Introduction to digital electronics. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Introduction to digital electronics. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Any symbol of information in digital devices is encoded in binary code, so signals can only take two values: high or low voltage level, the presence or absence of a voltage pulse, etc. A prerequisite for this is the possibility of confident recognition by elements of digital circuits of two signal values, corresponding to symbols 0 and 1, in conditions of changing ambient temperature, power supply voltage, and other destabilizing factors.

We have already said that the values of signal levels (Uc) by the elements of digital devices are not perceived continuously, but at discrete times, the interval between which is called the working cycle T. As a rule, one elementary transformation of the code input received at the input words. Time discretization is provided by special control devices that generate synchronizing pulses (SI). In discrete devices, two ways of presenting information are used: potential and pulse. With the potential method, the values of logical 0 and logical 1 correspond to low and high level voltages. If logical 0 corresponds to a low-level voltage, and logical 1 to a high one, then such logic is called positive, and vice versa, if logical 0 is a high-level voltage, and logical 1 is a low-level voltage, then such logic is called negative or inverse.

In the following, the terms "high and low level voltage" (high and low level signals) corresponding to the logic 1 and logic 0 levels are mainly used.

Information in digital devices can be represented in serial and parallel codes. When using a serial code, each cycle corresponds to one bit of the binary code. The number (of the bit is determined by the number of the cycle, counted from the cycle, coinciding with the beginning of the code representation. The graphs shown in Figure 1 illustrate the serial code of the byte binary number 10011011 with potential and pulse methods of information presentation. In the first method, Figure 1, and the signal remains low or high levels for one or more cycles.At the moment of signal transition from one level to another, its value is uncertain.In the pulsed way of representing digital information in Figure 1b, the presence or absence of a pulse of finite duration corresponds to the unit and zero values of a binary variable.In a serial code number, all its digits can be fixed on one element and transmitted over one information transmission channel.To transmit the entire number, eight cycles are required (Figure 1, c).

Introduction to digital electronics

Parallel code can significantly reduce the processing time and transmission of information. For example, Figure 3 illustrates the parallel code of the seven-bit number 1101101. In this case, both in the pulsed (Figure 2, a) and potential (Figure 2, b) ways of presenting information, all bits of the binary code are presented in one time cycle, can be recorded by separate elements and transmitted over separate channels (bit tires). Digital devices that process and transform the information received at their inputs are called digital automata. The task of constructing a digital automaton that performs certain actions on binary signals is to select elements and a method for their connection that provide a given transformation. These problems are solved by mathematical logic or algebra of logic (Boolean mathematics). Devices that form the functions of Boolean mathematics are called logical or digital and are classified according to various distinguishing features. Digital devices according to the nature of the information at the inputs and outputs are divided into devices of serial, parallel and mixed action.

To implement a parallel action device that performs a similar function, two groups of inputs with eight bits in each group and eight outputs (according to the word width of the output word) are required. Mixed-type devices are also known, in which, for example, the input word is represented in parallel form, and the output word is in serial form (this is code conversion).

According to the circuit design and the nature of the connection between the input and output variables, taking into account their changes in cycles of operation, combinational and sequential digital devices are distinguished. In combinational devices, the set of signals at the inputs and outputs at each particular moment of time is completely determined by the input signals acting at that moment at its inputs. If the input and output functions in the n-cycle are denoted as Xⁿ and Yⁿ, then the relationship between them will be determined by the expression

Yⁿ=L(Xⁿ),

where L is the sign of the logical transformation performed by the device. Digital devices, unlike analog devices, allow almost any kind of conversion. In serial type digital devices, the value of the output variables Yⁿ in the n-cycle is determined not only by the value of the input variables Xⁿoperating at a given time, but also depend on the internal state of the device Cⁿ. In turn, the internal state of the device depends on the values of the variables acting at the input in the previous cycles. The operation of a serial device can be written as

Yn=ƒ(Xⁿ,Cⁿ); Cⁿ=F(X^n-1,C^n-1),

where X^n-1 and C^n-1 - respectively, a set of input variables and internal states of the device in the previous cycle.

An example of a serial device can be a pulse counter, the state of the outputs of which depends on the total number of pulses received at its input.

Basic logical elements perform the following logical operations. "AND" - logical multiplication, "OR" - logical addition, "NOT" - negation (inversion). More details will be discussed in the next chapter.

Author: -=GiG=-, gig@sibmail; Publication: cxem.net

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