Difference Between Blocking and Non-blocking#
Blocking Assignment "="#
The next statement is executed only after the blocking assignment is completed; once the assignment is done, the variable value on the left side of the equal sign changes immediately. In the same block, the order of writing blocking assignment expressions affects the result of the assignment. There is no corresponding circuit in hardware.
That is, serial assignment, statements are executed in order from top to bottom and take effect immediately.
Non-blocking Assignment "<="#
The value on the right side of the expression is calculated at the start of the assignment, and the assigned variable is updated only at the falling edge of the clock in this simulation cycle, meaning the assignment does not take effect immediately; non-blocking assignments allow other statements in the block to execute simultaneously. In the same block, the order of writing non-blocking assignment expressions does not affect the result of the assignment. There is a corresponding circuit in hardware.
Parallel execution, not taking effect immediately, statements execute simultaneously.
Summary#
The hardware circuit corresponding to blocking assignment is not related to the clock trigger edge but only to the change in input levels; blocking assignment corresponds to flip-flops, and assignments occur only at the trigger edge.
- Use non-blocking assignment when modeling sequential circuits.
- Use non-blocking assignment when modeling latches.
- Use blocking assignment when establishing combinational logic in an always block.
- Use non-blocking assignment when establishing both sequential and combinational logic in the same always block.
- Do not use both blocking and non-blocking assignments in the same always block.
- Do not assign values to the same variable in more than one always block.
- Do not introduce delays in assignments.