Signal assignment sequential

The second architecture illustrates an alternative approach that uses an If-Else statement. This sequentially evaluates each expression in the construct tmtil it finds one diat is true and then executes the associated sequence of actions. This sequential statement is similar to the conditional signal assignment in that each possible two-way branch is evaluated in series, generating a multiplexer tree. Like the Case statement, the If-Else can initiate any number of actions within each brandv unlike its concurrent equivalent. 

Section 4.2 has illustrated a number of different ways in which a simple multiplexer can be designed in VHDL. Not all of these approaches are recommended for such a circuit, but die nature of the example has allowed a number of concurrent and sequential language statements to be introduced, not least various Signal assignment statements and the Case and If statements. For structural style architectures, a number of different techniques that allow components to be used in a design have also been demonstrated. 

Those that evaluated every condition sequentially but still did not verify that only one of them was TRUE. These circuits contained Component instantiations or Concurrent signal assignments. 

Those that evaluated each condition sequentially until one was found to be TRUE. These circuits contained If statements or Conditional signal assignments. 

The keyword Unaffected can be used in certain concurrent signal assignment statements to ensure that there is no change in the value of a signal given a particular condition. In this respect it is similar to the Null statement in sequential statement sections. Its use therefore implies the existence of feedback around the combinational logic. In some circuits this coxild possibly lead to hazardous conditions and so it should be used with caution. 

A conditional signal assignment statement is similar to an If-Else statement. The former is executed concurrently and the latter sequential (inside a process). 

Signal assignments can be performed in a concurrent or a sequential manner. For the designer, the difference between these two methods is that... 

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 ... 

A function is very similar to a Process statement in its operation except that it cannot contain signal assignment or Wait statements. This means that a function caimot be used to create sequential logic. As the RIPPLE functions illustrate, variables that are local to the function can be declared in the declarative part and its execution is sequential. All objects declared locally are discarded when the final result is returned. The s)mtax of a function and other important details are given in Box 6.4. 

Any sequential statements can be used in a function except a Signal assignment and a Wait This means that a function cannot infer a sequential block of logic. 

The sequence-specific assignment, however, can be made from NOE spectra (see Figures 18.17 and 18.18b) that record signals from H atoms that are close together in space. In addition to the interactions between H atoms that are far apart in the sequence, these spectra also record interactions between H atoms from sequentially adjacent residues, specifically, interactions from the H atom attached to the main chain N of residue number i -r 1 to H atoms bonded to N, Ca, and Cp of residue number i (Figure 18.19a). 

Figure 4 Sequential assignment of the backbone atoms for the segment Pro-109 to Val-113 of inhibited sfSTR by 4-D HCANNH and 4-D HCA(CO)NNH. Four planes are shown from each spectrum. The assigned backbone atoms are indicated in (A). In (B) the upper four planes in solid lines are from the 4-D HCANNH and the lower four planes in dashed lines are from the 4-D HCA(CO)NNH. The chemical shifts for the four correlated nuclei in each case are shown. The correlations continue for the segment Pro-109 to Pro-129. As Pro lacks a protonated N, this residue serves as a "stop" signal. The correlation of 19 residues with Pro at the N- and C-terminal ends is unique for this segment in the sequence of sfSTR, therefore these backbone atoms are specifically assigned without having to further assign side chain atoms. (From Ref. 5.)...

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