Verilog Non Blocking Assignment

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:

LED_on = 0; count = count + 1; LED_on = 1;

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   


always @(posedge i_clock) begin r_Test_1 <= 1'b1; r_Test_2 <= r_Test_1; r_Test_3 <= r_Test_2; end

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:

always @(posedge i_clock) begin r_Test_1 = 1'b1; r_Test_2 = r_Test_1; r_Test_3 = r_Test_2; end

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.



I had a hard time over this too.

But firstly, you should understand that non-blocking or blocking is actually nothing to do with whether a latch/ff would be created!

For their difference you could understand it simply(at beginning) by this point: i. If use blocking, sentences after it could not be executed until block sentence LHS assigned value, since what changed to LHS of it could be updated and used if the variable is used. However, for non-blocking, it don't block following sentence like parallel with following sentence(actually RHS calculation should be done first, but it doesn't matter, ignore it when you confuse). The LHS don't change/updated for this time's execution (updated next time when always block trigged again). And following sentence use the old value, as it updated at the end of execution cycle.

One key point is to find whether in you code (always block) there is any case variable not assigned value but could happen. If you don't pass value to it and that case occurs, then latch/ff is created to keep the value.

For example,

Following could also create latch/ff:

--> latch/ffs created for in=1, b no assignment, in=0 a no assignment.

In addition, when you sense posedge of clk , it is bound to end with latch/ff. Because, for clk, there must exist negative edge, and you don't do anything, latch/ffs are created to keep all the old value!

answered Jun 25 '14 at 17:33

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