## VHDL Logical Operators and Signal Assignments for Combinational Logic

In this post, we discuss the VHDL logical operators, when-else statements , with-select statements and instantiation . These basic techniques allow us to model simple digital circuits.

In a previous post in this series, we looked at the way we use the VHDL entity, architecture and library keywords. These are important concepts which provide structure to our code and allow us to define the inputs and outputs of a component.

However, we can't do anything more than define inputs and outputs using this technique. In order to model digital circuits in VHDL, we need to take a closer look at the syntax of the language.

There are two main classes of digital circuit we can model in VHDL – combinational and sequential .

Combinational logic is the simplest of the two, consisting primarily of basic logic gates , such as ANDs, ORs and NOTs. When the circuit input changes, the output changes almost immediately (there is a small delay as signals propagate through the circuit).

Sequential circuits use a clock and require storage elements such as flip flops . As a result, changes in the output are synchronised to the circuit clock and are not immediate. We talk more specifically about modelling combinational logic in this post, whilst sequential logic is discussed in the next post.

## Combinational Logic

The simplest elements to model in VHDL are the basic logic gates – AND, OR, NOR, NAND, NOT and XOR.

Each of these type of gates has a corresponding operator which implements their functionality. Collectively, these are known as logical operators in VHDL.

To demonstrate this concept, let us consider a simple two input AND gate such as that shown below.

The VHDL code shown below uses one of the logical operators to implement this basic circuit.

Although this code is simple, there are a couple of important concepts to consider. The first of these is the VHDL assignment operator (<=) which must be used for all signals. This is roughly equivalent to the = operator in most other programming languages.

In addition to signals, we can also define variables which we use inside of processes. In this case, we would have to use a different assignment operator (:=).

It is not important to understand variables in any detail to model combinational logic but we talk about them in the post on the VHDL process block .

The type of signal used is another important consideration. We talked about the most basic and common VHDL data types in a previous post.

As they represent some quantity or number, types such as real, time or integer are known as scalar types. We can't use the VHDL logical operators with these types and we most commonly use them with std_logic or std_logic_vectors.

Despite these considerations, this code example demonstrates how simple it is to model basic logic gates.

We can change the functionality of this circuit by replacing the AND operator with one of the other VHDL logical operators.

As an example, the VHDL code below models a three input XOR gate.

The NOT operator is slightly different to the other VHDL logical operators as it only has one input. The code snippet below shows the basic syntax for a NOT gate.

• Mixing VHDL Logical Operators

Combinational logic circuits almost always feature more than one type of gate. As a result of this, VHDL allows us to mix logical operators in order to create models of more complex circuits.

To demonstrate this concept, let’s consider a circuit featuring an AND gate and an OR gate. The circuit diagram below shows this circuit.

The code below shows the implementation of this circuit using VHDL.

This code should be easy to understand as it makes use of the logical operators we have already talked about. However, it is important to use brackets when modelling circuits with multiple logic gates, as shown in the above example. Not only does this ensure that the design works as intended, it also makes the intention of the code easier to understand.

• Reduction Functions

We can also use the logical operators on vector types in order to reduce them to a single bit. This is a useful feature as we can determine when all the bits in a vector are either 1 or 0.

We commonly do this for counters where we may want to know when the count reaches its maximum or minimum value.

The logical reduction functions were only introduced in VHDL-2008. Therefore, we can not use the logical operators to reduce vector types to a single bit when working with earlier standards.

The code snippet below shows the most common use cases for the VHDL reduction functions.

## Mulitplexors in VHDL

In addition to logic gates, we often use multiplexors (mux for short) in combinational digital circuits. In VHDL, there are two different concurrent statements which we can use to model a mux.

The VHDL with select statement, also commonly referred to as selected signal assignment, is one of these constructs.

The other method we can use to concurrently model a mux is the VHDL when else statement.

In addition to this, we can also use a case statement to model a mux in VHDL . However, we talk about this in more detail in a later post as this method also requires us to have an understanding of the VHDL process block .

Let's look at the VHDL concurrent statements we can use to model a mux in more detail.

## VHDL With Select Statement

When we use the with select statement in a VHDL design, we can assign different values to a signal based on the value of some other signal in our design.

The with select statement is probably the most intuitive way of modelling a mux in VHDL.

The code snippet below shows the basic syntax for the with select statement in VHDL.

When we use the VHDL with select statement, the <mux_out> field is assigned data based on the value of the <address> field.

When the <address> field is equal to <address1> then the <mux_out> signal is assigned to <a>, for example.

We use the the others clause at the end of the statement to capture instance when the address is a value other than those explicitly listed.

We can exclude the others clause if we explicitly list all of the possible input combinations.

• With Select Mux Example

Let’s consider a simple four to one multiplexer to give a practical example of the with select statement. The output Q is set to one of the four inputs (A,B, C or D) depending on the value of the addr input signal.

The circuit diagram below shows this circuit.

This circuit is simple to implement using the VHDL with select statement, as shown in the code snippet below.

## VHDL When Else Statements

We use the when statement in VHDL to assign different values to a signal based on boolean expressions .

In this case, we actually write a different expression for each of the values which could be assigned to a signal. When one of these conditions evaluates as true, the signal is assigned the value associated with this condition.

The code snippet below shows the basic syntax for the VHDL when else statement.

When we use the when else statement in VHDL, the boolean expression is written after the when keyword. If this condition evaluates as true, then the <mux_out> field is assigned to the value stated before the relevant when keyword.

For example, if the <address> field in the above example is equal to <address1> then the value of <a> is assigned to <mux_out>.

When this condition evaluates as false, the next condition in the sequence is evaluated.

We use the else keyword to separate the different conditions and assignments in our code.

The final else statement captures the instances when the address is a value other than those explicitly listed. We only use this if we haven't explicitly listed all possible combinations of the <address> field.

• When Else Mux Example

Let’s consider the simple four to one multiplexer again in order to give a practical example of the when else statement in VHDL. The output Q is set to one of the four inputs (A,B, C or D) based on the value of the addr signal. This is exactly the same as the previous example we used for the with select statement.

The VHDL code shown below implements this circuit using the when else statement.

• Comparison of Mux Modelling Techniques in VHDL

When we write VHDL code, the with select and when else statements perform the same function. In addition, we will get the same synthesis results from both statements in almost all cases.

In a purely technical sense, there is no major advantage to using one over the other. The choice of which one to use is often a purely stylistic choice.

When we use the with select statement, we can only use a single signal to determine which data will get assigned.

This is in contrast to the when else statements which can also include logical descriptors.

This means we can often write more succinct VHDL code by using the when else statement. This is especially true when we need to use a logic circuit to drive the address bits.

Let's consider the circuit shown below as an example.

To model this using a using a with select statement in VHDL, we would need to write code which specifically models the AND gate.

We must then include the output of this code in the with select statement which models the multiplexer.

The code snippet below shows this implementation.

Although this code would function as needed, using a when else statement would give us more succinct code. Whilst this will have no impact on the way the device works, it is good practice to write clear code. This help to make the design more maintainable for anyone who has to modify it in the future.

The VHDL code snippet below shows the same circuit implemented with a when else statement.

## Instantiating Components in VHDL

Up until this point, we have shown how we can use the VHDL language to describe the behavior of circuits.

However, we can also connect a number of previously defined VHDL entity architecture pairs in order to build a more complex circuit.

This is similar to connecting electronic components in a physical circuit.

There are two methods we can use for this in VHDL – component instantiation and direct entity instantiation .

• VHDL Component Instantiation

When using component instantiation in VHDL, we must define a component before it is used.

We can either do this before the main code, in the same way we would declare a signal, or in a separate package.

VHDL packages are similar to headers or libraries in other programming languages and we discuss these in a later post.

When writing VHDL, we declare a component using the syntax shown below. The component name and the ports must match the names in the original entity.

After declaring our component, we can instantiate it within an architecture using the syntax shown below. The <instance_name> must be unique for every instantiation within an architecture.

In VHDL, we use a port map to connect the ports of our component to signals in our architecture.

The signals which we use in our VHDL port map, such as <signal_name1> in the example above, must be declared before they can be used.

As VHDL is a strongly typed language, the signals we use in the port map must also match the type of the port they connect to.

When we write VHDL code, we may also wish to leave some ports unconnected.

For example, we may have a component which models the behaviour of a JK flip flop . However, we only need to use the inverted output in our design meaning. Therefore, we do not want to connect the non-inverted output to a signal in our architecture.

We can use the open keyword to indicate that we don't make a connection to one of the ports.

However, we can only use the open VHDL keyword for outputs.

If we attempt to leave inputs to our components open, our VHDL compiler will raise an error.

• VHDL Direct Entity Instantiation

The second instantiation technique is known as direct entity instantiation.

Using this method we can directly connect the entity in a new design without declaring a component first.

The code snippet below shows how we use direct entity instantiation in VHDL.

As with the component instantiation technique, <instance_name> must be unique for each instantiation in an architecture.

There are two extra requirements for this type of instantiation. We must explicitly state the name of both the library and the architecture which we want to use. This is shown in the example above by the <library_name> and <architecture_name> labels.

Once the component is instantiated within a VHDL architecture, we use a port map to connect signals to the ports. We use the VHDL port map in the same way for both direct entity and component instantiation.

Which types can not be used with the VHDL logical operators?

Scalar types such as integer and real.

Write the code for a 4 input NAND gate

We can use two different types of statement to model multiplexors in VHDL, what are they?

The with select statement and the when else statement

Write the code for an 8 input multiplexor using both types of statement

Write the code to instantiate a two input AND component using both direct entity and component instantiation. Assume that the AND gate is compiled in the work library and the architecture is named rtl.

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## Signal Assignments in VHDL: with/select, when/else and case

Sometimes, there is more than one way to do something in VHDL. OK, most of the time , you can do things in many ways in VHDL. Let’s look at the situation where you want to assign different values to a signal, based on the value of another signal.

## With / Select

The most specific way to do this is with as selected signal assignment. Based on several possible values of a , you assign a value to b . No redundancy in the code here. The official name for this VHDL with/select assignment is the selected signal assignment .

## When / Else Assignment

The construct of a conditional signal assignment is a little more general. For each option, you have to give a condition. This means that you could write any boolean expression as a condition, which give you more freedom than equality checking. While this construct would give you more freedom, there is a bit more redundancy too. We had to write the equality check ( a = ) on every line. If you use a signal with a long name, this will make your code bulkier. Also, the separator that’s used in the selected signal assignment was a comma. In the conditional signal assignment, you need the else keyword. More code for the same functionality. Official name for this VHDL when/else assignment is the conditional signal assignment

## Combinational Process with Case Statement

The most generally usable construct is a process. Inside this process, you can write a case statement, or a cascade of if statements. There is even more redundancy here. You the skeleton code for a process (begin, end) and the sensitivity list. That’s not a big effort, but while I was drafting this, I had put b in the sensitivity list instead of a . Easy to make a small misstake. You also need to specify what happens in the other cases. Of course, you could do the same thing with a bunch of IF-statements, either consecutive or nested, but a case statement looks so much nicer.

While this last code snippet is the largest and perhaps most error-prone, it is probably also the most common. It uses two familiar and often-used constructs: the process and the case statements.

## Hard to remember

The problem with the selected and conditional signal assignments is that there is no logic in their syntax. The meaning is almost identical, but the syntax is just different enough to throw you off. I know many engineers who permanenty have a copy of the Doulos Golden Reference Guide to VHDL lying on their desks. Which is good for Doulos, because their name gets mentioned all the time. But most people just memorize one way of getting the job done and stick with it.

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• Finite State Machine (FSM) encoding in VHDL: binary, one-hot, and others (blog post)
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## Concurrent Conditional and Selected Signal Assignment in VHDL

This article will first review the concept of concurrency in hardware description languages. Then, it will discuss two concurrent signal assignment statements in VHDL: the selected signal assignment and the conditional signal assignment. After giving some examples, we will briefly compare these two types of signal assignment statements.

Please see my article introducing the concept of VHDL if you're not familiar with it.

## Concurrent vs. Sequential Statements

To understand the difference between the concurrent statements and the sequential ones, let’s consider a simple combinational circuit as shown in Figure 1.

## Figure 1. A combinational circuit.

If we consider the operation of the three logic gates of this figure, we observe that each gate processes its current input(s) in an independent manner from other gates. These physical components are operating simultaneously. The moment they are powered, they will “concurrently” fulfill their functionality. Note that while, in practice, the AND gate has a delay to produce a valid output, this does not mean that the OR gate will stop its functionality and wait until the output of the AND gate is produced. The OR gate will function all the time; however, its output will not be valid until its inputs have settled.

Now, let’s examine the VHDL description of Figure 1. This is shown below:

The main part that we are here interested in is the definition of the three gates:

Each of these lines describes a physical component in Figure 1. For example, the second line, which describes the OR gate, takes sig1 and c as inputs and produces the OR of these two values. We saw that the physical components of Figure 1 operate concurrently. Hence, it is reasonable to expect that the VHDL description of these gates should be evaluated in a concurrent manner. In other words, the above three lines of the code are executed at the same time and there is no significance to the order of these statements. As a result, we can rewrite the architecture section of the above code as below:

Since these statements are evaluated at the same time, we call them concurrent statements. This type of code is quite different from what we have learned in basic computer programming where the lines of code are executed one after the other. For example, consider the following MATLAB code:

This code produces out1=1 and out2=1 . However, if we change the order of the statements to the following, the program will stop working because we are trying to use sig1 before it is generated.

While the VHDL code describing Figure 1 was executed concurrently, the above MATLAB code is evaluated sequentially (i.e., one line after the other). VHDL supports both the concurrent statements and the sequential ones. It's clear that the concurrent VHDL statements will allow us to easily describe a circuit such as the one in Figure 1 above. In a future article, we'll see that the sequential VHDL statements allow us to have a safer description of sequential circuits. Furthermore, using the sequential VHDL, we can easily describe a digital circuit in a behavioral manner. This capability can significantly facilitate digital hardware design.

The following figure illustrates the difference between concurrent and sequential statements.

## Figure 2. The difference between concurrent and sequential statements. Image courtesy of VHDL Made Easy .

Now let's take a look at two concurrent signal assignment statements in VHDL: “the selected signal assignment statement” and “the conditional signal assignment statement”.

## Selected Signal Assignment or the “With/Select” Statement

Consider an n -to-one multiplexer as shown in Figure 3. This block should choose one out of its n inputs and transfer the value of this input to the output terminal, i.e., output_signal .

## Figure 3. A multiplexer selects one of its n inputs based on the value of the control_expression.

The selected signal assignment allows us to implement the functionality of a multiplexer. For example, the VHDL code describing the multiplexer of Figure 3 will be

Here, the value of the control_expression will be compared with the n possible options, i.e., option_1 , option_2 , …, option_n . When a match is found, the value corresponding to that particular option will be assigned to the output signal, i.e., output_signal . For example, if control_expression is the same as option_2 , then value_2 will be assigned to the output_signal .

Note that the options of a “with/select” assignment must be mutually exclusive, i.e., one option cannot be used more than once. Moreover, all the possible values of the control_expression must be included in the set of the options. The following example clarifies these points.

Example 1 : Use the "with/select" statement to describe a one-bit 4-to-1 multiplexer. Assume that the inputs to be selected are a , b , c , and d . And, a two-bit signal, sel , is used to choose the desired input and assign it to out1 .

The code for this multiplexer is given below:

Note that since the std_logic data type can take values other than “0” and “1” , the last line of the “with/select” statement needs to use the keyword “ others ” to take all the possible values of sel into account.

The following figure shows the simulation of this code using the Xilinx ISE simulator. (In case you’re not familiar with ISE, see this tutorial .) As shown in this figure, from 0 nanosecond (ns) until 300 ns the select input, sel , is 00, and, hence, out1 follows the input a . Similarly, you can verify the intended operation for the rest of the simulation interval.

## Figure 4. The ISE simulation for the multiplexer of Example 1.

Example 2 : Use the “with/select” statement to describe a 4-to-2 priority encoder with the truth table shown below.

The following VHDL code can be used to describe the above truth table:

The ISE simulation is shown in Figure 5.

## Figure 5. The ISE simulation for the priority encoder of Example 2.

Conditional signal assignment or the “when/else” statement.

The “when/else” statement is another way to describe the concurrent signal assignments similar to those in Examples 1 and 2. Since the syntax of this type of signal assignment is quite descriptive, let’s first see the VHDL code of a one-bit 4-to-1 multiplexer using the “when/else” statement and then discuss some details.

Example 3 : Use the when/else statement to describe a one-bit 4-to-1 multiplexer. Assume that the inputs to be selected are a , b , c , and d . And, a two-bit signal, sel , is used to choose the desired input and assign it to out1 .

The code will be

In this case, the expressions after “when” are evaluated successively until a true expression is found. The assignment corresponding to this true expression will be performed. If none of these expressions are true, the last assignment will be executed. In general, the syntax of the “when/else” statement will be:

We should emphasize that the expressions after the “when” clauses are evaluated successively. As a result, the expressions evaluated earlier has a higher priority compared to the next ones. Considering this, we can obtain the conceptual diagram of this assignment as shown in Figure 6. This figure illustrates a conditional signal assignment with three “when” clauses.

## Figure 6. The conceptual implementation of a “when/else” statement with three “when” clauses.

Let’s review the main features of the selected signal assignment and the conditional signal assignment.

## “With/Select” vs. “When/Else” Assignment

As mentioned above, the options of a “with/select” assignment must be mutually exclusive, i.e., one option cannot be used more than once. Moreover, all the possible values of the control_expression must be included in the set of options. While the “with/select” assignment has a common controlling expression, a “when/else” assignment can operate on expressions with different arguments. For example, consider the following lines of code:

In this case, the expressions are evaluating two different signals, i.e., reset1 and clk .

For the “when/else” assignment, we may or may not include all the possible values of the expressions to be evaluated. For example, the multiplexer of Example 3 covers all the possible values of sel ; however, the above code does not. The above code implies that out1 should retain its previous value when none of the expressions are true. This causes the inference of a latch in the synthesized circuit.

Another important difference between the “with/select” and “when/else” assignment can be seen by comparing the conceptual implementation of these two statements. The priority network of Figure 6 involves a cascade of several logic gates. However, the “with/select” assignment avoids this chain structure and has a balanced structure. As a result, in theory, the “with/select” statement may have better performance in terms of the delay and area (see RTL Hardware Design Using VHDL: Coding for Efficiency, Portability, and Scalability , Xilinx HDL Coding Hints , and Guide to HDL Coding Styles for Synthesis ).

In practice, we generally don’t see this difference because many synthesis software packages, such as the Xilinx XST, try not to infer a priority encoded logic. Though we can use the PRIORITY_EXTRACT constraint of XST to force priority encoder inference, Xilinx strongly suggests that we use this constraint on a signal-by-signal basis; otherwise, the constraint may guide us towards sub-optimal results. For more details see page 79 of the XST user guide .

• Concurrent statements are executed at the same time and there is no significance to the order of these statements. This type of code is quite different from what we have learned in basic computer programming where the lines of code are executed one after the other.
• The selected signal assignment or the "with/select" assignment allows us to implement the functionality of a multiplexer.
• The options of a “with/select” assignment must be mutually exclusive, i.e., one option cannot be used more than once. Moreover, all the possible values of the control_expression must be included in the set of the options.
• For the "when/else" statement, the expressions after the “when” clauses are evaluated successively. As a result, the expressions evaluated earlier has a higher priority compared to the next ones.
• One important difference between the “with/select” and “when/else” assignment can be seen by comparing the conceptual implementation of these two statements. The "when/else" statement has a priority network; however, the “with/select” assignment avoids this chain structure and has a balanced structure.

Featured image used courtesy of Parallella .

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Great content. The link to the ISE guide requires password. Can we get that posted again? Thanks!

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 Signal Assignment LRM §8.4, §9.5.

A signal assignment statement modifies the target signal

## Description:

A Signal assignment statement can appear inside a process (sequential statement) or directly in an architecture (concurrent statement). The target signal can be either a name (simple, selected, indexed, or slice) or an aggregate .

A signal assignment with no delay (or zero delay) will cause an event after delta delay, which means that the event happens only when all of the currently active processes have finished executing (i.e. after one simulation cycle).

The default delay mode ( inertial ) means that pulses shorter than the delay (or the reject period if specified) are ignored. Transport means that the assignment acts as a pure delay line.

VHDL'93 defines the keyword unaffected which indicates a choice where the signal is not given a new assignment. This is roughly equivalent to the use of the null statement within case (see Examples).

• Delays are usually ignored by synthesis tool.

Aggregate , Concurrent statement , Sequential statement , Signal , Variable assignment

## Chapter 4 - Behavioral Descriptions

Section 3 - signals and processes.

## Assignment Symbol in VHDL

VHDL assignments are used to assign values from one object to another. In VHDL there are two assignment symbols:

Either of these assignment statements can be said out loud as the word “gets”. So for example in the assignment: test <= input_1; You could say out loud, “The signal test gets (assigned the value from) input_1.”

Note that there is an additional symbol used for component instantiations (=>) this is separate from an assignment.

Also note that <= is also a relational operator (less than or equal to). This is syntax dependent. If <= is used in any conditional statement (if, when, until) then it is a relational operator , otherwise it’s an assignment.

One other note about signal initialization: Signal initialization is allowed in most FPGA fabrics using the := VHDL assignment operator. It is good practice to assign all signals in an FPGA to a known-value when the FPGA is initialized. You should avoid using a reset signal to initialize your FPGA , instead use the := signal assignment.

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## concurrent and conditional signal assignment (VHDL)

In VHDL, there are two types for signal assignment:

Problem is that some say that when...else conditions are checked line by line (king of sequential) while select...when...else conditionals are checked once. See this reference for example.

I say that when..else is also a sequential assignment because you are checking line by line. In other words, I say that there no need to say if..else within a process is equivalent to when..else . Why they assume when..else is a concurrent assignment?

• concurrency

• What is your question? –  mkrieger1 Commented Mar 7, 2017 at 0:06
• 1 And why down vote?... I edited the post –  mahmood Commented Mar 7, 2017 at 6:21

Where you are hinting at in your problem has nothing to do with concurrent assignments or sequential statements. It has more to do with the difference between if and case. Before we get to that first lets understand a few equivalents. The concurrent conditional assignment:

Is exactly equivalent to a process with the following code:

Likewise the concurrent selected assignment:

Is equivalent to a process with the following:

From a coding perspective, the sequential forms above have a little more coding capability than the assignment form because case and if allow blocks of code, where the assignment form only assigns to one signal. However other than that, they have the same language restrictions and produce the same hardware (as much as synthesis tools do that). In addition for many simple hardware problems, the assignment form works well and is a concise capture of the problem.

So where your thoughts are leading really comes down to the difference between if and case. If statements (and their equivalent conditional assignments) that have have multiple "elsif" in (or implied in) them tend to create priority logic or at least cascaded logic. Where as case (and their equivalent selected assignments) tend to be well suited for things like multiplexers and their logic structure tends to be more of a balanced tree structure.

Sometimes tools will refactor an if statement to allow it to be equivalent to a case statement. Also for some targets (particularly LUT based logic like Xilinx and Altera), the difference between them in terms of hardware effiency does not show up until there are enough "elsif" branches though.

With VHDL-2008, the assignment forms are also allowed in sequential code. The transformation is the same except without the process wrapper.

Concurrent vs Sequential is about independence of execution.

A concurrent statement is simply a statement that is evaluated and/or executed independently of the code that surrounds it. Processes are concurrent. Component/Entity Instances are concurrent. Signal assignments and procedure calls that are done in the architecture are concurrent.

Sequential statements (other than wait) run when the code around it also runs.

Interesting note, while a process is concurrent (because it runs independently of other processes and concurrent assignments), it contains sequential statements.

Often when we write RTL code, the processes that we write are simple enough that it is hard to see the sequential nature of them. It really takes a statemachine or a testbench to see the true sequential nature of a process.

• So please include this answer in the previous one. It is now clear –  mahmood Commented Mar 7, 2017 at 6:20

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## VHDL signal assignments

In architecture

so is this signal assignment in Architecture is correct or wrong

IS bit and std_logic having same condition and is it possible to assign the same to std_logic_vector as in assignment?

• 1 \\$\begingroup\\$ 1. Please use the preview before posting, especially for code. 2. Does the value of the question increase for every question mark added? \\$\endgroup\\$ –  pipe Commented Feb 13, 2018 at 11:49
• 2 \\$\begingroup\\$ This looks like homework so I'm not going to write an answer. It's incorrect, you need to convert at least one of the types. I have also edited the code to look nicer. In the future, remember to add four spaces on to the beginning of the parts that are code. \\$\endgroup\\$ –  stanri Commented Feb 13, 2018 at 11:56
• \\$\begingroup\\$ Also have a look here: electronics.stackexchange.com/questions/51848/… \\$\endgroup\\$ –  Oldfart Commented Feb 13, 2018 at 11:57
• \\$\begingroup\\$ @stanri, indentation advice is subjective but since we're here: I recommend to OP use 2 spaces, not 4. Avoids wasting line width. \\$\endgroup\\$ –  TonyM Commented Feb 13, 2018 at 12:21
• 3 \\$\begingroup\\$ @TonyM the four spaces are defined by the markup. If you use two it doesn't put the code in a code block. See electronics.stackexchange.com/editing-help : "Indent four spaces to create an escaped <pre> <code> block" \\$\endgroup\\$ –  stanri Commented Feb 13, 2018 at 12:22

VHDL is a strongly typed language. All vectors which you concatanate on the right side should be of same data type. And the data type of the result of the right side expression should match with the data type of the left side expression. In VHDL, "bit" is a 2-valued data type and "std_logic" is an 8-valued data type. They both are different.

• 1 \\$\begingroup\\$ std_logic is a resolved std_ulogic which is a character enumerated type having 9 values ('U', 'X', '0', '1', 'Z', 'W', 'L', 'H', '-') count 'em. Enumerated means the values are named, character means at least some names are character literals. The value is positional (which doesn't correspond between the two types). Those 'vectors' are single dimensional array types with an element type that is a character enumerated type. The concatenation operator is predefined for all single dimensional array types. \\$\endgroup\\$ –  user8352 Commented Feb 13, 2018 at 19:35

Required, but never shown

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1. VHDL Logical Operators and Signal Assignments for Combinational Logic

The VHDL code shown below uses one of the logical operators to implement this basic circuit. and_out <= a and b; Although this code is simple, there are a couple of important concepts to consider. The first of these is the VHDL assignment operator (<=) which must be used for all signals.

2. Variables vs. Signals in VHDL

Variables and Signals in VHDL appears to be very similar. They can both be used to hold any type of data assigned to them. The most obvious difference is that variables use the := assignment symbol whereas signals use the <= assignment symbol. However the differences are more significant than this and must be clearly understood to know when to ...

3. Signal Assignments in VHDL: with/select, when/else and case

With / Select. The most specific way to do this is with as selected signal assignment. Based on several possible values of a, you assign a value to b. No redundancy in the code here. The official name for this VHDL with/select assignment is the selected signal assignment. with a select b <= "1000" when "00", "0100" when "01", "0010" when "10 ...

4. PDF Concurrent Statements

A bus is a collection of wires related in some way by function or clock domain. Examples would be an address bus or data bus. In VHDL we refer to busses as a vector. For example: --8-bit bus consisting of 8 wires carrying signals of -- type std_logic --all these wires may be referred to by the name big_bus. SIGNAL big_bus : STD_LOGIC_VECTOR(7 ...

5. Concurrent Conditional and Selected Signal Assignment in VHDL

Conditional Signal Assignment or the "When/Else" Statement. The "when/else" statement is another way to describe the concurrent signal assignments similar to those in Examples 1 and 2. Since the syntax of this type of signal assignment is quite descriptive, let's first see the VHDL code of a one-bit 4-to-1 multiplexer using the ...

6. vhdl

Any variable which does not assign its value to a signal, but only to other variables, is a perfectly acceptable "wire". My understanding of the whole subject is this: A signal assignment inside a process will disregard other signal assignments made in the same process "instantiation".

7. vhdl

The Inside_process and Outside_process versions behave differently. If both designs work, it is mostly out of luck, because in this case Out_signal simply lags half a clock cycle when declared inside the process. Out_signal is assigned when the process triggers, which in this case occurs on rising and falling edges of clk.

8. Select Signal Assignment

Select statements are used to assign signals in VHDL. They can only be used in combinational code outside of a process. A selected signal assignment is a clear way of assigning a signal based on a specific list of combinations for one input signal. The syntax is demonstrated in the example below. The signal name after with is the signal whose ...

9. Signal Assignment

A Signal assignment statement can appear inside a process (sequential statement) or directly in an architecture (concurrent statement). The target signal can be either a name (simple, selected, indexed, or slice) or an aggregate. A signal assignment with no delay (or zero delay) will cause an event after delta delay, which means that the event ...

10. VHDL Tutorial

Section 3 - Signals and Processes. This section is short, but contains important information about the use of signals in the process statement. The issue of concern is to avoid confusion about the difference between how a signal assignment and variable assignment behave in the process statement. Remember a signal assignment, if anything, merely ...

11. PDF VHDL Syntax Reference

1 1. Bits, Vectors, Signals, Operators, Types 1.1 Bits and Vectors in Port Bits and vectors declared in port with direction. Example: port ( a : in std_logic; -- signal comes in to port a from outside b : out std_logic; -- signal is sent out to the port b c : inout std_logic; -- bidirectional port x : in std_logic_vector(7 downto 0); -- 8-bit input vector

12. VHDL Reference Guide

VHDL-93 defines an unaffected keyword, which indicates a condition when a signal is not given a new assignment: label: signal = expression_1 when condition_1 else expression_2 when condition_2 else unaffected ; The keywords inertial and reject may also be used in a conditional signal assignment.

13. Assignment Symbol

In VHDL there are two assignment symbols: <= Assignment of Signals. := Assignment of Variables and Signal Initialization. Either of these assignment statements can be said out loud as the word "gets". So for example in the assignment: test <= input_1; You could say out loud, "The signal test gets (assigned the value from) input_1.".

14. vhdl

1. Assignments in VHDL are neighter specified as registered or combinatorial. In VHDL the actual assignment type (the type of RTL logic generated) is just inferred. Registers in VHDL are created explicitly by assigning a signal on a clock edge, though just because a process has a clock it does not mean all signals in that block will be assigned ...

15. VHDL

Conditional signal assignment. Conditional signal assignment is a form of a concurrent signal assignment and plays the same role in architecture as the if then else construct inside processes. A signal is assigned a waveform if the Boolean condition supported after the when keyword is met. Otherwise, the next condition after the else clause is ...

16. VHDL internal signal assignment

1. I am new to VHDL and tried to implement a simple example from a book, what represents a 2bit register and the testbench. Compiling the files works well, and signals the stimulate the register (reset, clock and data) are correctly transmitted from the testbench to the register. But the assignment of internal signals to any value remains ...

17. concurrent and conditional signal assignment (VHDL)

A concurrent statement is simply a statement that is evaluated and/or executed independently of the code that surrounds it. Processes are concurrent. Component/Entity Instances are concurrent. Signal assignments and procedure calls that are done in the architecture are concurrent.

18. VHDL: Signal assignment question

TLDR: The specific ordering question I'm asking is: If the output signal generation function observes one of the intermediate signals as having changed before the other intermediate signals having changed, a "transient" output may be generated until the change in the second intermediate signal is observed. Does VHDL guarantee that this does not ...

19. [VHDL] Signals Assignment in Package -> how to?

But is it possible to assign the signals inside of the packages? I'd like to write something like following: package pkg_signals is. type plan_t is (A, B); signal plan : plan_t; signal sel : std_logic; end package pkg_signals; package body pkg_signals is. plan <= A when sel = '1' else B;

20. Why do we assign our outputs to signals first in VHDL?

4. It's because you couldn't read from ports of type OUT in VHDL. If the value driving an OUT port was to be read within the design, a signal had to used to hold it, then that signal assigned to the OUT port. The capability to do so was added in VHDL-2008.

21. VHDL signal assignments

VHDL is a strongly typed language. All vectors which you concatanate on the right side should be of same data type. And the data type of the result of the right side expression should match with the data type of the left side expression. In VHDL, "bit" is a 2-valued data type and "std_logic" is an 8-valued data type. They both are different.