1. 2/9/20031 ECE 551: Digital System Design & Synthesis Lecture Set 4 4.1: Verilog – Procedural Assignments &Scheduling Semantics 4.2: Verilog – More Behavioral Descriptions (In separate file)

2. 2/9/20032 ECE 601 - Digital System Design & Synthesis Lecture 4.1 – Procedural Assignments & Scheduling Semantics Overview  Blocking & Non-Blocking Assignments  The Verilog Stratified Event Queue  Determinism and Non-determinism  Guaranteed ordering  Ambiguous ordering  Blocking & Non-Blocking Assignments with Delays  Interacting Behaviors  Coding Guidelines

3. 2/9/20033 Procedural Assignments in General  Assignment consists of two parts:  Evaluation of the right hand side (RHS)  Assignment of evaluation to left hand side (LHS)  Delays can affect:  Time of execution of the current and subsequent assignments,  Time of evaluation of the RHS  Time of assignment to the LHS  Type of assignment (blocking, non-blocking) can affect the times as well

4. 2/9/20034 Blocking Assignments  Identified by =  Sequence of blocking assignments executes sequentially  An assignment must be completed before the next assignment is activated for consideration.  Inter-assignment delays block both evaluation and update  Intra-assignment delays block update but not evaluation

5. 2/9/20035 Nonblocking Assignments  Identified by <=  Nonblocking assignments without delays execute simultaneously  An assignment need not complete for consideration of subsequent assignments for execution.  Inter-assignment delays block both evaluation and update as well as consideration of subsequent assignments  Intra-assignment delays block update but do not block evaluation or consideration of subsequence assignments for evaluation  Nonblocking assignments for a given behavior are scheduled last.

6. 2/9/20036 Blocking & Non-Blocking Assignments – Examples - 1  Blocking reg[7:0] A, B, C, D always@(posedge clk) begin A = B + C; B = A + D; C = A + B; end  Non-Blocking reg[7:0] A, B, C, D always@(posedge clk) begin A <= B + C; B <= A + D; C <= A + B; end  initial begin A = 8’h1; B = 8’h2; C = 8’h3; D = 8’h4; end

7. 2/9/20037 Blocking & Non-blocking Assignment – Examples - 2  Simulation Results:

8. 2/9/20038 Simulation of Simultaneous Assignments  Scheduling semantics need to be defined for simulation  For consistency of simulation results and synthesized hardware behavior, these semantics need to be mimicked by synthesis tools  Scheduling semantics defined by:  Verilog Stratified Event Queue

9. 2/9/20039 Event Simulation Terminology [2]  Update event – every change in value of a net or register. Also a named event.  Processes execute in response to update events in arbitrary order.  Evaluation event – the evaluation of a process.  Simulation time is the time value used by the simulator to model actual time.  Events are kept in an event queue.  Scheduling an event – putting an event on a queue.

10. 2/9/200310 The Verilog Stratified Event Queue - 1  Five Regions  Region 1: Active Events – events that occur at the current simulation time and can be processed on any order.  Region 2: Inactive Events - events that occur at the current simulation time, but that shall be processed after all active events.  Region 3: Non-blocking Assign Update Events – Events that have been evaluated at some previous simulation time, but shall be assigned at this simulation time after all active and inactive events are processed.

11. 2/9/200311 The Verilog Stratified Event Queue - 2  Five Regions (Continued)  Region 4: Monitor Events – Events that shall be processed after all active, inactive, and nonblocking assign update events.  Region 5: Future Events – Events that occur at some future simulation time. Include future inactive events and future nonblocking assignment events.

12. 2/9/200312 More Terminology [2]  Simulation cycle – the processing of all active events.  Explicit zero delay (#0) – forces the process to be suspended and added as inactive event for the next simulation cycle.  Non-blocking assignment creates a nonblocking assign update event at the current or a later simulation time.  $monitor and $strobe system tasks create monitor events for their arguments  Re-enabled in every successive time step  Monitor events cannot create any other events  Certain PLI (Programming Language Interface) procedures are treated as inactive events.

13. 2/9/200313 Verilog Sim Reference Model - 1 T is current simulation time and all events are held in event queue, ordered by simulation time While (there are events){ if (no active events){ if (there are inactive events){ activate all inactive events; }else if (there are nonblocking assign update events){ activate all nonblocking update events; }else if (there are monitor events){ activate all monitor events; }else {advance T to the next event time; activate all inactive events for time T; }

14. 2/9/200314 Verilog Simulation Reference Model - 2 E = any active event; if (E is an update event){ update the modified object; add evaluation events for sensitive processes to the event queue; }else {/*shall be an evaluation event */ evaluate the process; add update events to the event queue; }

15. 2/9/200315 Determinism [2]  Standard guarantees a certain scheduling order:  Statements within a begin-end block shall be executed in the order in which they appear although the processing can be suspended in favor of other processes in the model.  Nonblocking assignments shall be performed in the order the statements are executed, i. e., they are queued, taken from the queue and performed in the order of the statements.

16. 2/9/200316 Nondeterminism [2]  Active events that be taken from the queue and processed in any order  Statements without time-control constructs (#,@) in behavioral blocks do not have to be executed as one event.  The simulator may suspend execution and place the partially-completed event as a pending active event on the event queue.  The ordering of this interleaving of execution is nondeterministic and not under user control.

17. 2/9/200317 Scheduling Implications of Assignments (Partial)  Continuous assignment – when value of expression changes schedules update event  Procedural continuous assignment – similar to continuous assignment  Blocking assignment (intra-assignment delay) – computes RHS, then causes process to be suspended and scheduled as a future event. When process returns, assigns LHS and enable events based on update of LHS.  Nonblocking assignment – computes updated value and schedules update as a nonblocking assign update event in current or future time step.

18. 2/9/200318 Blocking Assignment Example – Inter-assignment Delay  Delays both the evaluation and the update  Example: always @(posedge clk) begin b = a + a; # 5 c = b + a; # 2 d = c + a; end initial begin a = 3; b = 2; c = 1; end  What are the results for b, c and d and when do they occur?

19. 2/9/200319 Blocking Assignment Example - Intra-Assignment Delay  Delays the update, but not the evaluation  Example: always @(posedge clk) begin b = a + a; c = # 5 b + a; d = # 2 c + a; end initial begin a = 3; b = 2; c = 1; end  What are the results for b, c and d, and when do they occur?

20. 2/9/200320 Non-Blocking Assignment Example – Inter-assignment Delay  Delays both the evaluation and the update  Example: always @(posedge clk) begin b <= a + a; # 5 c <= b + a; # 2 d <= c + a; end initial begin a = 3; b = 2; c = 1; end  What are the results for b, c and d and when do they occur?

21. 2/9/200321 Non-Blocking Assignment - Intra-Assignment Delay  Delays the update, but not the evaluation  Example: always @(posedge clk) begin b = a + a; c = # 5 b + a; d = # 2 c + a; end initial begin a = 3; b = 2; c = 1; end  What are the results for b, c and d, and when do they occur?

22. 2/9/200322 Assignments with Delays – Simulation Results

23. 2/9/200323 Mixed Blocking/Nonblocking Assignment Example  Example: always @(posedge clk) begin b = a + a; c <= b + a; #2 d <= c + a; c = d + a; end  Initial begin a = 3; b = 2; c = 1; d = 0; end  Example: What are the results for b, c, and d, and when do they occur?

24. 2/9/200324 Mixed Blocking/Non-blocking Simulation Results

25. 2/9/200325 Interacting Behaviors - 1  Example: always @ (posedge clk) begin … A <= B; … end always @ (A or C) begin … D = C; … end  Non-blocking assignment execution causes blocking assignment execution to follow it in same timestep.

26. 2/9/200326 Interacting Behaviors - 2  Example: (from Cummings [1]) always @ (posedge clk or posedge rst) if (rst) y1 = 0; //reset else y1 = y2; always @ (posedge clk or posedge rst) if (rst) y2 = 1; // preset else y2 = y1;  If first always first after reset, y1 = y2 = 1. If second always first after reset, y2 = y1 = 0.  Thus results are order dependent and ambiguous, a Verilog “race condition.”

27. 2/9/200327 Interacting Behaviors - 3  Example: (from Cummings [1]) always @ (posedge clk or posedge rst) if (rst) y1 <= 0; //reset else y1 <= y2; always @ (posedge clk or posedge rst) if (rst) y2 <= 1; // preset else y2 <= y1;  Regardless of which always executes first, by the Verilog standard, the assignments for y1 and y2 occur in parallel at the end of the timestep, giving y1 = 1 and y2 = 0.  Thus results are order-independent.

28. 2/9/200328 Coding Guidelines [1]  #1: When modeling sequential logic, use nonblocking assignments.  #2: When modeling latches, use nonblocking assignments. (flip-flops?)  #3: When modeling combinational logic with an always block, use blocking assignments.  #4: When modeling both sequential and combinational logic within the same always block, use nonblocking assignments.

29. 2/9/200329 Coding Guidelines [1] (Continued)  #5: Do not mix blocking and nonblocking assignments in the same always block.  #6: Do not make assignments to the same variable from more than one always block.  #7: Use $strobe to display values that have been assigned using nonblocking assignments.  #8: Do not make assignments using #0 delays.

30. 2/9/200330 Coding Guidelines – Comments on Cummings [3]  Verilog’s event queue can be rerun in the same timestep if non-blocking events trigger always statements (CRK: can cause nonblocking statements not to be last in timestep).  Blocking assignments with delays, one per separate always statement is a viable alternative (CRK: But for synthesis? Seems like a bad idea.)  You can safely mix blocking assignments (combinational logic) and nonblocking assignments (flip-flops) in the same edge-triggered always statement, provided the nonblocking statements are last in the always (although may cause problems with false latch generation in synthesis).

31. 2/9/200331 References  1. Cummings, Clifford E., “Nonblocking Assignments in Verilog Synthesis, Coding Styles That Kill!, SNUG-2000.  2. IEEE, IEEE Standard Hardware Description Language Based on the Verilog Hardware Description Language, IEEE Std 1364-1995.  Campbell, Paul, “A note on Verilog assignments,” Verifarm, Inc., http://www.verifarm.com/assign.shtml