blocking and non blocking assignment in vhdl

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Does VHDL has blocking and non blocking assignments?

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jsavory (Member) asked a question.

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mcgett (Member)

jsavory (Member)

gszakacs (Member)

bassman59 (Member)

> -------------------------------------------------------------------------------- > > @bassman59 wrote: > > > To summarize: > >   > > NEVER use blocking assignments in a synchronous always block. > > --------------------------------------------------------------------------------

That's pretty good advice for anyone but the advanced user, who understands the

implications of using blocking assignments.  VHDL makes a distinction between

signals and variables, and prevents the use of blocking assignments on signals.

In Verilog any reg can have either blocking or non-blocking assignments but not

both for synthesis, making them either signals or variables depending on usage.

The preferred approach is to use blocking assignments only for local variables

(regs) within a clocked process (named always block), so they aren't inadvertently used

outside the process.  Even then you can get into trouble if you don't know what

you're doing.  Personally I have little use for this sort of coding, and find that there's

often a more elegant way to reach the same end without resorting to local variables.

Most often using a signal outside of the clocked block, either in a combinatorial

always block or in a continuous assignment can do the trick.

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Sequential (i.e. flip-flop and register) behaviour can be modelled using guarded blocks, but again for synthesis and readability it is better described using "clocked" processes.

In VHDL -93 the keyword block (or the guard condition, if there is one), may be followed by the keyword is , for consistancy.:

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Blocking and non-blocking assignment, blocking (the = operator), non-blocking (the <= operator), no comments:, post a comment.

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Blocking and Non-Blocking assignment

• Thread starter alam.tauqueer
• Start date Jan 31, 2008
• Jan 31, 2008

alam.tauqueer

Full member level 2.

vhdl blocking assignment Hi, Can any one tell me what is the difference between the below mention code ,and what would be the issues if we are using blocking statment in sequencial logic ////////////////////////////////////// always@(posedge clk) begin if(reset) begin q1 = 0; q2 = 0; end else begin q1 = d1; q2 = d2; end end ///////////////////////////////// always@(posedge clk) begin if(reset) begin q1 <= 0; q2 <= 0; end else begin q1 <= d1; q2 <= d2; end end ///////////////////////////////////  

Super Moderator

blocking assignments vhdl Hello, 1. there is no difference in this case, 2. behaviour would be different, if a assigned register value is used in another assignment effective on posedge clk 3. I don't see a particular issue, apart from function not behaving as intended. Both assignment techniques could be meaningful. An an implication of blocking assignment, logic function could depend on statement order, cause you can have mutiple assignments to a register in a sequence, each taking effect immediately. With non-blocking assignment, function is independant of statement order, cause any assignment takes effect in the next clock cycle. Regards, Frank  

Junior Member level 1

blocking and non-blocking assignments Hi, Correct me if I'm wrong. There's some guideline that we should follow: 1. When modeling sequential logic, use nonblocking assignments. 2. When modeling combinational logic with an always block, use blocking assignments. These can help you avoid all the mismatch between simulation and synthesis.  

blocking assignment vhdl Hello, I'm not sure in which situation you get "mismatch between simulation and synthesis". I think, a mismatch between programmers intention and the function actually coded is the more common case. You should know what you are doing. Your differentiation between sequential logic and combinational always blocks generally leads in the right direction. However, when describing synthesisable logic, the term "sequential" should be better replaced or supplemented by the term "synchronous", which characterizes the functional quality. Any assignment in the context of an "synchronous" always clock (posedge, negedge) instantiates a clock synchronous flipflop, which effectively operates non-blocking. If you use blocking statements in this context, this could have different meanings. As in your original example, it could have no particular meaning and then would be inadequate, possibly misleading. Or it could instantiate additional combinational logic executing before non-blocking assignments. In VHDL, you have to use a separate VARIABLE object instead of a SIGNAL (= reg) to achieve this functionality. In a combinational always block, non-blocking assignments don't have a reasonable purpose to my opinion. As a detailed Verilog description, I prefer the Synopsys HDL compiler reference manual (identical in parts with the Xilinx Foundation Express verilog reference) Regards, Frank  

• Feb 1, 2008

blocking and non blocking Well have a look at this. It's got a good explanation what we should use in each case.  

blocking assignments Hi Frank, I am fully agree with you but I was confuse like if we are using a blocking assignment in case of sequential always block as the example given above than how it is different from a non-blocking assignment. I have one more question here is there any simulation synthesis mismatch in case of blocking assignment? And what would be the harware ? It will be same hardware which we ll get through non-blocking assignment?  

santumevce1412

Junior member level 2.

verilog mixing blocking non-blocking For designing the sequential circuits, we define using non blocking statements because all the events r to be changed at the edge of the clock and for designing combinational circuits, we use blocking statements, as clk event is not a criteria..  

blocking non blocking mix Hello, thank you for linking the SNUG paper. It is really instructive and probably answers most questions raised in this discussion, also regarding possible mismatches between synthesis and simulation. It effectively could end the discussion. I have a minor differences regarding the mixed cases (Guideline #5) in synchronous always blocks, which can be meaningful in some cases to my opinion. But I share the authors viewpoint, that alternative constructs better avoid misunderstandings. In so far I also agree Guideline #5, others anyway. Regards, Frank  

vamsi_addagada

vhdl mixing blocking and non-blocking assignments hi ajay the systhesis out put is first code got one FF that is connect the d1,d2, second code getting the o/p is the 2FF vamsi  

• Feb 4, 2008

blocking and non blocking assignment vhdl Ok so in the above code we will not see any difference between blocking and non-blocking assignment. Can any one please tell me where would be difference come into picture if we will use blocking assignment in case of sequential circuit. It wil help us to get a better understanding. Tauqueer  

snug-verilog blocking,non-blocking Read the snug paper linked by NanhTrang. Would be a wast of time trying to explain the topic better.  

verilog blocking vs non blocking assignment Hi, @Frank: Maybe I'm not good enough to code mix blocking and non-blocking assignments so I just simply separate them. @Tauqueer: Please take a look at the paper I posted above. It have everything you are asking for.  

Maybe I'm not good enough to code mix blocking and non-blocking assignments so I just simply separate them. Click to expand...

blocking non blocking assignments vhdl I went through the paper now things are clear to me . Thanks alot to all. Regards Tauqueer  

Member level 2

snug paper, blocking and non blocking if u use non blocking assignment statements the values are assigned after current simulation,whereas in blocking values r assingned straight away one more thing in posedge of clock if u write blocking in synthesis u will get ff/latch  

• May 3, 2008

Newbie level 6

xilinx blocking vs non blocking Hi Alll Just go through verilog Basics by salman palnitkar, its good book for verilog basics. regards Mohi  

• May 5, 2008

Member level 5

the difference between blocking assignment check this pdf....hope it ll help u  

• May 13, 2008

blocking and non-blocking in vhdl Both statement blocks are correct. But using non blocing stmt assignment is good practice and using blocking stmt is bad coding style  

• May 20, 2008

Full Member level 4

blocking non-blocking assignment correct me if i am wrong.. Blocking assignment is similar to the variable assignment in vhdl... and non-blocking is similar to signal assignment in vhdl.. In blocking the data is assigned immediately but in non -blocking the data is assigend in the next clock cycle.. please let me know if i am wrong..  

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Verilog Blocking & Non-Blocking

Blocking assignment statements are assigned using = and are executed one after the other in a procedural block. However, this will not prevent execution of statments that run in a parallel block.

Note that there are two initial blocks which are executed in parallel when simulation starts. Statements are executed sequentially in each block and both blocks finish at time 0ns. To be more specific, variable a gets assigned first, followed by the display statement which is then followed by all other statements. This is visible in the output where variable b and c are 8'hxx in the first display statement. This is because variable b and c assignments have not been executed yet when the first $display is called.

In the next example, we'll add a few delays into the same set of statements to see how it behaves.

Non-blocking

Non-blocking assignment allows assignments to be scheduled without blocking the execution of following statements and is specified by a symbol. It's interesting to note that the same symbol is used as a relational operator in expressions, and as an assignment operator in the context of a non-blocking assignment. If we take the first example from above, replace all = symobls with a non-blocking assignment operator , we'll see some difference in the output.

See that all the $display statements printed 'h'x . The reason for this behavior lies in the way non-blocking assignments are executed. The RHS of every non-blocking statement of a particular time-step is captured, and moves onto the next statement. The captured RHS value is assigned to the LHS variable only at the end of the time-step.

So, if we break down the execution flow of the above example we'll get something like what's shown below.

Next, let's use the second example and replace all blocking statements into non-blocking.

Once again we can see that the output is different than what we got before.

If we break down the execution flow we'll get something like what's shown below.

Blocking (immediate) and Non-Blocking (deferred) Assignments in Verilog

There are Two types of Procedural Assignments in Verilog.

• Blocking Assignments
• Nonblocking Assignments

To learn more about Delay: Read  Delay in Assignment (#) in Verilog

Blocking assignments

• Blocking assignments (=) are done sequentially in the order the statements are written.
• A second assignment is not started until the preceding one is complete. i.e, it blocks all the further execution before it itself gets executed.

Non-Blocking assignments

• Nonblocking assignments (<=), which follow each other in the code, are started in parallel.
• The right hand side of nonblocking assignments is evaluated starting from the completion of the last blocking assignment or if none, the start of the procedure.
• The transfer to the left hand side is made according to the delays. An intra- assignment delay in a non-blocking statement will not delay the start of any subsequent statement blocking or non-blocking. However normal delays are cumulative and will delay the output.
• Non-blocking schedules the value to be assigned to the variables but the assignment does not take place immediately. First the rest of the block is executed and the assignment is last operation that happens for that instant of time.

To learn more about Blocking and Non_Blocking Assignments: Read Synthesis and Functioning of Blocking and Non-Blocking Assignments

The following example shows  interactions  between blocking  and non-blocking for simulation only (not for synthesis).

For Synthesis (Points to Remember):

• One must not mix “<=” or “=” in the same procedure.
• “<=” best mimics what physical flip-flops do; use it for “always @ (posedge clk..) type procedures.
• “=” best corresponds to what c/c++ code would do; use it for combinational procedures.

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Related posts:

• Synthesis and Functioning of Blocking and Non-Blocking Assignments.
• Delay in Assignment (#) in Verilog
• Ports in Verilog Module
• Module Instantiation in Verilog

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Blocking vs. Nonblocking in Verilog

The concept of Blocking vs. Nonblocking signal assignments is a unique one to hardware description languages. The main reason to use either Blocking or Nonblocking assignments is to generate either combinational or sequential logic. In software, all assignments work one at a time. So for example in the C code below:

The second line is only allowed to be executed once the first line is complete. Although you probably didn’t know it, this is an example of a blocking assignment. One assignment blocks the next from executing until it is done. In a hardware description language such as Verilog there is logic that can execute concurrently or at the same time as opposed to one-line-at-a-time and there needs to be a way to tell which logic is which.

<=     Nonblocking Assignment

=      Blocking Assignment

The always block in the Verilog code above uses the Nonblocking Assignment, which means that it will take 3 clock cycles for the value 1 to propagate from r_Test_1 to r_Test_3. Now consider this code:

See the difference? In the always block above, the Blocking Assignment is used. In this example, the value 1 will immediately propagate to r_Test_3 . The Blocking assignment immediately takes the value in the right-hand-side and assigns it to the left hand side. Here’s a good rule of thumb for Verilog:

In Verilog, if you want to create sequential logic use a clocked always block with Nonblocking assignments. If you want to create combinational logic use an always block with Blocking assignments. Try not to mix the two in the same always block.

Nonblocking and Blocking Assignments can be mixed in the same always block. However you must be careful when doing this! It’s actually up to the synthesis tools to determine whether a blocking assignment within a clocked always block will infer a Flip-Flop or not. If it is possible that the signal will be read before being assigned, the tools will infer sequential logic. If not, then the tools will generate combinational logic. For this reason it’s best just to separate your combinational and sequential code as much as possible.

One last point: you should also understand the semantics of Verilog. When talking about Blocking and Nonblocking Assignments we are referring to Assignments that are exclusively used in Procedures (always, initial, task, function). You are only allowed to assign the reg data type in procedures. This is different from a Continuous Assignment . Continuous Assignments are everything that’s not a Procedure, and only allow for updating the wire data type.

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