GENERATION OF CLOCK PULSE USING NAND GATE

Аннотация

This paper presents the design and functionality of a basic clock pulse generator using a NAND gate. Clock signals are essential in digital systems for timing and synchronization. The experiment focuses on generating a periodic waveform using simple components: a NAND gate (IC 7400), a resistor, and a capacitor. The paper explains the working principle of the oscillator circuit, emphasizing the charging and discharging behavior of the capacitor and how it influences the output waveform. Practical considerations such as duty cycle, waveform shape, hysteresis, and limitations of non-Schmitt trigger gates are discussed. The generated signal has applications in various digital systems where simple and cost-effective timing pulses are required. Keywords: Clock generator, NAND gate, IC 7400, timing signal, digital electronics, RC circuit, square wave, hysteresis, Schmitt trigger, pulse generation Introduction. Timing components are one of the most ubiquitous components in electronics. Since all logic operations in a synchronous machine occur in synchronism with a clock, the system clock becomes the basic timing unit. The system clock must provide a periodic wave form that can be used as a synchronous signal. The square waveform is a typical clock waveform used in a digital system. A clock generator is a circuit that produces a timing signal (known as a clock signal) for use in synchronizing a circuit's operation. The signal can range from a simple symmetrical square wave to more complex arrangements. Clock signal is a periodic signal that oscillates between a high and a low state. It’s ON time and OFF time need not be the same. The signal acts like a metronome which the digital circuit follows in time to coordinate its sequence of actions. Digital circuits rely on clock signals to know when and how to execute the functions that are programmed. The clock signal can be represented as a square wave when both its ON time and OFF time are same (As shown in Fig. 1). This clock signal is shown in the following figure. 228 SCIENCE SHINE INTERNATIONAL SCIENTIFIC JOURNAL Issue 10(45), Volume 1 | ISSN 3030-377X | 20.05.2025 Fig. 1 clock signal when both its ON time and OFF time are same This signal stays at logic High (5 V) for some time and stays at logic Low (0 V) for equal amount of time. This pattern repeats with some time period. In this case, the time period will be equal to either twice of ON time or twice of OFF time. The clock signal can be represented as train of pulses when ON time and OFF time are not same (As shown in Fig. 2). This clock signal is shown in the following figure. Fig. 2 clock signal when both its ON time and OFF time are not same This signal stays at logic High (5 V) for some time and stays at logic Low (0 V) for some other time. This pattern repeats with some time period. In this case, the time period will be equal to sum of ON time and OFF time. Clock signals are the heartbeats of digital systems. Hence, the stability of clock signals is highly important. Ideally clock signals should have minimum rise and fall times, specified duty cycles and zero skew. In reality clock signals have nonzero skews and noticeable rise and fall times, duty cycles can also vary. The reciprocal of the time period of clock signal is known as the frequency of the clock signal. Different clock generators can be developed by implementing different devices such as discrete devices, linear integrated circuits and digital integrated devices. These clock signals can be produced through many basic circuits. In this experiment, NAND gate, resistor(R) and capacitor(C) are being used to generate 455 KHz clock signal. NAND GATE. NAND stands for NOT-AND. An AND gate followed by a NOT circuit makes a NAND gate. Fig. 3 shows the circuit symbol of a 2-input NAND gate. The truth table of a NAND gate is obtained from the truth table of an AND gate by complementing the output entries. The output of a NAND gate is logic 229 SCIENCE SHINE INTERNATIONAL SCIENTIFIC JOURNAL Issue 10(45), Volume 1 | ISSN 3030-377X | 20.05.2025 ‘0’ when all its inputs are at logic ‘1’. For all other input combinations, the output is at logic ‘1’. NAND gate operation is logically expressed as :- Fig. 3 Graphic symbol of NAND Gate Table 1: Truth table of NAND gate IN OUT PUTS PUTS Y = A B (A.B)' 0 0 1 0 1 1 1 0 1 1 1 0 7400. The 7400 IC is a 14 Pin Quad 2-Input NAND Gate IC. This device contains four independent gates each of which performs the logic NAND function. NAND gates utilize advanced silicon-gate CMOS technology to achieve operating speeds similar to LS-TTL gates with the low power consumption of standard CMOS integrated circuits. All gates have buffered outputs. The pin diagram of IC 7400 is shown in following Fig. 4. 230 SCIENCE SHINE INTERNATIONAL SCIENTIFIC JOURNAL Issue 10(45), Volume 1 | ISSN 3030-377X | 20.05.2025 Fig. 4 Pin diagram of IC 7400 Clock Generator. The simple clock generator circuit as shown in Fig. 5 consists of a single TTL 7400 NA