In the example above, the initial execution of the block begins and always the execution of the block begins at moment 0. Always block waits for the event, here positive edge of the clock, while the initial block has executed only all the instructions in the start and end instructions without waiting. To allow the use of multiple statements where expected, the compound statement is provided: This section designs a 4×1 multiplex with the If-else statement. We are already seeing how the “if” statement works in Chapter 2. In lines 11 to 24 of list 4.3, the words “else if” and “else” are added to the “if” statement. Note that the If-else block can contain multiple “else if” statements between an “if” statement and an “else” statement. In addition, “begin – end” is added to line 12-15 of list 4.3, which is used to define multiple statements in the “if”, “else if” or “else” block. Fig. 4.5 shows the waveform created by Modelsim for listing 4.3. A procedure statement summarizes the purpose, scope and prescribed manner of complying with an established policy or completing a work unit. As required by the rigor and complexity of the previous sequence of steps, a procedural instruction can range from one sentence to several sections or paragraphs. If an active statement contains a blocking procedural mapping, the next statement is executed after the active statement is executed (in the next step of the simulation). Note that we can write the complete design with sequential programming (similar to C, C++ and Python codes).
But this can lead to a very complex material design, or one that cannot be synthesized at all. The best way to design is to create small units with “continuous assignment instructions” and “procedural assignment instructions” and then use the structural modeling style to create the mainframe. Statements that contain non-blocking procedural mappings are executed in the same simulation cycle. Remember: (see the words “conception”, “logic” and “statement” carefully) In both cases, the sentence is evaluated and, if true, the first subpoena is executed. In the second case, the second subpromiss is executed if the expression is false or @. As usual, the ambiguity “else” is solved by associating another with the last without any other if it is encountered. Verilog provides two loop instructions, `for` loop and `while` loop`. These loops are very different from software loops.
Suppose `for i = 1 to N is a loop`, then `i` is assigned a value in the software, i.e. first i=1, then the next cycle i=2 and so on. While Verilog implements N logic for this loop that runs in parallel. In addition, the software requires “N” cycles to complete the loop, while Verilog requires the loop to run in a single cycle. Since a procedural instruction is a summary of information relating to the progress of a procedure, it should be drawn up only after the completion of the procedure. Procedural instructions are executable instructions. Statements are executed sequentially, unless otherwise specified. The shift statement removes the first value from the list referenced by the lvalue.
Therefore, the list cannot be a null list. If the value is omitted, the list of arguments $ is assumed; Therefore, it can only be used in a functional body. The corresponding delete statements of the shift statements are: Begin: clk gets 0 after 1 unit of time, reset gets 0 after 11 units of time, activate after 16 units of time, data after 19 units. All statements are executed sequentially. Another form of procedural continuous assignment is provided by the declarations of force and release procedure. These statements have a similar effect on the Assign-Deassigned pair, but a force can be applied to both meshes and registers. If a procedure block contains more than one statement, these statements in the left side of a procedural mapping must be one of the following: The switch statement causes the control to be transferred to one of the following statements, depending on the value of an expression. It has the form When the switch statement is executed, its expression is evaluated and compared to each case constant. If one of the case constants is equal to the value of the expression, the control is passed to the statement that follows the corresponding case prefix. If no case constant matches the expression and there is a default prefix, the control is passed to the prefixed statement. If it is not case sensitive and there is no default value, none of the command-line option statements are executed. causes the termination of the smallest instruction while, doing, for or changing; is transmitted to the statement that follows the completed statement.
A statement block with non-blocking procedural assignments has similar functionality to a group of statements in a fork join block (Example 3). The expression of a blocking procedure assignment is evaluated and affected when the statement is found. In a group of sequential statements from start to end, the execution of the next statement is blocked until the assignment is complete. The assignment assessment is delayed by the delay if the delay is specified before the registry name. If the delay is specified before the expression, the expression evaluates when the statement is found and assigned in the time step specified by the timeout. In list 2.3, we have seen that simultaneous instructions are executed in parallel, that is, the order of the instruction does not matter. While list 2.6 shows the example of “sequential statements” where statements are executed one after the other. Here are the relationships between `Statements` and `Design Type`. The Verilog HDL contains two types of procedural mapping statements: blocking procedural assignments (Example 1) and non-blocking procedural assignments (Example 2). A function returns to its caller using the return statement, which takes one of the following forms: The subde statement is executed repeatedly as long as the value of the expression remains true.
The test takes place before each execution of the statement. Typically, expression statements are associations or function/procedure calls. For example, the case statement is displayed on lines 11 to 16 of list 4.4. “s” is used in the case statement on line 11; whose value is checked with the keyword `when` on lines 12 and 13, and so on. The value of the output y depends on the value of `s`, e.B. if `s` is `1`, then line 12 will be true, so the value of `i1` will be assigned to `y`. Note that we can use the “integer” notation (line 12) as well as the “binary” notation (line 13) in the “case” and “if” statements. The design generated by list 4.4 is shown in Fig.
4.6. A default template for how-to instructions should already exist. If not, one should be developed to ensure that all procedure instructions have the same “look and feel”. In this chapter, various statements on procedural tasks are discussed. Problems with loops are discussed and finally the loop is implemented with the “if” statement. Finally, it is demonstrated that Verilog designs may differ in simulation results and implementation results. Procedural assignments are used to update registries. There are two types of procedural blocks in Verilog: the statement is usually composed. Each statement in the statement can be marked with one or more case-sensitive prefixes as follows: In a non-blocking procedure assignment, the expression is evaluated when the statement is found and the assignment is moved to the end of the time step. In a group of sequential statements from start to finish, the execution of the next statement is not blocked and can be evaluated before the end of the assignment.
An instruction group with a non-blocking assignment has similar functionality to a statement group in a fork join block. Fig. 4.7 Loop with the `if` statement, listing 4.6 with N = 1, or to provide a null field for a loop instruction, such as: Instructions for assigning and decommissioning procedural assignments allow you to place continuous assignments in registers for controlled periods of time. Assigning procedures replaces assignments to a registry. Assigning the procedural statement terminates a continuous assignment to a registry. Then, as the next “always” statement (line 33), increase the number “count” by 1 if currentState is “continueState”; Otherwise, count for stopState is set to 0. Finally, the count on the exit is indicated by line 41. This allows us to implement the loops with the “always” statements. Procedural assignments can only be used in structured procedures (always, initial, task, function).
Most of the statements are expression instructions that have the following form: in Chapter 2, a 2-bit comparator with “procedural mappings” is designed. In this chapter, the keyword `if` was used in the `always` statement block. This chapter presents other keywords of this type that can be used in procedural tasks. A null statement is useful for placing a label just before the } of a compound statement, for example: the 4.6 list creates a loop with the `if` statement that counts the number up to the `x` entry. We do not need to define all possible cases in the `case-statement`, the keyword `default` can be used to provide output for undefined cases as shown in list 4.5. Only two cases are defined here, namely 7 and 3; For the rest of the cases, the default value (i2) is sent to the output. passes the control to the loop continuation part of the smallest while, do, or for statement.c is until the end of the loop. Block assignments run in the order in which they are encoded and are therefore sequential. Because they block the execution of the next statement, they are called assignment blocking until the active statement is executed. The assignment is made with the symbol “=”.
Example a = b; All statements in the Always block are executed sequentially. If the module contains multiple Always blocks, all Always blocks run in parallel, that is. . .