UNIT 5 – Sequential Circuits

Digital clock signal

A digital clock signal is a periodic electrical pulse or square wave used to measure time in a digital system. It has two stable states, high and low, and it switches between them at a fixed frequency, which determines the time resolution of the digital clock. The frequency of a digital clock signal is usually expressed in Hertz (Hz), and it determines the time it takes for a complete cycle of the signal, i.e., the time period.

Duty cycle

Duty cycle is a term used to describe the amount of time a periodic signal is in its high state relative to its total period. It is usually expressed as a percentage of the total period and can be used to describe the behavior of both analog and digital signals. The duty cycle of a digital signal is important because it determines the average value of the signal, which can be used to control the power consumption or the speed of certain digital circuits. In the case of a square wave, the duty cycle is the ratio of the high state duration to the total period of the wave.

Synchronous circuit

A synchronous circuit is a type of digital circuit in which all elements are synchronized to a common clock signal. The clock signal provides a regular timing reference that is used to coordinate the operation of the different elements in the circuit. In a synchronous circuit, all operations are performed in step with the clock, and the state of the circuit changes only on clock edges, ensuring that all elements in the circuit are working in a well-defined and predictable manner. This type of circuit is contrasted with asynchronous circuits, in which elements can operate independently of each other, without a common clock signal. Synchronous circuits are commonly used in high-speed digital circuits because they allow for accurate timing and predictable operation.

Asynchronous circuit

An asynchronous circuit is a type of digital circuit in which the different elements are not synchronized to a common clock signal. Instead, communication between elements is performed using signals that indicate the completion of specific operations or the availability of data. In an asynchronous circuit, the timing of operations is determined by the arrival of these signals, rather than by a shared clock signal. This allows for greater flexibility in the design of the circuit, but also requires more complex timing control and synchronization mechanisms. Asynchronous circuits are commonly used in low-power, low-frequency applications where a shared clock signal is not necessary or would consume too much power. They are also used in applications where multiple independent operations must be performed simultaneously, without the need for a common timing reference.

Edge-triggered circuit

An edge-triggered circuit is a type of digital circuit that responds to changes or "edges" in the voltage level of an input signal. It can be either a rising edge (when the voltage level changes from low to high) or a falling edge (when the voltage level changes from high to low). Edge-triggered circuits are commonly used in synchronous digital circuits, where a clock signal is used to coordinate the operation of different elements. In such circuits, the clock signal provides a regular timing reference, and the edge-triggered elements are triggered on specific clock edges, allowing for accurate and predictable operation. Edge-triggered circuits are contrasted with level-triggered circuits, which respond to the steady-state voltage level of an input signal, rather than changes in the voltage level.

Level-triggered circuit

A level-triggered circuit is a type of digital circuit that responds to the steady-state voltage level of an input signal, rather than changes or "edges" in the voltage level. It can be either high or low, depending on the design of the circuit. Level-triggered circuits are commonly used in asynchronous digital circuits; where there is no shared clock signal to coordinate the operation of different elements. In such circuits, the state of the input signal determines the state of the circuit, and the output of the circuit may change at any time, depending on the state of the input. Level-triggered circuits are contrasted with edge-triggered circuits, which respond to changes in the voltage level of an input signal, rather than its steady-state voltage level.

Sequential circuit

A sequential circuit is a type of digital circuit that contains memory elements, such as flip-flops, that store the state of the circuit. The state of a sequential circuit changes as a function of its current state and the input signals. This allows the circuit to perform complex operations and maintain information over time, making it useful for implementing state machines, counters, and other digital systems that require memory. Sequential circuits are commonly used in digital systems to process and store data, control the timing and flow of operations, and provide outputs that depend on the history of the inputs. The behavior of a sequential circuit is determined by its combination of combinational and sequential logic elements, which work together to perform the desired function.

Compare combinational and sequential circuits

Combinational and sequential circuits are two types of digital circuits that are used to implement different functions in digital systems. Combinational circuits are digital circuits that produce an output based solely on the current values of their inputs, without any memory elements to store the state of the circuit. The output of a combinational circuit depends only on the inputs and the logic gates or other components that perform the desired function. Combinational circuits are used to implement logic functions, arithmetic operations, and other functions that can be described using a truth table or Boolean equation. Sequential circuits, on the other hand, contain memory elements, such as flip-flops, that store the state of the circuit. The state of a sequential circuit changes as a function of its current state and the input signals. This allows the circuit to maintain information over time and perform complex operations, such as counting or implementing state machines. The behavior of a sequential circuit is determined by its combination of combinational and sequential logic elements, which work together to perform the desired function.

In summary, combinational circuits are used to implement functions that depend only on the current inputs, while sequential circuits are used to implement functions that depend on both the current inputs and the past state of the circuit.

Here is a point-by-point comparison between combinational and sequential circuits:

1. Function: Combinational circuits produce an output based solely on the current values of the inputs, while sequential circuits use the current inputs and the stored state to produce an output.

2. Memory: Combinational circuits do not have memory elements, while sequential circuits contain memory elements, such as flip-flops, that store the state of the circuit.

3. Operation: The output of a combinational circuit is determined only by the inputs and the logic gates, while the output of a sequential circuit depends on both the inputs and the stored state.

4. Timing: Combinational circuits do not have a concept of time, as their outputs change immediately in response to changes in the inputs. Sequential circuits, on the other hand, change state in response to both changes in the inputs and the clock signal.

5. Complexity: Combinational circuits are generally simpler and faster than sequential circuits, as they do not have to store and manage the state of the circuit. Sequential circuits are more complex, as they have to manage both the inputs and the stored state.

6. Applications: Combinational circuits are used to implement logic functions, arithmetic operations, and other functions that can be described using a truth table or Boolean equation. Sequential circuits are used to implement state machines, counters, and other digital systems that require memory.

SR flip-flop

An SR flip-flop is a type of sequential circuit that uses two input signals, S (Set) and R (Reset), to control the state of the flip-flop. The SR flip-flop is a basic building block of many digital systems, and it can be used to store a single bit of information.

The operation of the SR flip-flop is determined by the values of the S and R inputs. If S is high and R is low, the Q output will be set to 1, and if R is high and S is low, the Q output will be reset to 0. If both S and R are high, or if both S and R are low, the flip-flop will remain in its current state.

The SR flip-flop is considered a type of "level-sensitive" sequential circuit, as the inputs determine the state of the flip-flop based on the steady-state voltage levels, rather than changes in the voltage levels. The SR flip-flop is a type of bistable multivibrator, as it has two stable states that it can hold until the inputs change.

In summary, the SR flip-flop is a simple and versatile sequential circuit that is used in a wide range of digital systems to store and manipulate binary information.

JK flip-flop

A JK flip-flop is a type of sequential circuit that uses two input signals, J (Set) and K (Reset), to control the state of the flip-flop. The JK flip-flop is an improvement over the SR flip-flop, as it has the ability to change state without the risk of indeterminate outputs.

The operation of the JK flip-flop is determined by the values of the J and K inputs. If J is high and K is low, the Q output will be set to 1, and if K is high and J is low, the Q output will be reset to 0. If both J and K are high, the flip-flop will toggle, changing from 1 to 0 or from 0 to 1. If both J and K are low, the flip-flop will remain in its current state. The JK flip-flop is considered a type of "edge-sensitive" sequential circuit, as the inputs determine the state of the flip-flop based on changes in the voltage levels, rather than the steady-state voltage levels. The JK flip-flop is a type of bistable multivibrator, as it has two stable states that it can hold until the inputs change.

In summary, the JK flip-flop is a more versatile and robust type of sequential circuit that is used in many digital systems to store and manipulate binary information. The JK flip-flop is widely used in digital circuits, such as counters and state machines, due to its ability to change state and avoid indeterminate outputs.

T (Toggle) flip-flop

A T (Toggle) flip-flop is a type of sequential circuit that uses a single input signal, T (Toggle), to control the state of the flip-flop. The T flip-flop is a simpler and more efficient variant of the JK flip-flop, as it only has a single input and can toggle between two states with each rising edge of the clock signal. The operation of the T flip-flop is determined by the value of the T input. If T is high, the Q output will toggle on each rising edge of the clock signal, switching from 1 to 0 or from 0 to 1. If T is low, the Q output will remain in its current state. The T flip-flop is considered a type of "edge-sensitive" sequential circuit, as the input determines the state of the flip-flop based on changes in the voltage level of the T input, rather than the steady-state voltage level. The T flip-flop is a type of bistable multivibrator, as it has two stable states that it can hold until the T input changes.

In summary, the T flip-flop is a simple and efficient type of sequential circuit that is widely used in digital systems for its ability to toggle between two states and store a single bit of information. The T flip-flop is used in many digital circuits, such as counters, state machines, and memory elements, due to its versatility

and simplicity.

D (Data) flip-flop

A D (Data) flip-flop is a type of sequential circuit that uses a single input signal, D (Data), to control the state of the flip-flop. The D flip-flop is one of the simplest and most widely used types of flip-flops, and it is used to store a single bit of information.

The operation of the D flip-flop is determined by the value of the D input and the clock signal. The state of the Q output is set to the value of the D input on the rising edge of the clock signal. The Q output will hold its state until the next rising edge of the clock signal, at which point it will be updated based on the current value of the D input. The D flip-flop is considered a type of "edge-sensitive" sequential circuit, as the state of the Q output is updated based on changes in the clock signal, rather than the steady-state voltage level of the D input. The D flip-flop is a type of bistable multivibrator, as it has two stable states that it can hold until the next rising edge of the clock signal.

In summary, the D flip-flop is a simple and versatile type of sequential circuit that is widely used in digital systems for its ability to store a single bit of information. The D flip-flop is used in many digital circuits, such as counters, state machines, and memory elements, due to its simplicity and versatility.

A counter is a digital circuit that generates a sequence of binary numbers. There are two types of counters: asynchronous and synchronous.

1. 4-bit asynchronous counter:

An asynchronous counter, also known as a ripple counter, is a counter circuit in which the flip-flops are connected in such a way that the output of one flip-flop is connected to the clock input of the next flip-flop. As a result, the output of the first flip-flop changes every clock pulse, while the output of each subsequent flip-flop changes when the previous flip-flop output goes from high to low.

A 4-bit asynchronous counter can count from 0 to 15 (0000 to 1111) and requires four flip-flops. The circuit diagram of a 4-bit asynchronous counter is as follows:

2. 4-bit synchronous counter:

A synchronous counter is a counter circuit in which all the flip-flops are connected to a common clock signal. As a result, all the flip-flops change state simultaneously on each clock pulse. This type of counter is faster and more reliable than an asynchronous counter, but it requires more circuitry.

A 4-bit synchronous counter can also count from 0 to 15 (0000 to 1111) and requires four flip-flops. The circuit diagram of a 4-bit synchronous counter is as follows:

In both types of counters, the output of each flip-flop is connected to a binary decoder that converts the binary output of the counter into a decimal or hexadecimal display.

A shift register is a digital circuit that can store and shift binary data. There are different types of shift registers based on the direction of data transfer and the mode of operation. Here are three common types of shift registers:

1. 4-bit serial in-serial out shift register

2. Serial in-parallel out shift register

3. Parallel in-serial out shift register

1. 4-bit serial in-serial out shift register:

This shift register has four flip-flops that are connected in series, and the input data is applied to the first flip-flop. The data is shifted from one flip-flop to the next on each clock pulse. The output data is taken from the last flip-flop in the series. This shift register is called serial in-serial out because data is input and output in a serial (one bit at a time) manner.

2. Serial in-parallel out shift register:

This shift register also has four flip-flops, but the output of each flip-flop is connected to a common output line. The input data is still applied to the first flip-flop, but the output data is taken from all the flip-flops simultaneously when the parallel load input is activated. This shift register is called serial in-parallel out because data is input in a serial manner and output in a parallel (multiple bits at a time) manner.

3. Parallel in-serial out shift register:

This shift register has four input lines, one for each flip-flop, and the input data is applied to all the flip-flops simultaneously when the parallel load input is activated. The output data is taken from the first flip-flop, and the data is shifted from one flip-flop to the next on each clock pulse. This shift register is called parallel in-serial out because data is input in a parallel manner and output in a serial manner.