LOGIC FAMILY

Basically, there are two types of semiconductor devices: bipolar and unipolar. Based on these devices, digital integrated circuits have been made which are commercially available. Various digital functions are being fabricated in a variety of forms using bipolar and unipolar technologies. A group of compatible ICs with the same logic levels and supply voltages for performing various logic functions have been fabricated using a specific circuit configuration, which is referred to as a logic family.

1. 1.1 Bipolar Logic Families

The main elements of a bipolar IC are resistors, diodes and transistors. Basically, there are two types of operations in bipolar ICs: saturated and non-saturated. In saturated logic; the transistors in the IC are driven to saturation, whereas in the case of non-saturated logic, the transistors are not driven into saturation. The more used saturated bipolar logic family is Transistor-transistor logic (TTL). The non-saturated bipolar logic families are: Schottky TTL and Emitter-coupled logic(ECL).

2. 1.2 Unipolar Logic Families

MOS devices are unipolar devices and only MOSFETs are employed in MOS logic circuits. The MOS logic families are: PMOS, NMOS and CMOS. While in PMOS only p-channel MOSFETs are used and in NMOS only n-channel MOSFETs are used, in complementary MOS (CMOS), both p- and n-channel MOSFETs are employed and are fabricated on the same silicon chip.

3. 2 CHARACTERISTICS OF DIGITAL ICs

With the widespread use of ICs in digital systems and with the development of various technologies for the fabrication of ICs, it has become necessary to be familiar with the characteristics of IC logic families and their relative advantages and disadvantages. Digital ICs are classified either according to the complexity of the circuit, as the relative number of individual basic gates (2-input NAND gates) it would require to build the circuit to accomplish the same logic function or the number of components fabricated on the chip.

3.1. 2.1 Speed of Operation

The speed of a digital circuit is specified in terms of the propagation delay time. The delay times are measured between the 50 per cent voltage levels of input and out- put waveforms. There are two delay times: t pHL , when the output goes from the HIGH state to the LOW state and t pLH , corresponding to the output making a transition from the LOW state to the HIGH state. The propagation delay time of the logic gate is taken as the average of these two delay times.

3.2. 2.2 Power Dissipation

This is the amount of power dissipated in an IC. It is determined by the current, I CC , that it draws from the V CC supply, and is given by V CC x I CC . I CC is the average value of I CC (0) and I CC (1). This power is specified in milliwatts.

3.3. 2.3 Figure of Merit

The figure of merit of a digital IC is defined as the product of speed and power. The speed is specified in terms of propagation delay time expressed in nanoseconds. Figure of merit = propagation delay time (ns) x power (mW) It is specified in pico joules (ns x mW = pJ) A low value of speed-power product is desirable. In a digital circuit, if it is desired to have high speed, i.e. low propagation delay, then there is a corresponding increase in the power dissipation and vice-versa.

3.4. 2.4 Fan-Out

This is the number of similar gates that can be driven by a gate. High fan-out is advantageous because it reduces the need for additional drivers to drive more gates.

3.5. 2.5 Current and Voltage Parameters

The following currents and voltages are specified which are very useful in the design of digital systems.

High-level input voltage, V IH : This is the minimum input voltage which is recognized by the gate as logic 1.

Low-level input voltage, V IL : This is the maximum input voltage that is recognized by the gate as logic 0.

High-level output voltage, V OH : This is the minimum voltage available at the output corresponding to logic 1.

Low-level output voltage, V OL : This is the maximum voltage available at the output corresponding to logic 0.

High-level input current, I IH : This is the minimum current that must be supplied by a driving source corresponding to 1 level voltage.

Low-level input current, I IL : This is the minimum current that must be supplied by a driving source corresponding to 0 level voltage.

High-level output current, I OH : This is the maximum current that the gate can sink in 1 level.

Low-level output current, I OL : This is the maximum current which the gate can sink in 0 level.

High-level supply current, I CC (1): This is the supply current when all the outputs of the IC are at logic 1.

Low-level supply current, I CC (0): This is the supply current when all the outputs of the IC are at logic 0.

3.6. 2.6 Noise Immunity

Stray electric and magnetic fields may induce unwanted voltages, known as noise, on the connecting wires between logic circuits. This may cause the voltage at the input to a logic circuit to drop below V IH or rise above V IL and may produce undesired operation. The circuit’s ability to tolerate noise signals is referred to as the noise immunity, a quantitative measure of which is called noise margin (NM). The noise margins defined above are referred to as dc noise margins, and are calculated by: NMH= V OH – V IH and NML = V IL – V OL .

3.7. 2.7 Operating Temperature

It must be known the temperature range in which an IC functions properly. The accepted temperature ranges are: 0 to + 70 °C for consumer and industrial applications and –55 °C to + 125 °C for military purposes.

3.8. 2.8 Power Supply Requirements

The supply voltage(s) and the amount of power required by an IC are important characteristics required to choose the proper power supply.

3.9. 2.9 Flexibilities Available

Various flexibilities are available in different IC logic families and these must be considered while selecting a logic family for a particular job. Some of the flexibilities available are:

1. The breadth of the series: Type of different logic functions available in the series.

2. Popularity of the series: The cost of manufacturing depends upon the number of ICs manufactured. When a large number of ICs of one type are manufactured, the cost per function will be very small and it will be easily available because of multiple sources.

3. Wired-logic capability: The outputs can be connected together to perform additional logic without any extra hardware.

4. Availability of complement outputs: This eliminates the need for additional inverters.

5. Type of output: Passive pull-up, active pull-up, open-collector/drain, and tristate.