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Digital Electronics

Selection of Logic Gates or Digital Integrated Circuits

Factors to Assess the Performance of Logic Gates

Selection of Logic Gates or Digital Integrated Circuits- The various logic families own various characteristics or features and the decision regarding their selection for any specific assignment is done on the basis of its merits or characteristics. For example, speed may be a basic requirement in one system, whereas minimum dissipation of power may be important on some other systems. Therefore, it is immensely important to understand the characteristic of different logic families, so that a better selection could be made keeping in view its applications. As such, best possible results may be expected. Therefore, important factors or things which should kept in front during selection of logic gates or digital integrated circuits, are as follows. It has to be remembered that the efficiency of gates depends on these very factors.

  1. speed
  2. Fan -in
  3. Fan – out
  4. Noise – immunity
  5. Power dissipation
  6. Voltage levels
  7. Propagation delay time
  8. Speed – power product
  9. Noise margin
  10. Input and output currents




Speed

By speed of a logic is meant the time which is required for an input signal to be provided on logic gates’ input terminal and the resulting change on the logical gates’ output terminal. In other words, the time which is required for providing a signal on input terminal of a logic gate and the resulting change in the output state of the gate as a result of this signal, is known as speed of that logic gate.

Fan – in

By the fan–in of a logic gate (coming from the same or similar circuits) we mean the number of inputs, which the logic gate can control or handle properly. In other words, the total number of inputs being transmitted from the same or similar circuits on the logic gate, which this gate can drive or control easily, is known as fan–in.

Fan – out

By fan – out of a logic circuit, we mean that maximum number of outputs of similar circuits, which it can drive in a reliable manner. In other words, the circuits or units which could be driven easily through an output received from a logic gate, its number is called the fan – out of that circuit (i.e. the number of loads connected with the output of a digital circuit, is called fan – out of that circuit). For example, if the fan – out of a logic gate is 8, it implies that this gate can easily drive 8 units connected with its output, without reducing from its specific output voltage level (0-1) (or keeping its output voltage level within the confines of logic level). In simple words, the fan – out of any gate being 8 means, that this gate within the limits of its output voltage, or without affecting the limits of its logic levels, can quite reliably drive further 8 units through its one output.



Noise Immunity

Noise immunity is expressed as the maximum generated noise voltages, which a logic circuit can afford without a False change in its output state. In other words, maximum induced noise voltage on a logic circuit, which the circuit can easily tolerate or withstand without any sort of impact on its output (or without causing any false change on its output), is called noise immunity of this logic gate circuit. It has to be remembered that noise immunity is also referred to as noise margin. The higher the noise margin of any logic circuit, the better will be the performance of that logic circuit. For example, if the noise margin of any circuit is 5V, circuit rejects all noise pulses up to or below 5V.

Most often, lot of noise voltage is produced in logic circuit due to stray electric and magnetic fields, by means of which a false or wrong triggering may occur in logic circuit deviating from the logic levels. That’s when these generated noise voltages increase too much, the logic gate triggers automatically without providing any input pulse. This process is called false triggering. The limit of a logic circuit, where it can withstand maximum noise voltage without any sort of effect on its output, is known as its noise immunity. Or the quantity of noise voltage, which input of a logic circuit can afford without any sort of impact on the circuit’s output, is known as noise immunity. We have to remember that by noise we mean an unpleasant signal which is produced automatically within a circuit or a device or the signal produced by means of some external source, which mixes with the circuits’ logic levels and thus causes a change in it, as a result of which a false triggering occurs on the circuit.  

Power Dissipation

Whenever any circuit switches from one state to another (i.e. suppose from binary 0 to binary 1 state), or whenever the state of any circuit changes (change in a circuit from one state to another, is called switching), then owing to the flow of current through current-carrying resistors (circuits own resistance) fitted on it, power tends to dissipate in the form of heat (in milliwatts), which is known as power dissipation or AC power dissipation. It should be remembered that the best gate is one, in which minimum possible dissipation of power occurs. In other words, the power consumed by components fitted on a specific gate or circuit is known as the dissipation of that gate or circuit. Remember that high power dissipation, is meant the usage of high electrical energy or generation of high heat.



Voltage levels

The minimum and maximum voltage values provided for binary 0 and binary 1 states of any given digital integrated circuit, are known as voltage levels of that circuit. Remember that every manufacturer furnishes a data sheet with his manufactured circuit or device, in which, voltage levels of the circuit or device are being mentioned, apart from other necessary information.

Figure 2.36 

Selection Of Logic Gates

In other words, rated logic voltages of any logic circuit are called voltage levels of that circuit. We know that every logic gate or logic circuit has some specific voltage level5 or logic level, by means of which one can guess about its high or low voltages e.g. 0 or +5V etc.

Remember that in a typical situation, the output of the logic gate is 0V or +5V. However practically, sometimes the output value of the logic gate does not equal to 0V or +5V, therefore, manufacturers treat any voltage less than 0.4V as low output whereas 2.4V or above it, as high output.  This has been shown by a logic level block in figure 2.36. Such a situation is called worst case.



Propagation delay Time

When a signal passes (or propagates) through a logic circuit, a time delay always occurs (i.e. a signal always requires some time for producing some changes on output after passing through the gate). As change in output level occurs after some time as a result of change in input level, therefore a short time period of time during which change in output level always occurs, is known as propagation delay time. In simple words, the operational speed of a logic circuit is known as propagation time delay.

The propagation delay time is the time which is required for a change on a gates’ input level, by means of which change in its output level occurs. In other words, time required for producing a change on input of a logic circuit, by means of which output of the circuit changes, is called propagation delay time.  The time required for a corresponding change in gates’ output with respect to a change in gates’ inputs, is called propagation delay time. For example, if output of a logic circuit takes one micro second time (1 µSec) with respect to its input signal, it means that output pulse is has a delay of 1µSec with respect to input pulse (i.e. a time delay of 1µSec exists between output pulse and input pulse of the circuit). This term is known as propagation delay.

The higher the propagation delay time of any circuit, the lower the maximum frequency. Thus, a high-speed circuit is one, propagation delay time of which is very low. For example, if delay of a gate is 3ns, it will be speedier than a gate with 10ns. Remember, this factor is being attached a great importance at the time of designing of a digital circuit. Moreover, with an increase in the load (i.e. increasing the number of inputs), this delay time also increases. The lower the propagation delay time, the higher the operating speed of the device.

Speed Power Product

The product of power dissipation and propagation delay time tpd on a specific frequency, is called speed power product (SSP) or delay power product (DPP). Speed power product is in fact a measurement for assessing the performance of any logic circuit. A manufacturer describes its details on the product specification data sheet. As consumption of power increases with an increase in the speed of a logic circuit, therefore speed power product actually reflects a relation between speed and power.

Remember when there is an issue of selecting a certain type of logic for some specific purpose, this term is used for making a comparison between the gates. In such a situation, time is measured in seconds, whereas power is measured in watts. As watts are described as joules per second, therefore SSP is measured in joules or pekoe joules. Normally, values of both delay time as well as power dissipation should be low, for the purpose of a better performance. Therefore, merit of any logic gate for any specific operation, can be accessed via a speed power product (SSP). The gate with a low speed power product value is considered better. As compared to TTL, SSP values of CMOS circuits are very low.




Noise Margin

Noise margin, which is also known as noise immunity (which has been defined above), is the capacity of a logic gate through which it stops or withstands false signals or wrong triggering, is known as noise margin. In other words, the measurement of noise immunity of any circuit is called noise margin, which is expressed in terms of volts. There are always two specific noise margin values of any given logic circuit i.e. high-level noise margin and low-level noise margin. Noise margin is the difference between maximum possible low input (which a gate can sustain) and a driving gates’ maximum possible low output.

We know that a single logic gate is never independently used, rather during normal working conditions, output of one gate drives the input of the next gate. In worst case, a difference of 0.4v exists between driver output voltages and required input voltages, as can be seen from logic level block shown in the above figure.

V0L.max = 0.4V … Driver output

V1L.max = 0.8V … load input

The high values in worst case are as follows:

V0H.min = 2.4V … Driver output

V1H.min = 2V    … Load input 

The difference in both aforementioned situations is 0.4V. This difference is called noise margin. In simple words, quantity of noise voltage, which an input on a logic circuit can sustain without causing any false change in its output, is called noise margin. In case of transistor–transistor logic (TTL) its value is 0.4V, whereas in case of diode transistor logic (DTL), its value is 0.7V. It is to be reminded here that the higher the noise margin of a logic circuit, the better the performance of logic circuits

Input and Output Currents

By the input and output currents of any logic gate we mean those currents which operate gate as a source or supply in situation of low gate input and high gate output, whereas inversely, in situation of high gate input and low gate output, it operates gate as a sink (i.e. in such a situation, flow of current is inwards from load towards the gate instead of flow of current outwards from gate towards the load). Therefore, output circuit of a gate must be such that it supplies extensive current for the purpose of load capacitance charging (i.e. it operates as a source) and in case of load capacitance discharging, could take back maximum current from the load (i.e. it may also operate as a sink)



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