Electronic – Difference between blocking and nonblocking assignment Verilog

verilog

I was reading this page http://www.asic-world.com/verilog/verilog_one_day3.html when I came across the following:

We normally have to reset flip-flops, thus every time the clock makes the transition from 0 to 1 (posedge), we check if reset is asserted (synchronous reset), then we go on with normal logic. If we look closely we see that in the case of combinational logic we had "=" for assignment, and for the sequential block we had the "<=" operator. Well, "=" is blocking assignment and "<=" is nonblocking assignment. "=" executes code sequentially inside a begin / end, whereas nonblocking "<=" executes in parallel.

I was fairly sure that nonblocking assignments were sequential while blocking assignments were parallel. After all, you can make blocking assignments with assign statements outside of always blocks, and those all run in parallel. Is this a mistake, or is the behavior different inside an always block? And, if the behavior IS different inside an always block, can nonblocking assignments be made outside an always block?

Best Answer

was fairly sure that nonblocking assignments were sequential while blocking assignments were parallel.

Blocking assignment executes "in series" because a blocking assignment blocks execution of the next statement until it completes. Therefore the results of the next statement may depend on the first one being completed.

Non-blocking assignment executes in parallel because it describes assignments that all occur at the same time. The result of a statement on the 2nd line will not depend on the results of the statement on the 1st line. Instead, the 2nd line will execute as if the 1st line had not happened yet.