What is a Digital Integrated Circuit and How Do We Use It?

Catalog

I Introduction

Digital integrated circuits are designed and operated based on digital logic (Boolean algebra), which is used to process digital signals. According to the definition of integrated circuits, they can also be defined as the digital logic circuits or systems made by integrating components and wiring on the same semiconductor chip. According to the number of included gate circuits or components, digital integrated circuits can be divided into Small Scale Integrated (SSI) circuits, Medium Scale Integrated(MSI) circuits, Large Scale Integration (LSI) circuits, Very Large Scale Integrated (VLSI) circuits, and Ultra Large Scale Integration (ULSI) circuits.

The small-scale integrated circuit contains less than 10 gate circuits, or the number of components does not exceed 100; the medium-scale integrated circuit contains 10 to 100 gate circuits, or the number of components is between 100 to 1,000; Large-scale integrated circuits include more than 100 gate circuits, or the number of components is between 1,000 and 10,000; VLSI circuits include over 10,000 gate circuits, or the number of components is 100,000-1,000,000. The number of gate circuits of ULSI is more than 100,000, or the number of components is between 1,000,000 and 10,000,000.  

With the advancement of microelectronics technology, the scale of integrated circuits is getting larger and larger. Therefore, it doesn’t make much sense to simply divide the types by the number of integrated components.

II Logical Function and Internal Design of Digital Integrated Circuits

1. Logical Function of Digital Integrated Circuits

Digital logic circuits can be divided into two categories: combinational logic circuits and sequential logic circuits. In combinational logic circuits, the output at any time depends only on the input at that time rather than the previous working state of the circuit. The most commonly used combinational logic circuits are encoders, decoders, data selectors, demultiplexers, numerical comparators, full adders, parity checkers, and so on.

Figure 1. Combinational Logic Circuit

In a sequential logic circuit, the output at any time depends not only on the input at that time but also on the original state of the circuit. Therefore, the sequential logic circuit must have a memory function and must contain a memory cell circuit. Registers, shift registers, and counters are the most frequently used sequential logic circuits.

Figure 2. Sequential Logic Circuit

For the various application of the two kinds of logic circuits, they all have standardized and serialized integrated circuit products, which are usually called general-purpose integrated circuits. Correspondingly, those integrated circuits designed and manufactured for special purposes are called application-specific integrated circuits (ASICs).

2. Internal Design of Digital Integrated Circuits

The composition of the digital circuit includes a combinational logic and a register(flip-flop). Combinational logic is a function composed of elementary gates, and its output will only be related to the current input. The first diagram in figure 3 is a combinational logic, which only completes logical operations, while sequential circuits contain not only basic gates but also storage elements to save past information. The steady-state output of a sequential circuit is related to the current input and the state formed by the past inputs. While the logical operation is completed, the processing result can be temporarily stored for the next operation as shown in the second diagram.

Judging from the function, the internal digital integrated circuit can be divided into two parts: data path and control Logic. These two parts are integrated by plenty of sequential logic circuits, and most of them are synchronous sequential circuits. The sequential circuits are divided into several nodes by multiple registers, and these registers are working to the same beat under the control of the clock, which can simplify the design.

Figure 3. Internal Structure of Digital Integrated Circuit

In the long-term design process, many standard general elements have been accumulated, such as selectors (also called multiplexers, which can choose one output from multiple input data), comparators (for comparing the size of two numbers), adders, multipliers, shift registers, and so on. These unit circuits have a regular shape and are easy to integrate, which is why digital circuits are better developed in integrated circuits.

These units are connected together to form a data path according to the design requirements. The data to be processed passes from the input end to the output end through this path, and then the processed result can be obtained. At the same time, specially designed control logic and the various components that control the data path should work in accordance with their respective functional requirements and specific timing relationships.

III Models and Types of Digital Integrated Circuits

1. Models of Digital Integrated Circuits

Digital integrated circuit models generally consist of three parts: prefix, serial number, and suffix. The prefix represents the manufacturer, and the numbering includes the product serial number and component serial number. The suffix generally indicates the temperature level and package type.

2. Types of Digital Integrated Circuits

There are many types of digital integrated circuit products. According to the circuit structure, they can be divided into two series: TTL and MOS.

TTL digital integrated circuits conduct electricity with two kinds of carriers: electrons and holes, so they are also called bipolar circuits. MOS digital integrated circuits use only one type of carriers to conduct electricity. Those use electrons to conduct electricity are called NMOS circuits; those that conduct electricity with holes are named PMOS circuits. If a circuit composes of NMOS and PMOS circuits, we call it a CMOS circuit.

Figure 4. The symbol of (a) a PMOS transistor and (b) an NMOS transistor

Compared with TTL digital integrated circuits, CMOS digital integrated circuits have many advantages, such as a wide range of operating power supply voltage, low static power consumption, strong anti-interference ability, high input impedance, low cost, and so on.

There are many kinds of digital integrated circuits, including devices such as various gate circuits, flip-flops, counters, codecs, and memories.

IV Features of TTL and CMOS Digital Integrated Circuits

1. TTL Digital Integrated Circuits

● Power supply voltage range

The working power supply voltage range of the TTL circuit is very narrow. The voltage range of S, LS and F series is 5V±5%, while that of AS and ALS series is 5Y±10%.

● Frequency

● Voltage output

When the working voltage is 5V, the output high level is greater than 2.4V, and the input high level is over 2.0V; the output low level is less than 0.4V, and the input low level is less than 0.8V.

● Minimum output drive current

High current output TTL circuit:

● Fan-out capability (taking the number of LS-TTL loads as an example)

High current output TTL circuit:

For each series of TTL integrated circuits with the same funid, such as 7404, 74LS04, 74A504, 74F04, 74ALS04, their pin arrangement and logic function are exactly the same, but they have significant differences in circuit speed and power consumption.

2. CMOS Digital Integrated Circuits

● Power supply voltage range

The operating power supply voltage range of the CMOS digital integrated circuit is 3 to 18V, and that of the 74HC series is 2 to 6V.

● Power consumption

When the power supply voltage VDD = 5V：

● Input impedance

The input impedance of the CMOS circuit only lies on the leakage current of the protection diode at the input end, so its value is extremely high, reaching more than 108 to 1011Ω. Therefore, the CMOS circuit hardly consumes the power of the driving circuit.

● Anti-interference ability

Because the allowable range of the power supply voltage is large, their output high and low-level swings are large, and the anti-interference ability is strong. The maximum value of their noise tolerance is 45% VDD and the guaranteed value can reach 30% VDD. The higher the power supply voltage, the larger the tolerance value.

● Logical swing

The logic high level “1” output by the CMOS circuit is very close to the power supply voltage VDD, and the logic low level “0” approaches the power supply Vss. In the no-load condition, the output high-level VOH = VCC-0.05V and the output low-level VOL = 0.05V. Therefore, the CMOS circuit has the highest power utilization factor.

● Fan-out capability

In low-frequency operation, one output terminal can drive more than 50 CMOS devices.

Figure 5. Fan-out and Fan-in

● Radiation resistance

CMOS tubes are conductive devices where majority carriers are under control, and ray radiation has little effect on the majority carrier concentration. Therefore, the CMOS circuit is particularly suitable for devices in aerospace, satellites, and nuclear tests.

The CMOS integrated circuit has low power consumption and low internal heating value, which can greatly improve the integrated level. Besides, because of the complementary symmetrical structure of the circuit, its parameters will compensate for each other when the ambient temperature changes, so its temperature stability is good.

● Manufacturing process

The manufacturing process of the CMOS integrated circuit is simpler than that of the TTL integrated circuit, and it also occupies a smaller area on the silicon, which is particularly appropriate for manufacturing large-scale and ultra-large-scale integrated circuits.

V Precautions for Use

1. The circuit is not allowed to work when the parameters exceed the limit values, or it may work abnormally and easily cause damage.

The allowable variation range of TTL integrated circuit power supply voltage is relatively narrow, generally between 4.5 and 5.5V, so a + 5V stabilized power supply must be used; the working power supply voltage range of the CMOS integrated circuit is relatively wide so there is a larger choice.

When selecting the power supply voltage, in addition to not let the power supply voltage exceed the limit value, we must also note that the power supply voltage will affect the operating frequency and other performance of the circuit. If the power supply voltage is low, it will decrease the operating frequency or increase the transmission delay time. For example, when the power supply voltage of CMOS trigger drops from + 15V to +3V, its maximum operating frequency will drop from 10MHz to tens of kHz.

2. The polarity of the power supply voltage must not be reversed. If the positive and negative poles of the power supply are reversed and connected incorrectly, it will cause damage to the device due to excessive current.

3. In the CMOS circuit, the amplitude of the input signal cannot exceed VDD - VSS, which means VSS = V1 = VDD. When the voltage applied to the input of the CMOS circuit is too high (greater than the power supply voltage) or too low (less than 0V), or the power supply voltage changes suddenly, the circuit current may increase rapidly and the device may be burned out.

Figure 6. Equivalent Circuit of CMOS Latch-up

This phenomenon is called latch-up effect. The measures to prevent the latch-up effect are:

● The amplitude of the input signal cannot be greater than VDD and less than 0V

● Eliminate interference on the power supply;

● When conditions allow, reduce the power supply voltage as much as possible. If the operating frequency of the circuit is relatively low, it is best to use + 5V power supply;

● Take current limiting measures to limit the power supply current to no more than 30mA.

4. Settlement of redundant input ends. For the CMOS circuit, the spare input terminals can not be dangling somewhere; otherwise, the high voltage generated by electrostatic induction may easily damage the device. Instead, these redundant input terminals should be connected to yDD or yss, or connected in parallel with other input terminals according to the actual situation.

For the TTL circuit, the extra input terminal is allowed to be hanging. When hanging, the logic input state of this terminal is generally regarded as "1". Although the hanging is equivalent to a high level and does not affect the logical relationship between the AND gate and the NAND gate. However, it is easy to be interfered by dangling, which may cause the circuit to malfunction. Therefore, the redundant input side should be properly dealt with according to actual needs.

Figure 7-1 AND Gate Symbol

Figure 7-2 NAND Gate Symbol

For example, the redundant input terminals of the AND and NAND gate can be directly connected to the power supply, or one resistor could be shared by different input ends and connected to the power supply. Also, the redundant input terminals can be used in parallel.

5. The spare output terminals should be left hanging and must not be directly connected to VDD or VSS, or excessive short-circuit current will damage the device. Besides, the outputs of CMOS circuits with different logic functions cannot be directly connected together, otherwise, the conductive P-channel and N-channel MOS field-effect transistor will form a low-resistance path, causing a short circuit and device damage.

Except for tri-state gates and open-collector gates, the output terminals of TTL integrated circuits are not allowed to be used in parallel. If the output ends of several open-collector gate circuits are connected in parallel, a pull-up resistor should be connected between the output end and the power supply.

Figure 8. A simple schematic of an open collector of an integrated circuit (IC)

6. Due to the high input impedance, the CMOS circuit is prone to breakdown for electrostatic induction. In addition, to add a protection circuit inside, we should also notice the electrostatic shielding when using and storing the circuit. And when welding the CMOS circuit, the welding tool should be well grounded, the welding time should not be too long, and the welding temperature should not be too high. Besides, it is also forbidden to disassemble, unplug, and insert the integrated circuit while the power is on.

7. The digital circuits of multiple models can be directly used interchangeably. For example, the CC4000 series can be used interchangeably with the CD4000 series and MC14000 series. However, for some ICs with the same pin function and package, the electrical parameters are different, which we should notice before switching.

8. Pay attention to design technology and enhance anti-interference measures. When designing a printed circuit board, long leads should be avoided to prevent interference between signals and delays in signal transmission. In addition, the power cord must be wider, and the ground wire should be grounded in a large area to reduce ground noise interference. In the design of the CMOS logic system, the capacitive load should be minimized, as it will reduce the operating speed and increase the power consumption of the circuit.

Summarize

In this essay, we've learned the basic knowledge about the digital integrated circuit, which includes its logical function, internal design, and models. And the two typical types of digital integrated circuits一TTL and CMOS are detailedly discussed in the following. In the final chapter, several using tips were listed to help you use them correctly. Share this article with your friends if you like it!

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