Flip flop D-type

So far we have talked about synthesizing simple D-type flip-flops. What if we wanted to infer a flip-flop with asynchronous preset and clear To generate such a flip-flop, a special form of if statement has to be used. This is best shown with an example template. 

Here is another example. This is a 3-bit up-down counter that shows how a pre-built D-type flip-flop is used along with its remaining behavior. The key statements that are necessary to be added are the module instantiation statements. With such a model, a synthesis system retains the prebuilt component in the synthesized design to achieve the desired result this is shown in the synthesized netlist. 

FIGURE 8.89 Three standard cells. A two-input NAND gate is shown on the left, a D type flip-flop is in the middle, and a simple inverter is shown on the right. 

Figure 5.9 register description using instantiated D-type flip flops. 

Figure 5.10 Behavioural description of a D-type flip flop with an additional internal signal, INTERNAL Q.

The optimized circuits for the asynchronous architectures are, however, quite different. ASYNCJFI, shown in Figure 5.19, has imdergone a dramatic transformation that has doubled its cell count but actually reduced the number of transistors. This has been achieved by replacing each all-in-one multiplexer and flip flop element with simpler D-type flip flops and front-end combinational logic. The flip flops still retain their asynchronous initialization inputs and feedbadc connection but the multiplexed data inputs have gone. Note that the feedback connection now uses the inverted output of each flip flop. The result of area optimization is a slightly smaller... 

Let us look at some of the traps it is easy to fall into when making sequential signal assignments. The following examples are simple but bad versions of the D-type flip flop. The function of each should be clear without showing the associated entity ... 

Sequential logic can be instantiated but is usually inferred. The synthesizer will usually select D-type flip flops and use front-end combinational logic to create more complex sequential functions. 

The clocked processes infer storage elements. The counters designed in Chapter 5 investigated the types of D-type flip flops that are implemented under different circumstances. It should be easy to predict which will be used in these registers. 

This section looks at the components used in the structural style architectures of the shift register. These components are constructed in a behavioural style and describe D and JK-type flip flops. Both architectures, DFF and JKFF respectively, contain a synchronous reset, PC, and employ a Walt statement. 

The structural architectures demonstrated the use of manual and generated component instantiatiotrs in the creation of the 4-bit shift register A component instantiated from a vendor s library will always be optimized. It is therefore appropriate in these examples to instantiate the optimized versions of the D- and JK-type flip flops. 

Lipophilicity is intuitively felt to be a key parameter in predicting and interpreting permeability and thus the number of types of lipophilicity systems under study has grown enormously over the years to increase the chances of finding good mimics of biomembrane models. However, the relationship between lipophilicity descriptors and the membrane permeation process is not clear. Membrane permeation is due to two main components the partition rate constant between the lipid leaflet and the aqueous environment and the flip-flop rate constant between the two lipid leaflets in the bilayer [13]. Since the flip-flop is supposed to be rate limiting in the permeation process, permeation is determined by the partition coefficient between the lipid and the aqueous phase (which can easily be determined by log D) and the flip-flop rate constant, which may or may not depend on lipophilicity and if it does so depend, on which lipophilicity scale should it be based ... 

Table 9.5a. Intramolecular interglucose hydrogen bonds of the flip-flop type 0-( )D D( )-0 observed in, 8-eyclodextrin at room temperature (neutron diffraction). The occupational parameters of the oxygen atoms are 1.0, those of the deuterium atoms vary between 0.344 and 0.654. (For atom numbering scheme see Chapter 18 [454].)... 

Determines the type of flip flop that will be used in a sequential circuit. Options are generally for multiplexed data input type (M) or single input D-type (D),... 

The statistics for these two circuits are shown in Table 5.3. As expected, they indicate that the drcuit for the JK flip flop is three times as large as the one for the D-type. The delay figures given in this table compare the effects of the front-end combinational logic on each circuit. They indicate that the longest delay for an input signal to reach the latch in the JK flip flop is well over double that of the D-type. 

The optimization statistics in Table 5.3 show that the JK flip flop has had almost two-thirds of its logic stripped out whereas the transistor coimt for the D-type has only fallen by one-fifth. This demonstrates the highly circuit-dependent nature of the optimizer s success and that an efficient VHDL description does not guarantee that the synthesizer will produce a good circuit. 

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