Clocked always statement

The semantics of the always statement implies that all statements in are to be executed only when a rising edge or a falling edge of clock occurs. We shall call this special always statement as a clocked always statement. 

It is possible to have a single module that has multiple clocked always statements. Here is such an example of multiple clocks used in a single model. 

What if we want to model a flip-flop with synchronous preset and clear In such a case, simply describe the synchronous preset and clear logic within a clocked always statement (an always statement with a clock event). Here is an example. 

A task call can represent either combinational logic or sequential logic depending on the context under which the task call occurs. By this, we mean that the output parameters of a task call may imply memory depending on the context in which they are assigned. For example, if a task call occurs in a clocked always statement (always statement with a clock event), then an output parameter in a task call may be synthesized as a flip-flop this is determined by using the flip-flop inference rales. A synthesis system implements a task call by expanding the task call in-line with the rest of the code in effect, no separate hierarchy for the task call is maintained. 

In a Moore finite state machine, the output of the circuit is dependent only on the state of the machine and not on its inputs. This is described i pictorially in Figure 3-5. Since the outputs are dependent only on the j state, a good way to describe a Moore machine is to use an always state- j ment with a case statement. The case statement is used to switch between j the various states and the output logic for each state is described in the appropriate branch. The always statement can have the clock event in its] event list to indicate that it is a clocked always statement. This models the] condition of a finite state machine going from state to state synchronously j on every clock edge. The machine state itself is modeled using a reg vari-] able (a variable of reg data type). 

What if the assignment and reading of a variable all occur in the same clocked always statement In such a case, blocking assignments may be used. 

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. 

In all the above cases, a variable was assigned under the control of a clock and its value was used outside of the always statement, thus requiring its value to be saved in a flip-flop. 

What happens in the above case if we switch the order of the statements around In this case, since the value of Temp is used before its assignment, its value needs to be retained across multiple clock cycles, thereby inferring flip-flops for Temp. Temp models the internal state of the always statement. This is shown in the following example, where Temp is used before its assignment. 

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... 

Figure 2-50 Multiple clocks within an always statement.

This module has two always statements. The statements in the first always statement are controlled by a positive edge of clock Vt 15Clock, while the statements in the second always statement are controlled by the positive edge of clock Dsl Clock. 

A restriction usually imposed by a synthesis system is that a variable cannot be assigned under the control of more than one clock. For example, it would be illegal to assign to AddState in the second always statement. 

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. 

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. 

Here the intention appears to be to store the value of PresentState in a flip-flop (rising-edge-triggered). After synthesis, not only is there a flip-flop for PresentState, there are also four flip-flops for Zout. This is because Zout is assigned under the control of a clock. It may or may not be the intention to generate flip-flops for Zout. If not, then a case statement needs to be written in a separate always statement in which Zout is assigned, this... 

The synthesized netlist, as shown in Figure 5-4, evaluates on all changes of Read and Clock, while the always statement executes only on changes to Read. 

Recommendation For an always statement without a clock event (that is, when modeling combinational logic), include all variables read in the always statement in the event list. 

To understand the behavior of these processes, we associate one or more subscripted abstract variables to each declared object x in the HDL text, to hold current(x), the value of X when the description has stabilized (just before the clock edge), and the successively assigned values to the object in the process or always statement. 

Behavioral Compiler supports only using one clock within a process/always statement. 

The time difference t in IS is therefore always larger than the time difference r in IS, where the clock is at rest, i.e., a moving clock loses time as compared to a clock at rest. This phenomenon is called relativistic time dilation and might briefly be summarized by the statement that "proper time is always the shortest time between two events". 

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