bit  microprocessors  is  used  in  a  variety  of  applications  such  as appliances, automobiles, industrial process and control applications.

14.  Write the uses of microprocessors in Medical Instrumentation field?

Patient Monitoring in Intensive Care Unit, Pathological Analysis and the measurement of parameters like blood pressure and temperature.

15.  Define  Real Time Systems :

Real Time Systems are those in which timeliness is as important as the correctness of the outputs, although this does not mean that they have to be “fast systems”.

16.  List the limitations of 8 bit microprocessor:

•    Lower Execution Speed
•    It can address less memory size
•    Few instructions are available


17.  What do you mean ‘ Data Width’?

Data Width is the width of the ALU. An 8 bit ALU can add / subtract/ multiply etc.. two 8 bit numbers .  In many cases, the external data bus is the same width as the ALU, but not always. The 8088 had a 16 bit ALU and 8 bit bus , while the modern Pentiums fetch data 64 bits at a time for their 32 bit ALUs.

18.  Draw and specify the complete bit configuration of 8085  flag Register?




S- Sign Flag  .  If D7 =1 , then sign flag is set, otherwise rest.

Z-Zero  flag.    If  ALU  operation  results  in  zero,  then  this  flag  is  set, Otherwise it is reset.

AC-Auxilliary flag. In an arithmetic operation ,when a carry is generated by digit D3 and passed on to digit D4, the AC flag is set. Otherwise it is reset.

P-Parity Flag. If the result of an arithmetic or logic operation has an even number of 1’s then this flag is set. Otherwise it is reset.

CY-Carry Flag. If an arithmetic operation results in a carry, the carry flag
is set. Otherwise it is reset.


19.  List  the  four  operations  commonly  performed  by  MPU(  Micro  processing
Unit)?

•    Memory Read : Reads data (or instructions) from memory.
•    Memory Write: Writes Data (or instructions) into memory.
•    I/O Read: Accepts data from input devices.


•    I/O Write: Sends data to output devices.



20.  Write about RST pins in 8085?
In 8085 ,three RST pins are available, such as RST 7.5 ,RST 6.5 , RST
5.5.  RST  represents  Restart  Interrupts.  These  are  vectored  interrupts  that transfer the program control to specific memory locations. They have higher priorities than the INTR  interrupt.  Among these three,  the priority order is

7.5,6.5,5.5.

DELAY ROUTINE OF INTEL 8085

Delay routines are subroutines used for maintaining the timings of various operations in microprocessor.

In control applications, certain equipment needs to be ON/OFF after a specified time delay. In some applications, a certain operation has to be repeated after a specified time interval. In such cases, simple time delay routines can be used to maintain the timings of the operations.


DELAY ROUTINE PROCESS

A delay routine is generally written as a subroutine (It need not be a subroutine always. It can be even a part of main program). In delay routine a count (number) is loaded in a register of microprocessor. Then it is decremented by one and the zero flag is checked to verify whether the content of register is zero or not. This process is continued until the content of register is zero. When it is zero, the time delay is over and the control is transferred to main program to carry out the desired operation.

The delay time is given by the total time taken to execute the delay routine. It can be computed by multiplying the total number of T-states required to execute subroutine and the time for one T-state of the processor. The total number of T-states can be computed from the knowledge of T-states required for each instruction. The time for one T-state of the processor is given by the inverse of the internal clock frequency of the processor.

For example, if the 8085 microprocessor has 5 MHz quartz crystal then,

The internal clock frequency = 5 / 2 = 2.5 MHz

Time for one T-state= 1 / 2.5 x 106 = 0.4µsec 

•    For small time delays (< 0.5 msec) an 8- bit register can be used.
•    For large time delays (< 0.5 Sec) l6-bit register should be used.
•    For very large time delays (> 0.5 sec), a delay routine can be repeatedly called in the main program.

The disadvantage in delay routines is that the processor time is wasted. An alternate solution is to use dedicated timer like 8253/8254 to produce time delays or to maintain timings of various operations.

Two example delay routines are presented in this section with details of timing calculations.

EXAMPLE DELAY ROUTINE -1

Write a delay routine to produce a time delay of 0.5 msec in 8085 processor-based system whose clock source is 6 MHz quartz crystal.

Solution
The delay required is 0.5 msec, hence an 8-bit register of8085 can be used to store a Count value and then decrement to zero. The delay routine is written as a subroutine as shown below.

Delay routine

MVI D, N         ;  Load the count value, N in D-register.
Loop:    DCR D         ;  Decrement the count.
JNZ Loop         ;  If count is zero go to
RET             ;  Return to main program.

PROGRAMMING EXAMPLES:

TIMING DIAGRAM for various machine cycles of INTEL 8085

PROCESSOR CYCLES

•    The sequence of operations that a processor has to carry out while executing the instruction is called Instruction Cycle.

•     Each instruction cycle of a processor in turn consists of a number of machine cycles. The machine cycles are the basic operations performed by the processor.

•     To execute an instruction, the processor executes one or more machine cycles in a particular order. The machine cycles of a processor are also called Processor Cycles.

•     The manufacturer of microprocessors defines the timings and status of various signals during the processor cycles.

    In general, the instruction cycle of an instruction can be divided into two as Fetch and Execute. The fetch cycle is executed to fetch the opcode from memory and the execute cycle is executed to decode the instruction and to perform the work instructed by the instruction.

MACHINE CYCLES OF 8085

The 8085 microprocessor has 7 (seven ) basic machine cycles. They are

1. Opcode fetch cycle ( 4 Tor 6 T )
2. Memory read cycle (3 T )
3. Memory write cycle (3 T )
4. I/O read cycle (3 T)
5. I/O write cycle (3 T)
6. Interrupt acknowledge cycle (6 Tor 12 T )
7. Bus idle cycle. ( 2 Tor 3 T )

Each instruction of the 8085 processor consists of one to five machine cycles, i.e., when the 8085 processor executes an instruction, it will execute some of the machine cycles in a specific order.

The processor takes a definite time to execute the machine cycles. The time taken by the processor to execute a machine cycle is expressed in T -states. One T -state is equal to the time period of the internal clock signal of the processor. The T -state starts at the falling edge of a clock.





TIMING DIAGRAM

The timing diagram provides information about the various condition (high state or low state or high impedance state) of the signals while a machine cycle is executed. The timing diagrams are supplied by the manufacturer of the microprocessor. The timing diagrams are essential for a system designer. Only from the knowledge of timing diagrams, the matched peripheral devices like memories, ports, etc. can be selected to form a system with microprocessor as CPU .

The machine cycles are the basic operations performed by the processor, while instructions are executed. The time taken for performing each machine cycle is expressed in terms of  T-states. One T-state is the time period of one clock cycle of the microprocessor.

The various machine cycles are

1.  Opcode fetch ……………..    -  4 / 6 T
2.  Memory Read …………….    -  3 T
3.  Memory Write …………….    -  3 T
4.  I/O Read …………………..    -  3 T
5.  I/O Write ………………….    -  3 T
6.  Interrupt Acknowledge ……    -  6 / 12 T
7.  Bus Idle ……………………    -  2 / 3 T

The T -states required by the 8085 processor to execute each machine cycle are mentioned within brackets in the list of machine cycles, given above.

INTERRUPTS OF INTEL 8085

NEED FOR INTERRUPTS

Interrupt is a signal send by an external device to the processor, to the processor to perform a particular task or work. Mainly in the microprocessor based system the interrupts are used for data transfer between the peripheral and the microprocessor.
 
When a peripheral is ready for data transfer, it interrupts the processor by sending an appropriate signal to the interrupt pin of the processor. If the processor accepts the interrupt then the processor suspends its current activity and executes an interrupt service subroutine to complete the data transfer between the peripheral and processor. After executing the interrupt service routine the processor resumes its current activity. This type of data transfer scheme is called interrupt driven data transfer scheme.


TYPES OF INTERRUPTS
 
The interrupts are classified into software interrupts and hardware interrupts.

•    The software interrupts are program instructions. These instructions are inserted at desired locations in a program. While running a program, lf a software interrupt instruction is encountered, then the processor executes an interrupt service routine (ISR).


•    The hardware interrupts are initiated by an external device by placing an appropriate signal at the interrupt pin of the processor. If the interrupt is accepted, then the processor executes an interrupt service routine (ISR).


SOFTWARE INTERRUPTS OF 8085

The software interrupts are program instructions. When the instruction is executed, the processor executes an interrupt service routine stored in the vector address of the software interrupt instruction. The software interrupts of 8085 are RST 0, RST 1, RST 2, RST 3, RST 4, RST 5, RST 6 and RST 7.




The vector addresses of software interrupts are given in table below.

The software interrupt instructions are included at the appropriate (or required) place in the main program. When the processor encounters the software instruction, it pushes the content of PC (Program Counter) to stack. Then loads the Vector address in PC and starts executing the Interrupt Service Routine (ISR) stored in this vector address. At the end of ISR, a return instruction - RET will be placed. When the RET instruction is executed, the processor POP the content of stack to PC. Hence the processor control returns to the main program after servicing the interrupt. Execution of ISR is referred to as servicing of interrupt.


All software interrupts of 8085 are vectored interrupts. The software interrupts cannot be masked and they cannot be disabled. The software interrupts are RST0, RST1, … RST7 (8 Nos).


HARDWARE INTERRUPTS OF 8085

An external device, initiates the hardware interrupts of 8O85 by placing an appropriate signal at the interrupt pin of the processor. The processor keeps on checking the interrupt pins at the second T -state of last machine cycle of every instruction. If the processor finds a valid interrupt signal and if the interrupt is unmasked and enabled, then the processor accepts the interrupt. The acceptance of the interrupt is acknowledged by sending an INTA signal to the interrupted device.

The processor saves the content of PC (program Counter) in stack and then loads the vector address of the interrupt in PC. (If the interrupt is non-vectored, then the interrupting device has to supply the address of ISR when it receives INTA signal). It starts executing ISR in this address. At the end of ISR, a return instruction, RET will be placed. When the processor executes the RET instruction, it POP the content of top of stack to PC. Thus the processor control returns to main program after servicing interrupt. The hardware interrupts of 8085 are TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR.

Further the interrupts may be classified into VECTORED / NON-VECTORED and MASKABLE / NON-MASKABLE INTERRUPTS.

VECTORED INTERRUPT

In vectored interrupts, the processor automatically branches to the specific address in response to an interrupt.

NON-VECTORED INTERRUPT

But in non-vectored interrupts the interrupted device should give the address of the interrupt service routine (ISR).

In vectored interrupts, the manufacturer fixes the address of the ISR to which the program control is to be transferred. The vector addresses of hardware interrupts are given in table above in previous page.

•    The TRAP, RST 7.5, RST 6.5 and RST 5.5 are vectored interrupts.

•    The INTR is a non-vectored interrupt. Hence when a device interrupts through INTR, it has to supply the address of ISR after receiving interrupt acknowledge signal.


MASKABLE & NON-MASKABLE INETRRUPTS:

The hardware vectored interrupts are classified into maskable and non-maskable interrupts.

•    TRAP is non-maskable interrupt

•    RST 7.5, RST 6.5 and RST 5.5 are maskable interrupt.

Masking is preventing the interrupt from disturbing the main program. When an interrupt is masked the processor will not accept the interrupt signal. The interrupts can be masked by moving an appropriate data (or code) to accumulator and then executing SIM instruction. (SIM - Set Interrupt Mask). The status of maskable interrupts can be read into accumulator by executing RIM instruction (RIM - Read Interrupt Mask).


All the hardware interrupts, except TRAP are disabled, when the processor is resetted. They can also be disabled by executing Dl instruction. (Dl-Disable Interrupt).

•    When an interrupt is disabled, it will not be accepted by the processor. (i.e., INTR, RST 5.5, RST 6.5 and RST 7.5 are disabled by DI instruction and upon hardware reset).

•    To enable (to allow) the disabled interrupt, the processor has to execute El instruction (El-Enable Interrupt).

The type of signal that has to be placed on the interrupt pin of hardware interrupts of 8085 are defined by INTEL.

•    The TRAP interrupt is edge and level sensitive. Hence, to initiate TRAP, the interrupt signal has to make a low to high transition and then it has to remain high until the interrupt is recognized.

•    The RST 7.5 interrupt is edge sensitive (positive edge). To initiate the RST 7.5, the interrupt signal has to make a low to high transition an it need not remain high until it is recognized.

•    The RST 6.5, RST 5.5 and INTR are level sensitive interrupts. Hence for these interrupts the interrupting signal should remain high, until it is recognized.

INTERRUPT DRIVEN DATA TRANSFER SCHEME

The interrupt driven data transfer scheme is the best method of data transfer for effectively utilizing the processor time. In this scheme, the processor first initiates the I/O device for data transfer. After initiating the device, the processor will continue the execution of instructions in the program. Also at the end of an instruction the processor will check for a valid interrupt signal. If there is no interrupt then the processor will continue the execution.

When the I/O device is ready, it will interrupt the processor. On receiving an interrupt signal, the processor will complete the current instruction execution and saves the processor status in stack. Then the processor calls an interrupt service routine (ISR) to service the interrupted device. At the end of ISR the processor status is retrieved from stack and the processor starts executing its main program. The sequence of operations for an interrupt driven data transfer scheme is shown in figure below.

INSTRUCTION SET OF INTEL 8085

The 8085 instruction set can be classified into the following five functional headings.

Group I - DATA TRANSFER INSTRUCTIONS:

Includes the instructions that moves ( copies) data between registers or between memory locations and registers. In all data transfer operations the content of source register is not altered. Hence the data transfer is copying operation.

Ex: i) MOV A,B     ii) LDA 4600    iii)  LHLD 4200


Group II - ARITHMETIC INSTRUCTIONS:

Includes the instructions which performs the addition, subtraction, increment or decrement operations. The flag conditions are altered after execution of an instruction in this group.

Ex: i) ADD B     ii) SUB C    iii)  INR D    iv) INX H


Group III - LOGICAL INSTRUCTIONS:

The instructions which performs the logical operations like AND, OR, Exclusive-OR, complement, compare and rotate instructions are grouped under this heading. The flag conditions are altered after execution of an instruction in this group.

Ex: i) ORA B     ii) XRA A    iii)  RAR 


Group IV - BRANCHING INSTRUCTIONS:

The instructions that are used to transfer the program control from one memory location to another memory location are grouped under this heading. 

Ex: i) JZ 4200     ii) RST 7    iii)  CALL 4300


Group V - MACHINE CONTROL INSTRUCTIONS:

Includes the instructions related to interrupts and the instruction used to halt program execution.

Ex: i) SIM     ii) RIM    iii)  HLT


The 74 basic instructions of8085 are listed inTable-2.1. The opcode of each instruction, size, machine cycles, number of T -state and the total number of instructions in each type are also shown in table in next page. The instructions affecting the status flag are listed in table followed.

ADDRESSING MODES & INSTRUCTION FORMAT OF INTEL 8085

The 8085 have 74 basic instructions and 246 total instructions. The instruction set of 8085 is defined by the manufacturer Intel Corporation. Each instruction of 8085 has 1 byte opcode. With 8 bit binary code, we can generate 256 different binary codes. In this, 246 codes have been used for opcodes.

The size of 8085 instructions can be 1 byte, 2 bytes or 3 bytes.

•    The 1-byte instruction has an opcode alone.

•    The 2 bytes instruction has an opcode followed by an eight-bit address or data.

•    The 3 bytes instruction has an opcode followed by 16 bit address or data. While storing the 3 bytes instruction in memory, the sequence of storage is, opcode first followed by low byte of address or data and then high byte of address or data.


ADDRESSING MODES

Every instruction of a program has to operate on a data. The method of specifying the data to be operated by the instruction is called Addressing. The 8085 has the following 5 different types of addressing.

1. Immediate Addressing
2. Direct Addressing
3. Register Addressing
4. Register Indirect Addressing
5. Implied Addressing

Immediate Addressing

In immediate addressing mode, the data is specified in the instruction itself. The data will be apart of the program instruction. All instructions that have ‘I’ in their mnemonics are of Immediate addressing type.

Eg. MVI B, 3EH  - Move the data 3EH given in the instruction to B register.


Direct Addressing
In direct addressing mode, the address of the data is specified in the instruction. The data will be in memory. In this addressing mode, the program instructions and data can be stored in different memory blocks. This type of addressing can be identified by 16-bit address present in the instruction.

Eg. LDA 1050H  - Load the data available in memory location 1050H in accumulator.


Register Addressing

In register addressing mode, the instruction specifies the name of the register in which the data is available. This type of addressing can be identified by register names (such as ‘A’, ‘B’, … ) in the instruction.

Eg. MOV A, B -Move the content of  B register to A register.


Register Indirect Addressing

In register indirect addressing mode, the instruction specifies the name of the register in which the address of the data is available. Here the data will be in memory and the address will be in the register pair. This type of addressing can be identified by letter ‘M’ present in the instruction.

Eg. MOV A, M - The memory data addressed by HL pair is moved to A register.


Implied Addressing

In implied addressing mode, the instruction itself specifies the type of operation and location of data to be operated. This type of instruction does not have any address, register name, immediate data specified along with it.

Eg. CMA - Complement the content of accumulator.

INTEL 8085 ARCHITECTURE

The architecture of.8085 is shown in figure given below. The internal architecture of 8085 includes the ALU, timing and control unit, instruction register and decoder, register array, interrupt control and serial I/O control. 

OPERATIONS PERFORMED BY 8085

The ALU performs the arithmetic and logical operations.
The operations performed by ALU of 8085 are addition, subtraction, increment, decrement, logical AND, OR, EXCL U8IVE -OR, compare, complement and left / right shift. The accumulator and temporary register are used to hold the data during an arithmetic / logical operation. After an operation the result is stored in the accumulator and the flags are set or reset according to the result of the operation.


FLAG REGISTER:

There are five flags in 8085, which are sign flag (8), zero flag (Z), auxiliary carry flag (AC), parity flag (P) and carry flag (CY). The bit positions reserved for these flags in the flag register are shown in figure below.

After an ALU operation, if the most significant bit of the result is 1, then sign flag is set. The zero flag is set, if the ALU operation results in zero and it is reset if the result is non-zero. In an arithmetic operation, when a carry is generated by the lower nibble, the auxiliary carry flag is set. After an arithmetic or logical operation, if the result has an even number of 1 's the parity flag is set, other wise it is reset.

If an arithmetic operation results in a carry, the carry flag is set other wise it is reset. Among the five flags, the AC flag is used internally for BCD arithmetic and other four flags can be used by the programmer to check the conditions of the result of an operation.


TIMING & CONTROL UNIT:

The timing and control unit synchronizes all the microprocessor operations with the clock and generates the control signals necessary for communication between the microprocessor and peripherals.


INSTRUCTION REGISTER & DECODER:
When an instruction is fetched from memory it is placed in instruction register. Then it is decoded and encoded into various machine cycles.


REGISTER ARRAY:

•    Apart from Accumulator (A-register), there are six general-purpose programmable registers B, C, D, E, H and L.

•    They can be used as 8-bit registers or paired to store l6-bit data. The allowed pairs are B-C, D-E and H-L.

•    The temporary registers W and Z are intended for internal use of the processor and it cannot be used by the programmer.

•    STACK POINTER (SP):

The stack pointer SP, holds the address of the stack top. The stack is a sequence of RAM memory locations defined by the programmer. The stack is used to save the content of registers during the execution of a program.

•    PROGRAM COUNTER (PC):

The program counter (PC) keeps track of program execution. To execute a program the starting address of the program is loaded in program counter. The PC sends out an address to fetch a byte of instruction from memory and increment its content automatically. Hence, when a byte of instruction is fetched, the PC holds the address of the next byte of the instruction or next instruction


INSTRUCTION EXECUTION AND DATA FLOW in 8085

The program instructions are stored in memory, which is an external device. To execute a program in 8085, the starting address of the program should be loaded in program counter. The 8085 output the content of program counter in address bus and asserts read control signal low. Also, the program counter is incremented.

The address and the read control signal enable the memory to output the content of memory location on the data bus. Now the content of data bus is the opcode of an instruction. The read control signal is made high by timing and control unit after a specified time. At the rising edge of read control signals, the opcode is latched into microprocessor internal bus and placed in instruction register.

The instruction-decoding unit, decodes the instructions and provides information to timing and control unit to take further actions.