Variable inferred assignment

How is a flip-flop inferred It depends on the modeling style being followed and the context under which a variable is assigned a value. This... 

In the synthesized netlist, the output of the AND2 gate is the variable 77 so is the output of the OR2 gate. In this example, each assignment to the integer variable infers a unique wire. 

A latch may be inferred for a variable assigned in a case statement, just like in an if statement. If a variable is not assigned a value in all possible executions of the always statement, such as when a variable is assigned a value in only some branches of a case statement, a latch is inferred for that variable. See the following example. 

Hip-flop inference rule is simple If a variable is assigned a value under the control of a clock edge, a flip-flop is generated an exception to this rule is when a variable is assigned and used only locally within an always statement as an intermediate variable. 

Integer variables assigned under the control of a clock edge are also inferred as flip-flops. Here is an example where an integer variable is assigned under clock control. Four flip-flops are inferred for the variable IntState) the other high-order bits of the variable are optimized away (since they are not used). 

If a variable is assigned a value z in an always statement in which the variable is also inferred as a flip-flop, then it becomes necessary to save the enabling logic of the three-state also in a flip-flop. Here is the same example as above except that the always statement is controlled by a clock event. 

Here is the behavioral model for the transmitter block TX. This model is a synthesizable model. Rising-edge-triggered flip-flops are inferred for variables TBR, TR, TRE, TBRE, DOUT, CBTT and PA this is because these variables are assigned values under the control of clock CK. 

If the signal (or variable) infers a storage element (in a synchronous section elsewhere) then the logic of the assignment becomes part of the asynchronous combinational input logic to the storage element. 

Interaction of C02 with petroleum ether solutions of cis-Mo(N2)2 (PMe3)4 results in the formation of the pale yellow complex trans-Mo (C03) (PMe3)4 and of variable amounts of the carbonyl-carbonate derivatives Mo(C03) CO(PMe3)4 and Mo(C03) CO(PMe3)3 2. The yellow complex is a micro-crystalline, moderately air stable solid, which can be heated at 40-50°C for several hours without noticeable decomposition. Its infrared spectrum displays bands at 1670, 1155 and 1100 cnr1, which by comparison with the spectrum of a sample 50% enriched in C02 can be assigned to vibrations associated with the coordinated C02 molecules, but no conclusions as to the coordination mode of the C02 ligand can be inferred from this data. 

The variable Fox is not assigned in the else-branch of the conditional statement. Consequently, a latch is inferred for Fox since it needs to retain its value when Sat is true. The circuit synthesized in shown in Figure 1-10. 

A general rule for latch inferencing is that if a variable is not assigned in all possible executions of an always statement (for example, when a variable is not assigned in all branches of an if statement), then a latch is inferred. 

In this example, what should be the value of Grade if Marks has the value 12 It may be intended to be a don t care, but from the language semantics viewpoint, the variable Grade retains its last value, since no value is assigned to the variable explicitly when Marks has the value 12. Therefore a latch is inferred for Grade in keeping with the simulation semantics of a reg variable. 

In the previous section, we saw that a latch may be inferred for a variable that is not assigned a value for all possible values of a case expression. Sometimes it is the case that the designer does not expect the case expression to have any value other than those listed in the case items. Here is an example. 

A synthesis tool on encountering such a directive on a case statement understands that all possible values (that can occur in the design) of the case expression have been listed and no other values are possible. Consequently, a variable assigned in all branches of the case statement will never infer a latch. Here is the case statement in the NextStateLogic module with the directive specified. 

An alternative way to avoid latches in the above example is to specify a default branch in the case statement or to make a default assignment to all variables assigned in a case statement (in this example, NextToggle), prior to the case statement. Here is an example that uses a default branch to avoid inferring latches. 

The variable NextState is assigned a value only when ClockA is 1. If ClockA is 0, NextState retains its previous value, thus inferring a latch. 

A variable declared locally within an always statement is also inferred as a latch if it is incompletely assigned in a conditional statement (if statement or case statement). This is shown in the following module. 

If a variable is used before it is assigned in an incompletely specified conditional statement, then a latch is inferred. Here is such a module. 

If a variable, that is inferred as a latch, is assigned constant values in some branches of a conditional statement, bits that are 1 get assigned to the preset terminal of the latch, while those with 0 get assigned to the clear terminal. This is shown in the following example. 

A flip-flop is inferred from a variable when it is assigned a value in a special form of always statement. This always statement is of the form ... 

The assignment to output Zee occurs only at the falling edge of the clock. Variable Zee is inferred to be a falling-edge-triggered flip-flop. The flip-flop shown in the figure has a data-select, that is, the input Control selects either A or B as the data for the flip-flop. 

Figure 2-48 Flip-flops inferred from a variable assigned under clock control.

No flip-flops are inferred for Temp since it is locally declared within the always statement and a value is assigned to the variable and used immediately in the same clock edge. Flip-flops are inferred for NextState (as this... 

The synthesized netlist is the same as in Figure 2-48. Notice that on every clock edge, NextState always get the value of Temp assigned in the previous clock cycle, but not so in the synthesized netlist. The recommendation here is to avoid using locally declared variables in this fashion. Hopefully a synthesis tool will issue a warning if no flip-flops are inferred for Temp. 

Two flip-flops are inferred to hold the value of the variable MealyState with the specified state assignment. The default branch in the case statement can be avoided by specifying the case statement as full case , as shown next. 

Synthesis infers four flip-flops for this model, three for variable Previous and one for SeqFound. However, optimization reveals that one of the flip-flops for Previous is not necessary and hence it is removed. In this model, the output is latched since it is assigned a value under the control of a clock edge. If a latched output is not desired, then the assignment to SeqFound must be done outside the always statement. Such a module is shown next. 

A variable that does not have a value assigned in all branches of a case statement or an if statement can lead to a latch being built. This is because in Verilog HDL, a reg variable (assigned within an always statement) infers memory, and thus if the variable is not assigned a value in all branches of a conditional statement, the value needs to be saved in memory. Here is an example. 

Sampling. Statistical analysis relies on a clear distinction between random variability and variation in results due to deliberate manipulation of known factors in an experiment. Treatments are assigned systematically to subjects while individual subject characteristics contribute to random variability. It is important to understand that studies are not performed to learn what happened to the participants—they are conducted to provide a basis for predicting what will likely happen to an entire future subject population if a product is approved and broadly marketed. Inferences from a sample to a larger population are only possible when certain statistical principles are followed in the selection of that sample. Those principles are rarely followed in clinical research today, and inferences to future subject populations are therefore not generally supported from a statistical standpoint. 

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