Edge events are queued and then queued by @(…) Guards serves instead of @(?) being a guard waiting for Edge events and blocking them until an Edge event occurs. A process can contain a sensitivity list in parentheses after the keyword process. The sensitivity list identifies a set of signals that the process monitors for events. If the sensitivity list is omitted, the process must contain one or more waiting instructions. On the other hand, if the sensitivity list is included, the process body cannot contain waiting instructions. Instead, there is an implicit wait statement, just before the end-of-process keywords, that contains the signals listed in the sensitivity list as signals in an on clause. We may also have used a more complex definition of the function rising_edge than necessary (or may be available in any simulators or synthesis tools). The alternative simple method is to use the clock in the sensitivity list and then check whether the clock value is 1 for the ascending edge or 0 for the descending edge. The equivalent VHDL for an up-edge D-type rocker is specified. Note that we used the implicit sensitivity list (with a wait on clk statement) as opposed to the explicit sensitivity list, although we can use the two interchangeably.

1. Regular monitoring of events: The execution of the instruction is carried out in case of signal changes or positive or negative signal transitions. For example, the installation of a clock and the negation of the reset, etc. There are two types of synchronization controls in Verilog: delay and event expressions. Delay control is just a way to add a delay between when the simulator encounters the instruction and when it actually executes it. Event expression is used to delay the statement until a simulation event occurs, which can be a change in the value of a mesh or variable (implicit event), or an explicitly named event that is raised in another procedure. In Verilog, the @ sign specifies an edge-dependent event control that is blocked until a value transition (an edge) occurs for one of the event identifiers. A block without edge constructs in the sensitivity list represents either a pure combination circuit or the block outputs can be set up. A simple example could derive a single GOLD door as follows: We have used processes in examples in this chapter and previous chapters quite widely, so we have seen most of the details of how they are written and used. In summary, let`s look at the formal syntax of a process statement and verify how the processes work. The syntax rule is Note that in this case, d does not appear in the sensitivity list because it is not required. The toggle will only do anything if an ascending edge occurs on the clock signal (clk).

There are a number of different methods to describe this feature, all of which are equivalent. In this case, we have explicitly defined the clk signal in the sensitivity list. For example, the Always block and the initial second block are synchronized by a_event. Events can be reported as arrays, as in the case of b_event, which is a size 5 array, and index 3 is used for triggering and waiting purposes. This tells the simulator to update the feature whenever A or B changes. A simple mistake with this coding style is to leave a signal from the list, which gives the impression that the design simulation has a memory effect. In the example above, if B were not included in the sensitivity list, changes made to B without a change in A would not result in a change in output C. The memory effect is shown in Figure 3.4. Output Y, which is the result of a function with an incomplete sensitivity list, does not change when B changes. It appears that the previous values are stored. Verilog now allows you to replace the sensitivity list with *, which is a handy shortcut that eliminates these problems by adding all the meshes and variables read by the statemnt, as shown below.

An event controls the execution of a statement or block of a statement. Changes in the value of variables and meshes can be used as a synchronization event to trigger the execution of other procedural statements and are an implicit event. If an expression is evaluated at the same result, it cannot be considered an event. There are several types of event-based controls. Synthesizers ignore combined sensitivity lists. The list is only checked to determine whether the generated logic should be combinatorial or sequential. Thus, while the two outputs in Example 3.4 simulate differently, they synthesize on exactly the same hardware. This is an example of a simulation-synthesis divergence that can occur with an incomplete sensitivity list. In the sensitivity list, list all the signals that you want the code to evaluate in the process when the state changes. For example, a sensitivity list often uses clock or master reset.

Whenever the reset or clock state changes, the process code is executed. .