Digital Electronics Unit 4 PDF

DIGITAL ELECTRONICS UNIT 4 Sequential Circuits: Basic Latch, Gated SR Latch, gated D Latch, Master-Slave edge triggered flip- flops, T Flip-flop, JK Flip-flop, Excitation tables. Registers, Counters, Verilog code for flip-flops INTRODUCTION A digital circuit's input-output relationship can be rep...

DIGITAL ELECTRONICS UNIT 4 Sequential Circuits: Basic Latch, Gated SR Latch, gated D Latch, Master-Slave edge triggered flip- flops, T Flip-flop, JK Flip-flop, Excitation tables. Registers, Counters, Verilog code for flip-flops INTRODUCTION A digital circuit's input-output relationship can be represented as a truth table. An equivalent high-level circuit uses logic gates, each represented by a different shape (standardized by IEEE/ANSI 91-1984). A low-level representation uses an equivalent circuit of electronic switches (usually transistors). Most digital systems divide into combinational and sequential systems. The output of a combinational system depends only on the present inputs. However, a sequential system has some of its outputs fed back as inputs, so its output may depend on past inputs in addition to present inputs, to produce a sequence of operations. Simplified representations of their behavior called state machines facilitate design and test. Sequential systems divide into two further subcategories. "Synchronous" sequential systems change state all at once when a clock signal changes state. "Asynchronous" sequential systems propagate changes whenever inputs change. Synchronous sequential systems are made using flip flops that store inputted voltages as a bit only when the clock changes. Synchronous systems A 4-bit ring counter using D-type flip flops is an example of synchronous logic. Each device is connected to the clock signal, and update together. Main article: synchronous logic The usual way to implement a synchronous sequential state machine is to divide it into a piece of combinational logic and a set of flip flops called a state register. The state register represents the state as a binary number. The combinational logic produces the binary representation for the next state. On each clock cycle, the state register captures the feedback generated from the previous state of the combinational logic and feeds it back as an unchanging input to the combinational part of the state machine. The clock rate is limited by the most time-consuming logic calculation in the combinational logic. Sequential Circuits: Sequential circuits are digital circuits that output a value based on both the current input values and the previous output values. They have internal memory and can store information from one input to the next. Examples of sequential circuits include registers, counters, and flip-flops. 1|Page  DIGITAL ELECTRONICS UNIT 4 Sequential Circuits Sequential circuits, used for state machines, timers, counters, and memory elements, store and use previous state information to determine their next state, with two types: finite state machines and synchronous sequential circuits. Sequential circuits produce output based on current and previous input variables, with memory elements storing binary information, and latches storing one bit of information. The figure demonstrates two types of inputs in combinational logic: external, uncontrolled, internal, secondary, and outputs, with external inputs being state variables and secondary outputs being excitations. Types of Sequential Circuits: There are two types of sequential circuits: Type 1: Asynchronous sequential circuit: Asynchronous sequential circuits use input pulses without a clock signal, providing faster operation and independence from internal clock pulses, making them ideal for high-speed applications. But these circuits are more difficult to design and their output is uncertain. Type2: Synchronous sequential circuit: Circuits use clock signals and level inputs, with output pulses the same duration as clock pulses. They are slower than asynchronous circuits, with level output changing state. Synchronous sequential circuits are utilized in the design of counters, registers, RAM, MOORE/MEALY state management machines, and other state retaining machines. Advantages: Memory, Timing, State machine implementation, Error detection. Dis-advantages: Complexity, Timing constraints, Testing and debugging 2|Page  DIGITAL ELECTRONICS UNIT 4 Basic Latch Flip flops are fundamental building blocks of digital systems, used to store binary data in sequential logical circuits. They have two stable states and different working types. Gated SR Latch A Gated SR Latch has three inputs: Set, Reset, and Enable, activated by an ENABLE input connected to a switch, enabling the operation of the SET and RESET inputs. Truth Table Circuit Diagram Gated D Latch The Gated D Latch is a gated latch with two inputs, DATA and ENABLE, designed using gated SR latch and connected using an inverter. Circuit Diagram 3|Page  DIGITAL ELECTRONICS UNIT 4 Master-Slave edge triggered flip- flops "JK Flip Flop" introduces a race-round condition where inputs and CLK are set to 1 for a long time, causing uncertain or unreliable output. Explanation The master-slave flip flop is a series configuration of two JK flip flops, with the first flip flop acting as the master and the second as the slave, and an inverter or NOT gate. Working: The slave flip flop is isolated until the clock pulse is true, and the system's state is affected by inputs J and K. When inputs J and K are 1, the master flip flop works as reset, while the slave flip flop toggles on the clock's negative transition. Timing Diagram of a Master Flip Flop: Master flip flop output remains 1 until clock pulse is set to 1, while slave flip flop output remains 0 until clock pulse is set to 0. T Flip-flop 4|Page  DIGITAL ELECTRONICS UNIT 4 T flip flop is a type of JK flip-flop with a single input with clock input, connecting both JK flip-flop inputs together as a single input. The T flip flop is also known as Toggle flip-flop. These T flip-flops are able to find the complement of its state. Truth Table: JK Flip-flop The JK flip flop improves the S-R flip flop by modifying the SR flip flop, resulting in accurate output when S and R input is set to true. The J-K flip flop is a digital system's Set or Reset Flip flop, adjusting its output Y based on the difference in inputs. Truth Table: 5|Page  DIGITAL ELECTRONICS UNIT 4 Excitation tables The excitation table, derived from the truth table, consists of columns for present and next states, and each input, depending on the type of flip-flop. SR flip flop An SR flip-flop is a sequential circuit with two inputs (S for Set and R for Reset) and two outputs (Q and Q'), with an excitation table describing required input conditions. Current State (Qn) Next State (Qn+1) S (Set) R (Reset) 0 0 0 X 0 1 1 0 1 0 0 1 1 1 X 0 In this table: X means the input value is irrelevant or don't-care, as it doesn't affect the transition. 0 means the input should be set to logic 0. 1 means the input should be set to logic 1. JK Flip-Flop A JK flip-flop is a sequential circuit with two inputs (J and K) and two outputs (Q and Q'), with an excitation table describing the necessary input conditions for transitioning between states. Current State (Qn) Next State (Qn+1) J (Set) K (Reset) 0 0 0 X 0 1 1 X 1 0 X 1 1 1 X 0 In this table: X means the input value is irrelevant or don't-care. 0 means the input should be set to logic 0. 1 means the input should be set to logic 1. Excitation tables are crucial in sequential circuit design, ensuring intended state transitions and avoiding race conditions or undefined states, while maintaining circuit stability and functionality. Registers 6|Page  DIGITAL ELECTRONICS UNIT 4 Registers are essential data storage elements in digital electronics, storing binary data and holding intermediate results during operations. They execute machine instructions, manage data, and contribute to system performance. Here's an overview of registers and their significance: Data Storage: Registers are used to store binary data, typically in the form of binary numbers. These data can represent values, addresses, instructions, or intermediate results generated during computations. Fast Access: Registers are the fastest storage elements in a computer system. They are located within the CPU itself, making them easily accessible by the processor for fast read and write operations. This speed is essential for efficient data manipulation and execution of instructions. Size and Types: Registers come in various sizes, including 8-bit, 16-bit, 32-bit, and 64-bit, depending on the architecture of the processor. Different types of registers serve specific purposes, such as general-purpose registers (GPRs), special-purpose registers (SPRs), and control registers. Functionality: Registers perform various functions in a processor, including: Accumulator: Used for arithmetic and logical operations. Counter: Used for counting. Program Counter (PC): Stores the address of the next instruction to be fetched and executed. Memory Address Register (MAR): Holds the address of the memory location to read from or write to. Memory Buffer Register (MBR): Temporarily stores data read from or to be written to memory. Instruction Execution: During the execution of machine instructions, registers play a critical role. Data may be loaded into registers from memory, manipulated in registers, and then stored back in memory or used as operands for subsequent instructions. Data Transfer: Registers facilitate data transfer between different components of a computer system, such as the CPU, memory, and input/output devices. They act as temporary storage buffers for data movement. Parallelism: Modern processors use multiple registers to enable parallel processing and instruction pipelining, allowing multiple instructions to be executed simultaneously or in stages. Register File: In many processor architectures, registers are organized into a register file, a collection of registers that can be read from and written to. The register file provides a fast and efficient means of accessing and storing data. Context Switching: Registers are saved and restored during context switching, which occurs when the CPU switches from executing one task or process to another. Saving and restoring registers is essential to maintaining the state of a task. Counters Counters are digital circuits used in digital electronics, embedded systems, and microcontrollers for counting and tracking events, contributing to functionality and control in both simple and complex systems. Here's an overview of counters and their types: Types of Counters: 1. Asynchronous (Ripple) Counters: Asynchronous counters use multiple flip-flops (usually D-type flip-flops) to count events or states. Each flip-flop represents a binary digit (bit) of the count. 7|Page  DIGITAL ELECTRONICS UNIT 4 Counting is asynchronous, meaning that the flip-flops are triggered by the output of the preceding flip-flop. Asynchronous counters are straightforward to design but may suffer from longer propagation delays as the count increases due to ripple carry. 2. Synchronous Counters: Synchronous counters use a common clock signal to update all flip-flops simultaneously. They are often designed using JK or T flip-flops to minimize propagation delays. Synchronous counters are suitable for applications requiring precise timing and minimal clock skew. 3. Up Counters: Up counters increment the count value as events occur. The count increases with each clock cycle. When the maximum count value is reached, some up counters wrap around to zero, while others may have additional control logic to stop or reset. 4. Down Counters: Down counters decrement the count value as events occur. The count decreases with each clock cycle. When the count reaches zero, some down counters wrap around to the maximum count value, while others may have additional control logic to stop or reset. 5. Binary Counters: Binary counters count in binary form, which means that each flip-flop represents a binary digit (0 or 1). They are the most common type of counters and are used in various digital systems. 6. BCD (Binary-Coded Decimal) Counters: BCD counters count in binary-coded decimal form, which represents decimal digits (0-9) using four binary bits. They are used in applications that require direct decimal counting and display. Applications of Counters: Frequency Counters: Used to measure the frequency of input signals, such as clock signals or waveforms. Timer and Delay Generators: Utilized in various timing and delay applications, such as generating precise time delays or controlling events. Programmable Dividers: Used to divide a clock signal by a specified value, which is useful for generating lower-frequency clock signals from a higher-frequency source. Address Generation: In microcontrollers and microprocessors, counters are used to generate memory addresses for data retrieval and storage. Sequence Control: Counters are used in sequence control systems to generate patterns, sequences, or control signals based on the count value. Pulse and Event Counting: Counters can be used to count pulses, events, or occurrences in various systems and processes. Verilog code for flip-flops Verilog code examples for D, JK, and T flip-flops can be instantiated in Verilog designs, connecting clock signal, data inputs, and outputs, and simulating or synthesizing as needed. These examples provide RTL (Register-Transfer Level) descriptions of these flip-flops. 8|Page  DIGITAL ELECTRONICS UNIT 4 D Flip-Flop: A D flip-flop, also known as a Data or Delay flip-flop, stores one bit of data and is sensitive to the clock edge, as demonstrated in a Verilog implementation. module d_flip_flop ( input wire clk, // Clock input input wire d, // Data input output wire q // Output ); always @(posedge clk) begin q