Example: Multiplexer using conditional signal assignment
Example: Multiplexer using selected signal assignment
The structure of a VHDL source code is based on the inclusion of libraries, packages, and the description of VHDL modules. Depending on how it implements a design, the easiest way to imagine how the modules communicate is to compare it to an electrical circuit. A module has input and output signals which provide the connection between the modules. Depending on the EDA tool, the link between two or more modules can be made by typing the ports directly, as shown in the example below, or by using tools that provide the modules as parts, similar to a spice program for the analog simulation. | |
Module 1 | |
Module 2 | |
Module 3 | |
… |
Libraries are a collection of packages that define the operators and operations in the logic utilized to describe a digital system. The IEEE library is commonly used; it defines a binary representation consisting of values of 0 and 1, and a standard logic representation (std_logic) with nine distinct logic values, of which the most important are: 0, 1, X (unknown) and Z (high impedance). To include a library in VHDL, the reserved word library is used in an instruction of this form:
Packages are a collection of elements, like data types, subprograms, constants, or operators, within a library. The reserved word for the inclusion of a package from a library is use, the instruction is written as:
For the IEEE library, to use the logic primitive’s description, it is required to include the package IEEE.std_logic_1164. For this, write the following code sequence:
The declaration for interface-signal uses this form:
For example, the next lines of code are the declaration for the entity of an and-gate with two inputs and one output:
The architecture descriptor – architecture – describes the function of the digital circuit. In this part of a module, one or more abstraction level presented above are combined to create an architecture. The code can set out how the circuit behaves using functional blocks and the connection between them, or a mathematical description of the logical functions (truth table, transition diagram, algorithm). The declaration of an architecture uses this form:
In the declaration, the signals and constants are assigned if they are needed. To describe logical primitives, the body of VHDL module architectures consists of:
Example: The and-gate architecture with two inputs described with fundamental logic and is:
Module instantiation is used to connect circuit elements. At the top level, the connection is made between modules, and the EDA tool creates the netlist for all the components and connections that are used. A VHDL design description written exclusively with component instantiation is known as “Structural VHDL”. This method is very similar to the schematic entry; in this case, instead of using graphic elements, it is implemented as text. One or more components can be declared in the architecture body. To instantiate a component inside a design, the following two steps are needed:
The declaration section of the architecture is included between the keyword “is” and “begin”:
For example, to instantiate the and-gate with two outputs modules in another architectural model, the simplest way is to copy the entity of and2 and paste it to the architectural body. After that, change the “entity” to “component”.
bit | ‘0’ or ‘1’ |
Boolean | FALSE or TRUE |
integer | -(231-1) to (231+1) |
real | -1.0E38 to 1.0E38 |
character | “printable character”. Ex ‘d’, ‘A’, ‘4’, ‘+’ |
std_logic time | ‘1’ Logic 1 ‘0’ - Logic 0 ‘Z’ - High impedance ‘W’ - Weak signal, can’t tell if 0 or 1 ‘L’ - Weak 0, pulldown ‘H’ - Weak 1, pullup ‘-‘ - Don’t care ‘U’ - Uninitialized ‘X’ - Unknown, multiple drivers Integer + unit (fs, ps, ns, us, ms, sec, min, hr) |
* users defined data, for example, enumeration:
VHDL is a strongly typed language, so different data types cannot be combined in the same instruction.
When designing with a hardware description language, data buses are often used. Data buses comprise multiple bit buses. The data types presented above allow single bits to be modeled in a design. A vector consists of one or more bits that are modeled similarly to one of the predefined data. The most basic and encountered types of vectors in VHDL are made up of several bit or std_logic types. The code below is an example of how to declare a vector type signal in VHDL.
The [ range ] field is used to determine the number of bits in the vector and the location of the most significant and least significant bits. The key word used to describe the range value in VHDL are: downto and to. The next lines present a declaration of an 8-bit vector using the keywords:
When writing VHDL code, it is often necessary to do a data conversion. Two general methods to convert between data types are available. The first method is to cast the signal to the correct type. In this method, it is converted between the signed, unsigned and std_logic_vector.
The code below is an example demonstrating the syntax to cast signals or data.
The second way employs functions. This method is used to convert between the signed or unsigned types and the integer type. To use a suitable conversion function, it is needed to include either the numeric_std or the std_logicarith packages. These packages are available in the IEEE library.
When assigning data to a vector type, VHDL uses quotation marks (“) instead of apostrophes. Also, it can specify to use a hexadecimal notation by adding an ‘x’ to the start of the data. This works if the number of bits is a factor of four. Using octal values can be done by adding an ‘o’ to the start of the data. This works only with VHDL-2008. The code below gives some examples of how to assign data to vector types in VHDL.
Because VHDL is similar to other programming languages, it allows variables. These are declared in the architecture declaration area.
Example: In this example, several variables of each of the above types are declared and used in a small code combination:
VHDL has a set of operators that can be used in creating designs. Predefined operators:
binary operators | and | logical and |
or | logical or | |
and | the logical complement of and | |
nor | the logical complement of or | |
xor | logical exclusive of or | |
xnor | the logical complement of exclusive of or | |
relational operators | = | test for equality |
/= | test for inequality | |
< | test for less than | |
⇐ | test for less than or equal | |
> | test for greater than | |
>= | test for greater than or equal | |
shifting operators | sll | shift left logical |
srl | shift right logical | |
sla | shift left arithmetic | |
sra | shift right arithmetic | |
rol | rotate left | |
ror | rotate right | |
additive operators | + | addition |
- | subtraction | |
& | concatenation | |
sign operators | + | unary plus |
- | unary minus | |
multiplicator operators | * | multiplication |
/ | division | |
mod | modulo | |
rem | remainder | |
other operators | not | compliment |
abs | absolute value | |
** | exponentiation | |
assignment operator | <= | assignment of signals |
:= | assignment of variables and signal initialization | |
timing operators | after, wait |
Example 1: After declaring the variables, f is assigned the value between a and b using a binary operator. The binary operator “and” can be replaced with another binary operator according to the designer's need. This operation is performed after the delay; for this task, the timing operator “after” is used.
Example 2: Uses the timing operator “wait” to implement a breakpoint and wait for an event specified to happen:
Example 3: Uses different shift operators to make changes at a bit level.
Example 4: The combined use of several operators:
Get the latest updates on new products and upcoming sales
courses:system_design:vhdl_language_and_syntax:data_types
In VHDL, signals must have a data type associated with them that limits the number of possible values. This type has to be fixed when the signal is declared, either as entity port or an internal architecture signal, and can not be changed during runtime. Whenever signal values are updated, the data types on both sides of the assignment operator ’⇐’ have to match.
A number of data types are already defined in the standard package which is always implicitly referenced.
’ boolean ’ is usually used to control the flow of the VHDL execution while ‘ bit ’ uses level values (’ 0 ’, ’ 1 ’) instead of truth values (’ false ’, ’ true ’) and is therefore better suited to model wires. Number values can be communicated via signals of type ’ integer ’ or ’ real ’.
The actual range and accuracy depends on the platform implementation and only lower bounds are defined, e.g. integers are guaranteed to be at least 32 bits wide. Floating point operations can not be synthesized automatically, yet, i.e. the use of ’ real ’ data types is restricted to testbench applications.
The same applies to ’ character ’ and ’ time ’.
Real types are not synthesizeable:
’ time ’ is a special data type as it consists out of a numerical value and a physical unit.
It is used to delay the execution of statements for a certain amount of time, e.g. in testbenches or to model gate and propagation delays. Signals of data type ’ time ’ can be multiplied or divided by ’ integer ’ and ’ real ’ values. The result of these operations remains of data type ’ time ’.
The internal resolution of VHDL simulators is set to femto-seconds (fs).
Arrays are useful to group signals of the same type and meaning.
Two unconstrained array data types, i.e. whose range is not limited, are predefined in VHDL:
Please note that the array boundaries have to be fixed during signal declarations, e.g. ’bit_vector(3 downto 0)’.
Only constrained arrays may be used as entity ports or architecture signals.
EXAMPLE_1 of DATATYPES is signal SEL : bit; signal A, B, Z : integer range 0 to 3; begin A <= 2; B <= 3; process(SEL,A,B) begin if SEL = ‘1’ then Z <= A; else Z <= B; end if; end process; end EXAMPLE_1; | EXAMPLE_2 of DATATYPES is signal SEL : bit; signal A, B, Z : bit_vector(1 downto 0); begin A <= "10"; B <= "11"; process(SEL,A,B) begin if SEL = ‘1’ then Z <= A; else Z <= B; end if; end process; end EXAMPLE_2; |
Integer signals will be mapped to a number of wires during synthesis.
These wires could be modelled via bit vectors as well, yet ’ bit_vector ’ signals do not have a numerical interpretation associated with them.
Therefore the synthesis result for the two example architectures would be the same. The process models a simple multiplexer which selects the input A as source for its output Z when the select signal SEL is ’1’ and the input B otherwise.
Please note that the multiplexer process is exactly the same for both data types!
Special care is necessary when signal assignments with arrays are carried out.
Although the data type and the width of the signals have to match, this is not true for the order of the array elements.
The values are assigned according to their position within the array, not according to their index.
Therefore it is highly recommended to use only one direction (usually ’ downto ’ in hardware applications) throughout your designs.
The specification of signal values is different for the base types ’character’ and ’bit’ and their corresponding array types ’string’ and ’bit_vector’. Single values are always enclosed in single quotation marks (’), while double quotation marks (“) are used to specify array values.
As bit vectors are often used to represent numerical values, VHDL offers several possibilities to increase the readability of bit vector assignments.
First, a base for the following number may be specified. Per default binary data consisting of ’0’s and ’1’s is assumed. Please note that the values have to enclosed in double quotation marks even though only a single symbol might be necessary when another base is used! Additionally, underscores (_) may be inserted at will to split long chains of numbers into smaller groups in order to improve readability.
Since VHDL’93, the same rules apply to the enhanced bit vector types ’ std_(u)logic_vector ’, which will be discussed later on, as well.
Concatenation operator: &
Resulting signal assignment:
As signal assignments require matching data types on both sides of the operator it is sometimes necessary to assemble an array in the VHDL code.
The concatenation operator ’ & ’ groups together the elements on its sides which have to be of the same data type, only.
Again, the array indices are ignored and only the position of the elements within the arrays is used. The concatenation operator may be used on the right side of signal assignments, only!
Another way of assigning signals which does not suffer from this limitation is via the aggregate construct.
Here, the signals that are to build the final array are enclosed in a ’(’ ’)’ pair and separated by ’,’.
Instead of a simple concatenation, it is also possible to address the array elements explicitly by their corresponding index, as shown in the last signal assignment statement of the aggregate example.
The keyword ’ others ’ may be used to select those indices that have not been addressed, yet.
The inverse operation of concatenation and aggregation is the selection of slices of arrays, i.e. only a part of an array is to be used.
The range of the desired array slice is specified in brackets and must match the range declaration of the signal!
Of course, it is possible to select only single array elements.
Chapters of System Design > VHDL Language and Syntax
Stack Exchange network consists of 183 Q&A communities including Stack Overflow , the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Q&A for work
Connect and share knowledge within a single location that is structured and easy to search.
What is the meaning of "combinational assignment" and "registered assignment" to signals? In particular what are the differences between these two types of assignments?
Essentially, the difference boils down to whether the signal gets assigned on a clock edge or not.
Combinational code, like A <= B + 1 would have A being assigned B+1 "immediately," whereas
would result in A being assigned B+1 only on a rising clock edge. With code like this, other blocks can use the value of A being guaranteed that its value will be stable and unchanging after a clock edge.
Registers, or clock gating in general I suppose, are really what make designs of any complication possible. You create a pipeline of operations by putting a register at the border between operations. For example, the input operands to an ALU must be stable - in a register- so that the ALU can execute properly, and the result of the ALU's execution should be in a register so that whatever block uses it does not 'see' the changing values inside the ALU as the calculations take place, but only the stable last result.
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 on every edge.
Also registers can be inferred without a clock, if some path through a process does not assign a signal, then VHDL assumes you meant to latch that signal between successive passes. This is called an inferred latch (and should be avoided).
VHDL does not know about the technology that you are going to be using. It does not know whether your synthisis engine can generate T,D,JK,SR, or any other sort of latch. For that reason it just suggests a latch, it is up to the synthisis enging to decide which latch fits the bill or if it is simply impossible. Similarlay the fitter might say that a particular latch requested by the synthisis enging is not available or there are no enough of them.
Sign up or log in, post as a guest.
Required, but never shown
By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .
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.
Learn Verilog
Save my name, email, and website in this browser for the next time I comment.
1. introduction, 2. combinational circuits, signal assignments in vhdl.
library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.STD_LOGIC_ARITH.ALL; use IEEE.STD_LOGIC_UNSIGNED.ALL; entity fullAdder is port( A,B: in std_logic; -- input bits for this stage Ci: in std_logic; -- carry into this stage S: out std_logic; -- sum bit Co: out std_logic -- carry out of this stage ); end fullAdder; architecture a1 of fullAdder is begin S <= A xor B xor Ci; Co <= (A and B) or ((A xor B) and Ci); end a1;
if a = '0' then x <= a; y <= b; elsif a = b then x <= '0'; y <= '1'; else x <= not b; y <= not b; end if;
Every signal that is assigned a value inside a process must be defined for all possible conditions.
Structural vhdl, 3. sequential circuits.
busy is high when the circuit is in the middle of performing an operation; while busy is high, the insert and delete inputs are ignored; the outputs are not required to have the correct values when busy is high empty is high when there are no pairs stored in the priority queue; delete operations are ignored in this case full is high when there is no room for any additional pairs to be stored; insert operations are ignored in this case
entity priQueue is Port (clk, reset : in std_logic; insert, delete : in std_logic; key, value : in std_logic_vector(wordSize-1 downto 0); smallValue : out std_logic_vector(wordSize-1 downto 0); busy, empty, full : out std_logic ); end priQueue; architecture a1 of priQueue is constant rowSize: integer := 4; -- local constant declaration type pqElement is record dp: std_logic; key: std_logic_vector(wordSize-1 downto 0); value: std_logic_vector(wordSize-1 downto 0); end record pqElement; type rowTyp is array(0 to rowSize-1) of pqElement; signal top, bot: rowTyp; type state_type is (ready, inserting, deleting); signal state: state_type; begin process(clk) begin if rising_edge(clk) then if reset = '1' then for i in 0 to rowSize-1 loop top(i).dp <= '0'; bot(i).dp <= '0'; end loop; state <= ready; elsif state = ready and insert = '1' then if top(rowSize-1).dp /= '1' then for i in 1 to rowSize-1 loop top(i) <= top(i-1); end loop; top(0) <= ('1',key,value); state <= inserting; end if; elsif state = ready and delete = '1' then if bot(0).dp /= '0' then for i in 0 to rowSize-2 loop bot(i) <= bot(i+1); end loop; bot(rowSize-1).dp <= '0'; state <= deleting; end if; elsif state = inserting or state = deleting then for i in 0 to rowSize-1 loop if top(i).dp = '1' and (top(i).key < bot(i).key or bot(i).dp = '0') then bot(i) <= top(i); top(i) <= bot(i); end if; end loop; state <= ready; end if; end if; end process; smallValue <= bot(0).value when bot(0).dp = '1' else (others => '0'); empty <= not bot(0).dp; full <= top(rowSize-1).dp; busy <= '1' when state /= ready else '0'; end a1;
package commonConstants is constant lgWordSize: integer := 4; constant wordSize: integer := 2**lgWordSize; end package commonConstants; library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.STD_LOGIC_ARITH.ALL; use IEEE.STD_LOGIC_UNSIGNED.ALL; use work.commonConstants.all; entity firstOne is Port (a: in std_logic_vector(0 to wordSize-1); x: out std_logic_vector (lgWordSize downto 0) ); end firstOne; architecture a1 of firstOne is procedure encode(x: in std_logic_vector(0 to wordSize-1); indx: out std_logic_vector(lgWordSize-1 downto 0); errFlag: out std_logic) is -- Unary to binary encoder. -- Input x is assumed to have at most a single 1 bit. -- Indx is equal to the index of the bit that is set. -- If no bits are set, errFlag bit is made high. -- This is conceptually simple. -- -- indx(0) is OR of x(1),x(3),x(5), ... -- indx(1) is OR of x(2),x(3), x(6),x(7), x(10),x(11), ... -- indx(2) is OR of x(4),x(5),x(6),x(7), x(12),x(13),x(14(,x(15),... -- -- but it's tricky to code so it works for different word sizes. type vec is array(0 to lgWordSize-1) of std_logic_vector(0 to (wordSize/2)-1); variable fOne: vec; variable anyOne: std_logic_vector(0 to wordSize-1); begin -- fOne(0)(j) is OR of first j bits in x1,x3,x5,... -- fOne(1)(j) is OR of first j bits in x2,x3, x6,x7, x10,x11,... -- fOne(2)(j) is OR of first j bits in x4,x5,x6,x7, x12,x13,x14,x15,... for i in 0 to lgWordSize-1 loop for j in 0 to (wordSize/(2**(i+1)))-1 loop for h in 0 to (2**i)-1 loop if j = 0 and h = 0 then fOne(i)(0) := x(2**i); else fOne(i)((2**i)*j+h) := fOne(i)((2**i)*j+h-1) or x(((2**i)*(2*j+1))+h); end if; end loop; end loop; indx(i) := fOne(i)((wordSize/2)-1); end loop; anyOne(0) := x(0); for i in 1 to wordSize-1 loop anyOne(i) := anyOne(i-1) or x(i); end loop; errFlag := not anyOne(wordSize-1); end procedure encode; function firstOne(x: std_logic_vector(0 to wordSize-1)) return std_logic_vector is -- Returns the index of the first 1 in bit string x. -- If there are no 1's in x, the value returned has a -- 1 in the high order bit. variable allZero: std_logic_vector(0 to wordSize-1); variable fOne: std_logic_vector(0 to wordSize-1); variable rslt: std_logic_vector(lgWordSize downto 0); begin allZero(0) := not x(0); fOne(0) := x(0); for i in 1 to wordSize-1 loop allZero(i) := (not x(i)) and allZero(i-1); fOne(i) := x(i) and allZero(i-1); end loop; encode(fOne,rslt(lgWordSize-1 downto 0),rslt(lgWordSize)); return rslt; end function firstOne; begin x <= firstOne(a); end a1;
Find centralized, trusted content and collaborate around the technologies you use most.
Q&A for work
Connect and share knowledge within a single location that is structured and easy to search.
Get early access and see previews of new features.
Consider a type
Why do I get a compiler error when I try to create a constant value of this type, for example. Note that I have only tried using Altera's Quartus II.
Error: VHDL Type mismatch error at [filename.vhd](line number): cFoo type does not match string literal.
However everything is ok if my type is
with an example assignment to a constant of:
Moreover consider that I try to assign index 0 with another constant. For example.
To which the error the compiler spits out is now:
VHDL error at [filename.vhd](line number): type of identifier "cBar" does not agree with its usage as "foo" type.
So basically its telling me that the compiler does not realize that the assignment is to the index of 0 but instead an assignment to the the array type.
How can I let the compiler know that the assignment is to only index 0?
IEEE Std 1076 9.3.3 Aggregates, 9.3.3.1 para 4:
Both named and positional associations can be used in the same aggregate, with all positional associations appearing first (in textual order) and all named associations appearing next (in any order, except that it is an error if any associations follow an others association). Aggregates containing a single element association shall always be specified using named association in order to distinguish them from parenthesized expressions.
With an MCVE :
You need to use named association with a single element. The first example specifies index 0, the second any elements.
Para 3 of the above quoted subsection:
Each element association associates an expression with elements (possibly none). An element association is said to be named if the elements are specified explicitly by choices; otherwise, it is said to be positional. For a positional association, each element is implicitly specified by position in the textual order of the elements in the corresponding type declaration.
It's helpful to see the BNF:
aggregate ::= ( element_association { , element_association } ) element_association ::= [ choices => ] expression choices ::= choice { | choice } choice ::= simple_expression | discrete_range | element _simple_name | others
Reminder: Answers generated by artificial intelligence tools are not allowed on Stack Overflow. Learn more
Post as a guest.
Required, but never shown
By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .
IMAGES
COMMENTS
bit Type in VHDL. The bit type is the simplest of all types in VHDL. We use this type to model a single logical value within our FPGA. The bit type can only ever have a value or either 1b or 0b. The code snippet below shows the method we use to declare a bit type signal in VHDL. signal <signal_name> : bit;
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.
We can create our own array types in VHDL. To do this, we include the array keyword in the type definition. We must also declare the number of elements in the array. The code snippet below shows the general syntax we use to declare an array type in VHDL. type <type_name> is array (<range>) of <type>;
Variable assignments are generally synthesisable, providing they use types and operators acceptable to the synthesis tool. In a "clocked process", each variable which has its value read before it has had an assignment to it will be synthesised as the output of a register. ... In VHDL-93, a variable assignment may have a label: label: variable ...
Here is some basic VHDL logic: 1. 2. signal and_gate : std_logic; and_gate <= input_1 and input_2; The first line of code defines a signal of type std_logic and it is called and_gate. Std_logic is the type that is most commonly used to define signals, but there are others that you will learn about.
Integer data type. It can hold an integer number ranging from - (2 31 - 1) to + (2 31 - 1). Interestingly two subtypes of integers are also defined in the standard library of VHDL. Now, what are subtypes you ask, in short, a subtype is a datatype which has constrained values of its base type.
The most basic of complete VHDL statements, a signal assignment is likely also one of the most common. Syntax: < signal_name > <= < expression >; -- the expression must be of a form whose result matches. the type of the assigned signal. Examples: std_logic_signal_1 <= not std_logic_signal_2; std_logic_signal <= signal_a and signal_b;
Each VHDL objects must be classified as being of a specific data type. VHDL includes a number of predefined data types, and allows users to define custom data types as needed. Predefined Scalar Data Types (single objects) ... Each signal assignment results in the corresponding event queue being modified to schedule the new event. signal line x ...
Understanding VHDL Basics, Structure, Data Types and Operators 1. VHDL Hardware Description Language Hardware description languages (HDL) are used to design digital and electronic systems. Today, the languages most widely employed are VHDL and Verilog. These description languages allow the user to write a program that describes the behavior of the circuit.
In VHDL, signals must have a data type associated with them that limits the number of possible values. This type has to be fixed when the signal is declared, either as entity port or an internal architecture signal, and can not be changed during runtime. Whenever signal values are updated, the data types on both sides of the assignment operator ...
Aggregates are a grouping of values to form an array or record expression. The first form is called positional association, where the values are associated with elements from left to right: signal Z_BUS : bit_vector (3 downto 0); signal A_BIT, B_BIT, C_BIT, D_BIT : bit; ...
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 ...
VHDL Data Types Predefined Data Types Specified through the IEEE 1076 and IEEE 1164 standards The IEEE Standard 1076 defines the VHSIC Hardware Description Language or VHDL - Developed by Intermetrics, IBM and Texas Instruments for United States Air Force. - 1076-1987 was the first version - Revised in 1993, 2000, 2002, and 2008
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.".
The standard package defines built-in VHDL data types that can be used for designing and associated operations that go along with them. The textio package provides support for file operations, for example reading and writing to external data files. ... The final type of assignment is known as a selected signal assignment. A selected assignment ...
The signal assignment operator specifies a relationship between signals. In other words, the signal on the left side of the signal assignment operator is dependent upon the signals on the right side of the operator. (Source: Digital_Mclogic_Design by Bryan Mealy, Section: The Signal Assignment Operator: "<=", page 339)
As an example, we look at ways of describing a four-bit register, shown in Figure 2-1. Using VHDL terminology, we call the module reg4 a design entity, and the inputs and outputs are ports. Figure 2-2 shows a VHDL description of the interface to this entity. This is an example of an entity declaration.
An array contains multiple elements of the same type. When an array object is declared, an existing array type must be used. An array type definition can be unconstrained, i.e. of undefined length. String, bit_vector and std_logic_vector are defined in this way. An object (signal, variable or constant) of an unconstrained array type must have ...
Designing Circuits with VHDL 1. Introduction VHDL is a hardware description language that can be used to design digital logic circuits. VHDL specifications can be automatically translated by circuit synthesizers into digital circuits, in much the same way that Java or C++ programs are translated by compilers into machine language. While VHDL code bears a superficial resemblance to programs in ...
1. I have an array in VHDL of the form, type CacheArray is array(0 to 15) of std_logic_vector(33 downto 0); signal cache_array: CacheArray := (others => (others => '0')); I wish to assign values to this array such that only one bit of each index is initialized. I suspected something like this will work,
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.
type foo is array (0 downto 0) of std_logic_vector(7 downto 0); with an example assignment to a constant of: constant cFoo : foo := ( x"00", x"11" ); Moreover consider that I try to assign index 0 with another constant. For example. type foo is array (0 downto 0) of std_logic_vector(7 downto 0); constant cBar : std_logic_vector(7 downto 0);