D Type Flip Flops
D type flip-flops are fundamental building blocks in digital circuits. They are capable of storing a single bit of data and are commonly used for sequential logic operations. The state of a D type flip-flop is determined by its input and clock signal, making it useful for applications such as data storage, synchronization, and control in computer systems.
6 Key excerpts on "D Type Flip Flops"
eBook - ePub- J. Gibson(Author)
- 2013(Publication Date)
- Routledge(Publisher)
...This is shown schematically in Fig. 4.5 Fig. 4.5 The D-type flip-flop Because R is always the hazardous condition S = R = 1 cannot arise; the complete action of the flip-flop is simpler than that of the SR flip-flop and is given in Table 4.2. The D-type flip-flop will act as a storage element for a single binary digit (bit). The logic states 1 and 0 may be used to represent the two binary digits 1 and 0 resepctively. If the bit to be stored is presented as the appropriate logic level at the D (data) input and a clock pulse is applied to the flip-flop with the input maintained at D as long as the clock input is at 1, then Q will become the same as D. Thereafter Q will remain at this value until a new binary digit is input in the same way. Therefore a bit input at D is held (stored) by the flip-flop even when it no longer exists at D. Table 4.2 Some integrated circuit D-type bistables do not operate in the manner which has been described. These alternative devices are edge-triggered types and the behaviour of such devices is described later. To distinguish between the two forms of D-type bistable the version already described, that is a simple clocked SR bistable with an inverter between the S and R inputs, is often called a latch. If a number of D-type flip-flops are connected to the same source of clock pulses then the group of flip-flops will store several binary digits which may be used to represent a multiple digit binary number. This group of digits is called a word and the group of flip-flops is called a register. This grouping can be arranged in many ways and there are many types of register. 4.5 The serial shift register A serial shift register consists of a number of storage elements (bistables, flip-flops) arranged in a circuit with a single data input, a single clock input, and several outputs which are denoted here as OP A, OP B, OP C, …, etc. A new value is set up at the data input while the clock input is at 0...
eBook - ePub- J. Crowe, Barrie Hayes-Gill(Authors)
- 1998(Publication Date)
- Butterworth-Heinemann(Publisher)
...6 Flip-flops and flip-flop based circuits 6.1 INTRODUCTION Flip-flops 1 are vital ingredients in all except purely combinational logic circuits and are therefore extremely important. The SR (Set-Reset) flip-flop was introduced in the last chapter and illustrates an important point, namely that all flip-flops are asynchronous sequential logic circuits. However, by controlling their use they can be considered as synchronous circuit elements, which is exactly the approach taken here. Rather than providing a detailed description of how flip-flops are designed and operate, they are presented as discrete circuit elements (e.g. like a multiplexer or full adder) to be used in subsequent designs. In general, flip-flops possess data inputs (e.g. the S and R inputs), an output, Q (and its complement,), and also (as we will see) a ‘clock’ input which controls the activation, or clocking, of the flip-flop. That is the timing of the change in the flip-flop’s output in response to its inputs. 6.1.1 Flip-flop types The SR flip-flop can be set, or reset, or held in the same state via control of its inputs. However, it cannot be made to change state (i.e. its output give the complementary value) or toggle. Further thought reveals that if it could its operation would be unpredictable since it is an asynchronous circuit and therefore if made to toggle it would do so continuously (i.e. oscillate) until new inputs were presented. However, by gating the inputs to an SR flip-flop via AND gates under control of the flip-flop’s complementary outputs (Q and) it is possible to produce a flip-flop whose output can be made to toggle (i.e. go from 0 to 1 or vice versa) when activated (see Problem 6.1)...
eBook - ePub- Brian Holdsworth, Clive Woods(Authors)
- 2002(Publication Date)
- Newnes(Publisher)
...6 Latches and flip-flops 6.1 Introduction A digital logic circuit or system is usually made up of combinational elements such as NAND and NOR gates and memory elements which may, for example, be discrete flip-flops or latches. Alternatively, an interconnection of these devices may be found in a shift register, a counter, or in a variety of MSI and LSI packages. With the introduction of memory elements as components in digital systems, an additional variable, time, has been introduced and must be taken into account when designing digital systems. In effect, logic operations can be performed sequentially, information being stored in a memory element and released at some specified instant later so that it can take part in a controlled combinational operation. Systems operating in this way are called sequentially operated systems. There has always been considerable confusion over the use of the terms latch and flip-flop. It will be assumed in this book that a flip-flop is a device which changes its state at times when a change is taking place in the clock signal. The flip-flop is said to be either leading edge or trailing edge triggered, the edges referred to being those of the clock signal. On the other hand an asynchronous latch, without a control line, is continuously monitoring the input signals and changes its state at times when an input signal is changing. A synchronous latch is also continuously monitoring the input signals but in this case a change of state at the output can only occur when the control signal is active. In both cases the latch is driven by events, but for the synchronous latch the control signal has to be high before the input can be translated into a change at the output. 6.2 The bistable element By cross coupling a pair of NAND gates which are both connected as inverters, a bistable element is formed. There are two possible states for the element: (a) Q = 0, = 1 and (b) Q = 1, = 0 (see Figure 6.1)...
eBook - ePub- John Morris(Author)
- 2013(Publication Date)
- Routledge(Publisher)
...4 Sequential Logic ‘All sequential logic circuits rely on the existence of a basic memory or storage device, usually a bistable.’ The bistable Often called the ‘flip-flop’ this is a device that has two stable states. If a flip-flop is triggered so that its output is say logic 1 it will remain in this state until it is made to change, i.e. it has a memory. A glance at a catalogue or set of data sheets will reveal the type of flip-flops available, there is a bewildering array. Flip-flops These fall into a number of categories: the RS type; the D type; the T type; the JK type; the master-slave JK type; and the Edge triggered type. In order to help gain an understanding of the various types of flip-flop it is a good idea to take a very brief look at the operation of the basic type, the RS (or SR) bistable. The RS flip-flop The basic logic circuit using NOR gates is shown in Fig. 4.1(a) with a block diagram in Fig. 4.1(b). The truth table is shown in Fig. 4.2. Note that the state of the output is determined by the inputs, but when both inputs are at Logic 0 the previous state is held – or stored. The situation where both inputs can be at Logic 1 results in an indeterminable output (impossible to predict). Fig. 4.1a NOR gate RS flip-flop Fig. 4.1b Symbols for NOR gate RS flip-flop Fig. 4.2 Truth table for NOR gate RS flip-flop An RS flip-flop can also be made using NAND gates as shown in Fig. 4.3(a) and 4.3(b). Note The circuit symbol has circles drawn at the inputs to show this device triggers from a Logic 0, i.e. it is active low and is thus the opposite to the NOR RS type. Fig. 4.3a NAND gates RS flip-flop Fig. 4.3b Symbols for NAND gate RS flip-flop THOUGHT Does this mean that to set Q to ‘1’, S must be ‘0’? Yes. Study the table of Fig 4.4. Fig...
eBook - ePub- M. Michael Vai(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...In this case, t 2 has to be lengthened according to the time of changing X so that this change can propagate completely through the combinational logic circuit. This prevents incorrect state transitions from occurring. The above requirements are quite restrictive since t 1 must be long enough to correctly latch the new state variable values, but not so long that erroneous values can be stored. In general, it is very difficult to operate a circuit in such a manner properly. 5.4 D-Flip-Flop The design of a clock signal to be used in a sequential logic circuit can be significantly simplified if we can ensure that the memory element is never (or almost never) allowed to be transparent. This requires the use of an edge-triggered memory element, which responds to the rising or falling edge of the clock signal. The D-flipflop is such a device. Unlike a level-controlled device, an edge-triggered memory element captures data into storage at the extremely short time intervals when the clock goes through its transitions. The D-flip-flop shown in Fig. 5.15 is implemented by cascading two D-latches controlled by complementary signals (L and L ¯). This structure is known as a master-slave structure, in which the master D-latch receives a signal from outside and passes it, under the control of the clock, to the slave D-latch. Since the master and slave D-latches are controlled by complementary signals, the path from D to Q is never allowed to be transparent. 4 Fig. 5.15 Master-slave structure to form a D-flip-flop. Fig. 5.16 illustrates the behavior of this D-flip-flop. The latch delay times are ignored in this timing diagram. The output of the master D-latch (K) follows the changes of input D when clock L is high. The slave D-latch accepts K as its input. As noted, since the two D-latches are controlled by complementary signals, K is blocked from entering the slave D-latch while it is changing...
eBook - ePubDigital Design
Basic Concepts and Principles
- Mohammad A. Karim, Xinghao Chen(Authors)
- 2017(Publication Date)
- CRC Press(Publisher)
...The memory elements are also referred to as bistable electronic circuits; that is, they exist indefinitely in one of two binary states. Binary data are stored in a memory element by transitioning it into the 1 state to store a 1 and the 0 state to store a 0. The one or more inputs feeding the memory circuits are known as excitation inputs since they excite the circuit to reach a newer steady state. The two commonly used sequential memory elements are latches and flip-flops. Typically, the flip-flop circuit output indicates the latest state of the memory element. There are a number of different flip-flops available, each differing from one another in the number of inputs they have and in the manner in which its binary state is affected by the inputs. The changes in the values of the outputs of flip-flops often are a direct consequence of the frequency with which the circuit inputs change their values. However, there exists a special class of sequential memory device, known as a monostable multivibrator, whose output is often independent of the changes or rate of changes in the inputs. In this chapter, we introduce the characteristics of the various flip-flops. 7.2 Latches A latch is a bistable memory unit whose state is determined by its excitation inputs. It is the fundamental building block of a flip-flop. The latch is basically a combinatorial circuit that has one or more of its outputs fed back as its inputs. If an input signal drives the latch output to a 1, the memory element is called a set latch. On the other hand, it is referred to as a reset latch when the excitation inputs force the output to a 0. If the device has both set and reset excitation inputs, then the memory element is referred to as a set-reset latch. A flip-flop, on the other hand, differs from a latch in that it has a triggering signal called clock. The clock signal triggers a command that allows the flip-flop to change its state in accordance with its excitation inputs...
