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Variable declaration assignment, net declaration assignment, assign deassign, force release.
- Procedural continuous
Legal LHS values
An assignment has two parts - right-hand side (RHS) and left-hand side (LHS) with an equal symbol (=) or a less than-equal symbol (<=) in between.
The RHS can contain any expression that evaluates to a final value while the LHS indicates a net or a variable to which the value in RHS is being assigned.
Procedural assignments occur within procedures such as always , initial , task and functions and are used to place values onto variables. The variable will hold the value until the next assignment to the same variable.
The value will be placed onto the variable when the simulation executes this statement at some point during simulation time. This can be controlled and modified the way we want by the use of control flow statements such as if-else-if , case statement and looping mechanisms.
An initial value can be placed onto a variable at the time of its declaration as shown next. The assignment does not have a duration and holds the value until the next assignment to the same variable happens. Note that variable declaration assignments to an array are not allowed.
If the variable is initialized during declaration and at time 0 in an initial block as shown below, the order of evaluation is not guaranteed, and hence can have either 8'h05 or 8'hee.
Procedural blocks and assignments will be covered in more detail in a later section.
This is used to assign values onto scalar and vector nets and happens whenever there is a change in the RHS. It provides a way to model combinational logic without specifying an interconnection of gates and makes it easier to drive the net with logical expressions.
Whenever b or c changes its value, then the whole expression in RHS will be evaluated and a will be updated with the new value.
This allows us to place a continuous assignment on the same statement that declares the net. Note that because a net can be declared only once, only one declaration assignment is possible for a net.
Procedural Continuous Assignment
- assign ... deassign
- force ... release
This will override all procedural assignments to a variable and is deactivated by using the same signal with deassign . The value of the variable will remain same until the variable gets a new value through a procedural or procedural continuous assignment. The LHS of an assign statement cannot be a bit-select, part-select or an array reference but can be a variable or a concatenation of variables.
These are similar to the assign - deassign statements but can also be applied to nets and variables. The LHS can be a bit-select of a net, part-select of a net, variable or a net but cannot be the reference to an array and bit/part select of a variable. The force statment will override all other assignments made to the variable until it is released using the release keyword.
Programmable Logic/Verilog Operators
This page is going to talk about some of the verilog operators.
- 1 Arithmetic Operators
- 2 Logical Operators
- 3.1 Example: Full Adder
- 4 Assignment Operators
- 5 Shift and Rotate
- 6 Concatenation and Replication
- 7 Reduction Operators
- 8 Conditional Operator
- 9 Operator Precedence
Arithmetic Operators [ edit | edit source ]
The arithmetic operators are as follows:
In practice, the division and modulus operators are typically only usable in simulation, not synthesis. Division is a particularly complicated operation, and most programmable chips do not have dedicated divider modules.
Logical Operators [ edit | edit source ]
There are a number of logical operators. Logical operators act on an entire value (multiple bits), and treat the values of "zero" as being "false", and "non-zero" as being "true".
The reduction operators are
- && AND
What happens is that Verilog converts the whole number into either a 1 (if it contains a nonzero bit) or 0 (if it only contains zero), then performs the equivalent bitwise operation. Thus, the answer is also one bit.
!0000 = ~0 = 1 !1101 = ~1 = 0 !1000 = ~1 = 0
!(any nonzero binary number) = ~1 = 0
0000 && 1101 = 0 & 1 = 0 0010 && 1101 = 1 & 1 = 1
0000 || 0110 = 0 | 1 = 1 0000 || 0000 = 0 | 0 = 0
Bitwise Operators [ edit | edit source ]
There are a number of bitwise operators to perform boolean operations on each individual bit in a bus.
Example: Full Adder [ edit | edit source ]
This should help to demonstrate how these bitwise operations are performed.
Assignment Operators [ edit | edit source ]
there are three assignment operators, each of which performs different tasks, and are used with different data types:
The continuous assignment is typically used with wires and other structures that do not have memory. A continuous assignment is happening continuously and in parallel to all other computational tasks. The order of continuous assignments, or their location in the code do not matter.
Non-blocking assignments are assignments that occur once, but which can all happen at the same time. These assignments are typically used with registers and integer data types, and other data types with memory. The following two code snippets with non-blocking assignments are equivalent:
All non-blocking assignments in a single code block occur simultaneously. They happen only once, and the input values for all such assignments are read before the operation takes place (which requires the use of additional latch structures in synthesis).
Blocking assignments are also used with registers and integers (and other memory data types). Blocking assignments occur sequentially, and the code after the assignment will not execute until the assignment has occured.
Shift and Rotate [ edit | edit source ]
if you want to shift right assign shr = a >> 1 // shift a right by 1 bit.
example a=8'b10011011 then x=8'b01001101
assign shr = a >> 2 // shift a right by 2 bits.
example a=8'b10010101 then x=8'b00100101
if you want to shift left assign shl = a << 1 // shift a left by 1 bit.
example a=8'b10011011 then x=8'b00110110
assign shl = a << 2 // shift a left by 2 bits.
example a=8'b10011011 then x=8'b01101100
Concatenation and Replication [ edit | edit source ]
Combines 2 or more than 2 operands
Reduction Operators [ edit | edit source ]
These are the same as the bitwise operations above.
Conditional Operator [ edit | edit source ]
If the Condition is true, the value of <if true> will be taken, otherwise the value of <else> will be taken. Example:
ram[EAB[9:0]] will be assigned to EDB in case ram_rd_en is true.
Operator Precedence [ edit | edit source ]
- Book:Programmable Logic