After executing the instructions inside the body of the “process”, the “process” is stopped, which means that it waits for the next edge of the clk. Sometimes using sequential instructions is not only easier, but also safer and more efficient. For example, when sequential instructions are used to describe certain circuits such as rockers, the synthesis software detects the intended circuit and uses optimized structures to implement it. However, these optimized circuits are not used when we describe a rocker with logic gates. For more information on the danger of deriving a memory element from primitive doors, see section 8.3 of this book. The VHDL description of a type D rocker is explained later in this article. A process statement is listed in the architecture declaration. The process statement begins with the reserved keyword process and follows the begin statement of the architecture. The process statement also contains a begin statement.
The process statement ends with a statement to end the process. The process can have a label that also appears at the end of the process statement. The structure of the process instruction consists of a declaration section and an instruction section. Sequential instructions allow us to describe the abstract behavior of a circuit, rather than using low-level components such as different logic gates to build the circuit. This abstract description of behavior can sometimes simplify circuit design. This is because human thought and algorithms resemble a sequential process. Previously, only signals were used in if statements. The same rules apply to the use of variables, with one difference. If, as with a signal, a variable is assigned only in certain branches of the if statement, the feedback retains the previous value. Unlike a case where a signal is used, reading and writing a variable in the same process results in a return only if the read operation occurs before writing. In this case, the read value is the previous value of the variable. When a signal is used, reading and writing in the same process always leads to feedback.
This observation is used to create registers or counters using variables. Remember that a sequential process is interpreted synthetically by placing a toggle or register on each signal assigned in the process. This means that normally no variables are written in flip-flops or registers. However, if there is a return of a previous variable value, that return is implemented through a toggle or register to make the process synchronous. Example 6.21 describes a performance counter that uses the unsigned integer type. If an unsigned value is incremented and the value is the highest value in the range, the lowest value in the range is obtained. Line 9 of the code indicates the beginning of the “process”. The clk clock signal is specified in the sensitivity list of this “process”. This means that whenever clk changes, whether from bottom to top or vice versa, the sequential instructions of the “process” are executed. The sensitivity_list is the list of signals that are constantly monitored by the “process” construction. Each time a signal from the sensitivity_list changes, the “process” is activated and the sequential instructions of the “process” are executed line by line. Then the “process” waits for another change in the sensitivity_list.
In this case, the “process” is said to be suspended. Thus, a “process” has two states: either it is activated due to a change in sensitivity_list, or it rests and waits for a change in the signals of the sensitivity_list. In a previous article, we looked at some concurrent VHDL instructions. This article will attempt to develop a better understanding of the meaning of sequential utterance. Unlike concurrent statements, sequential statements are executed on a line-by-line basis. Therefore, we need to separate these two types of code. To do this, wrap the sequential statements in a VHDL construct called a process. A “process” can appear anywhere after the “begin” statement of “architecture”.
This algorithm is essentially based on human reasoning to count the number of leading zeros in a sequence of bits. For this reason, the method in Figure 4 appears to be easier to understand than Figure 2. As this example shows, our reasoning is usually sequential, that is, we examine the elements of the input vector one by one and make the appropriate decision. As the above structure suggests, sequential statements are placed between the keywords “begin” and “end”. In this part of the code, the order of the instructions is important, as is the code of traditional computer programming. With these simultaneous instructions, we can implement different logic gates. Therefore, simultaneous utterances are theoretically sufficient to describe any digital circuit. In summary, the algorithm in Figure 4 is sequential and uses abstract descriptions. Since it resembles human thought, it is easier to understand.
However, unlike the block diagram in Figure 2, the algorithm in Figure 4 gives us no clues about the underlying hardware structure. Since each of these two methods has its advantages and disadvantages, VHDL offers us the tools to use these two techniques. In the “process”, there are two “if” statements. VHDL`s “if” instructions are similar to conditional structures used in computer programming languages. If the condition after the if keyword is true, the statements are executed after the then keyword. Otherwise, the statements are executed after the “else” keyword. However, the “else” branch is optional. For example, in the code above, the external “if” statement does not have an “else” branch, unlike the internal “if” statement. Now let`s take a look at the VHDL description of a DFF with a positive edge. Please note that future articles will cover successive statements in detail and the purpose of this example is to familiarize yourself with some of the concepts discussed above. Sequential VHDL instructions allow the designer to describe the operation or behavior of a circuit as a sequence of related events. Sequential instructions reside in processes, functions, and procedures.
Sequential instructions differ from concurrent instructions in that they have an order dependency that may or may not involve a sequential circuit (with memory elements). The VHDL process statement is the primary method of input to a sequential statement. A process statement, including all the declarations and sequential statements it contains, is actually a single concurrent instruction within a VHDL architecture. This means that the designer can write as many concurrent processes and other instructions as necessary to describe a design without worrying about the order in which the simulator processes each instruction. Therefore, the process statement represents the behavior statement in VHDL. The example in Figure 4.23 illustrates the general structure of the process instruction. This statement can be read as “sQ set the value of sQ_next in the next simulation cycle. However, for a number of reasons, VHDL provides us with several sequential instructions where the order of the instructions is important: case instructions are often synthesized as multiplexers. In future articles, we will discuss the examples of sequential VHDL statements in more detail. However, to familiarize yourself with the concepts discussed in this article, let`s take a look at the VHDL description of a D-type toggle (DFF): Sequential instructions are used in processes to specify how signals are affected. The process is executed as a whole in order. After all the sequential instructions in the process have been executed, the new values are assigned to the signals.
This example reads the previous value of the variable count from the else branch of the if statement to calculate the next value. The result is feedback. Note that this example actually creates two registers. According to the feedback rules, the number of variables is recorded. The result of the signal is also recorded, because all signals assigned in a sequential process are recorded. This additional tab always contains the same value as the registry for the number of variables. The synthesis tool usually eliminates this redundant register. VHDL is a hardware description language that can be used to tell synthesis software which physical components should be added to the design and how those components are connected to each other. For example, we can use VHDL to describe the circuit in Figure 1.