Verilog HDL continuous assignment statement, process assignment statement, process continuous assignment statement

1. Continuous assignment statement （Continuous Assignments）

The continuous assignment statement is Verilog The basic statement of data flow modeling , Used for line network wire Assign a value , Equivalent to gate level description , yes Describe the circuit from a higher abstract point of view . Consecutive assignment statements must begin with key word assign Start .

assign     LHS_target = RHS_expression  ;

The main feature of continuous replication is ：

（1） Lvalue of continuous assignment statement , It can be one of the following types ：

        ① Scalar network

        ② Vector network

        ③ An element of a matrix （ The matrix can be scalar wire net type , It can also be of vector line network type ）

        ④ Some bit of vector line network

        ⑤ Partial bit of vector line network

And the various types of splices mentioned above , however , Cannot be a vector or a vector register .

（2） Continuous assignment statements are always active . As long as one operand changes , The expression will be immediately recalculated , And assign the result to the net to the left of the equal sign .

（3） Operands can be scalar or vector nets or registers , It can also be a function call .

（4） The assignment delay is used to control the time when a new value is assigned to the network , Explain according to the simulation time unit . The assignment delay is similar to the gate delay , It is very important to describe the timing in actual circuits .

Verilog It also provides another pair of wire A simple method of type assignment , That is to say wire Type variables are assigned values when declared .wire Type variables can only be assigned once , Therefore, this continuous assignment method can only be used once . For example, the following assignment method and the assignment method of the above assignment example , The results are consistent .

wire      A, B ;
wire      Cout = A & B ;

2.  Procedure assignment statement （Procedural Assignments）

    The update object of procedure assignment statement is register 、 Integers 、 Real numbers or time variables . These types of variables are assigned , Its value will remain unchanged , Until a new value is assigned by another procedure assignment statement . Procedure assignment statements only work when they are executed . Procedure assignment statements can only be used in initial or always Assignment within a statement , Only variable data types can be assigned , meanwhile initial and always Only procedure assignment statements can be used in . The lvalue of a procedure assignment statement can be one of the following types ：

①reg、 Plastic number 、 Real type number 、 Time register variable or memory unit

② The above various types of bit selection （ for example ：addr[3]）

③ The above various types of domain selections （ for example ：addr[31:16]）

④ The above three types of splicing

    Procedural assignment statements include two types of assignment statements ： Block assignment （=） And non blocking assignment （<=）（ The main differences are detailed in various Verilog Reference books , No more details here ）.

3.  Procedure continuous assignment statement （Procedural Continuous Assignments）

    Process continuous assignment is to assign variables in the process block Or continuous assignment of wire network data , It is a procedural assignment , In other words , Procedural continuous assignment statement is a kind of statement that can be used in always or initial Statements that appear in statement blocks . This assignment can be rewritten （Override） All other statements assign values to nets or variables . This assignment allows assignment expressions to be continuously driven into variables or nets .

    There are two types of procedure continuous assignment statements ：

①assign and deassign Procedural statement ： Assign values to variables .

②force and release Procedural statement ： It is mainly used to assign values to the network , It can also be used to assign values to variables .

3.1 assign and deassign sentence

    assign and deassign Statements constitute a class of procedural continuous assignment statements , It can only be used for continuous assignment of register type variables , It cannot be used for continuous assignment of network type data .

①assign sentence

grammar ：assign < Register type variable > = < Assignment expression >;

    assign When executed , Register type variables will be continuously driven by assignment expressions , That is, enter the continuous assignment state . If there is an ordinary procedure assignment statement to perform procedure assignment on the register variable , Because the procedure is continuous, the assignment statement assign The priority of is higher than that of ordinary procedure assignment statements , Therefore, the register variable in continuous assignment state will ignore the process assignment operation of ordinary process assignment statement , Its logical state is still determined by the assignment expression in the continuous assignment statement of the process .

    If there are two assign Statement to continuously assign values to the same register variable , So the second one assign The execution of will override the first assign The implementation effect of .

②deassign sentence

grammar ：deassign < Register type variable >;

    deassign Statement is a statement that cancels a continuous assignment , Used to end the continuous assignment of variables . When deassign After statement execution , Originally by assign The continuous assignment of the variable by the statement will be invalidated , The state in which the register variable is continuously assigned will be released , The variable can be assigned again by the ordinary process assignment statement . Here's a little bit of attention , When this statement is executed to undo the continuous assignment of a register variable , The register variable will remain the original value before using the statement .

③ Example ： 

 The execution process of the above statement is as follows ：

s0： stay 0 moment ,out To be an assignment 0, And keep this value ;

s1： stay 10 moment ,s1 Start execution , The implementation of variable out Continuous assignment operation , So from 10 The moment begins ,out Will be in a continuous assignment state ;

s2： stay 20 moment ,s2 Start execution , Will cover s1 The effect of , So from 20 The moment begins ,out Will be made by c & d Continuous drive ;

s3：s3 Operation covers s2 operation ;

s4： When deassign The statement is executed , Variable out The continuous assignment state is released , Its value will remain the last assign Statement , namely “e & f”;

3.2 force and release sentence

    force and release Statements and assign and deassign Statements like , It is also a procedure continuous assignment statement . This set of assignment statements can not only have an effect on register type variables , It can also continuously assign values to the network type data .

①force sentence

grammar ：force < Register variable or wire network data > = < Assignment expression >>;

force Statement when applied to a register type variable , It's in force After statement execution , This register variable will be forced by < Assignment expression > Continuous drive , Enter the state of continuous assignment , At this time, the assignment of other lower priority assignment statements to the register variable will be ignored , Until the execution of one release Statement to release successive assignments to the register variable .

force When the statement is applied to network data , be force After statement execution , The corresponding network data will be < Assignment expression > Continuous drive , Drivers with lower priority on the network data will be ignored , Until there is one release Until the statement is executed .

②release sentence

grammar ：release  < Register variable or wire network data >;

    release After statement execution , Originally by force The continuous assignment of a procedure imposed by a statement on a variable or network will be invalidated , The variable will be released from the state of being continuously assigned , The assignment operation of the assignment statement with lower priority will be valid .

③ Example ： 

s0： Implementation of the variable var_reg1 Procedure assignment operation , namely var_reg1 To be an assignment 3'b000;

s1： perform assign Procedure continuous assignment statement , Used to implement the control of variables var_reg2 Continuous assignment of , thus var_reg2 Will be consecutively assigned to 3'b001;

s2： When executing this statement ,var_reg1 Not by assign Statement has carried out continuous assignment operation , therefore var_reg1 By force Continuous assignment is 3'b100;

s3： After executing this statement ,var_reg2 By force Continuous assignment is 3'b100;

s4： When executing this statement , Because of the variable var_reg1 Will exit the state of continuous assignment , because var_reg1 Never been assign Statement has carried out continuous assignment operation , so var_reg1 The value remains unchanged , That is to keep force The value in the State 3'b100;

s5： When executing this statement , because var_reg2 In execution s3 It's been done by s1 Continuous assignment is realized , So in this sentence s5 After execution , Variable var_reg2 Will be restored to by assign sentence s1 Determined continuous assignment state , namely 3'b001;

4.  The difference between assignment statements

4.1  The difference between continuous assignment statements and procedure assignment statements

• The continuous assignment statement consists of assign To mark , The procedure assignment statement cannot contain this keyword ;
• The data type on the left side of the continuous assignment statement must be the line network data type , The assigned data type in the procedure assignment statement must be a variable of register type ;
• Continuous assignment statements cannot appear in procedure blocks （initial or always） in , The procedure assignment statement can ;
• Continuous assignment statement is mainly used to model combinational logic circuit and describe the connection between line network data , The process assignment statement is mainly used to describe the behavior of sequential logic circuits ;
• The assignment of continuous assignment statement to the assigned wire network data is “ continuity ” Of （ That is, any change in the assignment expression will immediately reflect the value of online data ）, And procedural assignment statements , The assignment operation is performed only when the procedure assignment statement is executed , After the statement is executed, the value of the assigned variable is no longer affected by the assignment expression （ Note that once here means ： stay initial In block , Procedural assignments are performed only once in sequence , And in the always In block , Every time you meet always The condition of , The... Should be executed once in sequence always Statement in block .）.
• The difference between continuous assignment and process assignment ：

	Process assignment	Continuous assignment
assign	nothing assign（ Except for procedural continuous assignment ）	Yes assign
Symbol	Use “=” or “《=”	Use only “=”
Location	stay always Sentence or initial ... can appear in any statement	Do not appear in always Statement and initial sentence
conditions for execution	Related to other statements around	When the value of the operand at the right end of the equal sign changes
purpose	Drive register	Drive wire network

4.2  The difference between procedure continuous assignment statement and continuous assignment statement

• Procedure continuous assignment statements can only be used in procedure blocks （initial The process is fast or always Process block ） Inside , Continuous assignment statements cannot appear in procedure blocks .
• The procedure continuous assignment statement can continuously assign values to register type variables （ among force-release Statement can also continuously assign values to the network ）, However, the assignment target cannot be a variable or one or more bits of the network , The continuous assignment statement can only assign values to the network data , The assignment target can be one or several bits of wire mesh data .

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