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Slide 1 7. Verilog: Combinational always statements. VHDL: Combinational Processes: To avoid (I.E. DO NOT What in your HDL code?) Cases that generate Synthesis.

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Presentation on theme: "Slide 1 7. Verilog: Combinational always statements. VHDL: Combinational Processes: To avoid (I.E. DO NOT What in your HDL code?) Cases that generate Synthesis."— Presentation transcript:

1 Slide 1 7. Verilog: Combinational always statements. VHDL: Combinational Processes: To avoid (I.E. DO NOT What in your HDL code?) Cases that generate Synthesis Warnings

2 Slide 2 1.Make INCOMPLETE ASSIGNMENTS 2.Exclude based on the SENSITIVITY LIST 3.Make COMBINATIONAL LOOPS Why these are not treaten as an error by the synthesizer? DO NOT: Verilog for Synthesis: Combinational always statements

3 Slide 3 1. INCOMPLETE ASSIGNEMNTS in combinational always statements Example module NOT_GOOD( input A, input B, input SEL, output reg Q); always @ (A or B or SEL) if (SEL) Q<=A; endmodule WHAT WILL BE THE VALUE OF Q WHEN SEL = 0? The Synthesizer will not be able to determint the value of Q when SEL = 0. Therefore: WILL RETAIN THE VALUE OF Q! (by default will consider else Q<=Q;) THEREFORE: WILL INFER A TRANSPARENT LATCH!

4 Slide 4 1 INCOMPLETE ASSIGNEMNTS in combinational always statements: Example: module CORRECT1( input A, input B, input SEL, output reg Q); always @ (A or B or SEL) Q <=B; if (SEL) Q<=A; endmodule SIGNALS ARE GETTING THEIR NEW VALUE AT THE END OF THE ALWAYS STATEMEMNT! (Their value can be read only after the always statement is ended and relaunched) In tis case, XST CAN DETERMINE THE VALUE OF Q when SEL=0, so it will infer a multiplexer!

5 Slide 5 1. INCOMPLETE ASSIGNEMNTS in combinational always statements Same for the code below (the more known version) module CORRECT2( input A, input B, input SEL, output reg Q); always @ (A or B or SEL) if (SEL) Q<=A; else Q <= B; endmodule So, if possible, for describing simple logic, use assign! Less lines to write In the case of an incomplete assignment the syntax checker will generate an error: assign Q = (SEL==1) ? A;

6 Slide 6 1. INCOMPLETE ASSIGNEMNTS in combinational always statements Note: The following is also an incomplete assignment! Example: module AND_2( input A, input B, output reg F); always @ (A or B) if (A&&B) F<=1; else F <= 1; //It should be 0! //This can be from a typo! endmodule Keep in mind, that the Synthesizer MINIMIZES!

7 Slide 7 1. INCOMPLETE ASSIGNEMNTS in combinational always statements The synthesizer minimizes! Generally: For an input that is NOT READ, it means that is NOT USED. The synthesizer generates a warning message such as: WARNING:Xst:647 - Input is never used. Note A signal is read also in a condition, such as if (B)…., or (B==1), not only …<= B;! For an output which NEVER CHANGES during circuit operation, the synthesizer generates a warning such as: WARNING:Xst:xxxx – Output Q never changes during circuit operation. Q is connected to VCC Or: WARNING:Xst:xxxx – Output Q is constant Or, if Q changes during circuit operation ONLY ONCE: WARNING:Xst:xxxx – The register/latch FDXX hinder the constant Q cleaning in line XX

8 Slide 8 1. INCOMPLETE ASSIGNEMNTS in combinational always statements The synthesizer minimizes! WARNING:Xst:xxxx – The register/latch FDXX hinder the constant Q cleaning in line XX This can happen when Q is initialized to a value and then in the circuit is set to another CONSTANT value Example: reg Q = 0; always @ (posedge CLK) //even for sequential statements, //because Q will change only once! Q <=1; Q will be 0 at the beginning and then constantly 1! Where is the greatest danger of making constant outputs? Look carefully at the shift register code!: Shift left: Q <= {Q[6:0], In} or Q <= {Q[7:1], In} ? Shift right: Q<= {In, Q[7:1]} or Q <= {In, Q[6:0]} ?

9 Slide 9 1. INCOMPLETE ASSIGNEMNTS in combinational always statements The above examples are relatively simple and the mistake is visible But do you like to use nested if statements? Example: always @ * begin if (A) if (B) begin if (C) Q<=3’b010; end else Q <=3’b000; //else for which if? //it is else for if (B) //What if C = 0? It will infer a LATCH else if (B) Q<=3’b111; //this else goes then to which if? //to if (A) else if ….//and this one…? // to else if (B)! ALIGN “elses” to “ifs”! Better: Concat A, B, C into a bus and use assign or case Sometimes, nested ifs can not be avoided. Example: Transform an 8-bit counter to a 2-digit BCD counter (LAB exercise!)

10 Slide 10 1. INCOMPLETE ASSIGNEMNTS in combinational always statements Frequently appear in CASE statements The “default” statement is not enough! Example: Logic describing transitions to a state machine: StC represents the currents state, StN the next state always @ * begin case (StC) Idle: if (In1) StN <= St1; else StN <= Idle; St1: if (!In1) StN <=St2; //HERE IS NO ELSE! //THE SYNTHESIZER WILL INFER A //LATCH FOR StN! St2: StN <= St3; St3: if (In2) StN <= St2; else StN <= Idle; endcase

11 Slide 11 1. INCOMPLETE ASSIGNEMNTS in combinational always statements Frequently appear in case statements Solution that also allows for more compact description always @ * begin StN <= StC; // by default, STAY IN THE CURRENT //STATE case (StC) Idle: if (In1) StN <= St1; St1: if (!In1) StN <=St2; St2: StN <= St3; St3: if (In2) StN <= St2; //here are two choices, we need else else StN <= Idle; default: StN <= Idle; //default DOES NOT TOTALLY SOLVE // INCOMPLETE ASSIGNMENTS, only for the CASE branches! // It also helps for safe implementation of a state machine endcase

12 Slide 12 1.. INCOMPLETE ASSIGNEMNTS in combinational always statements Why important to avoid? Because can lead to inference of unwanted latches One reason for simulation differences between behavioral and post-translate A latch is a memory element, it will not always act as a combinational circuit! The syntesizer generates a warning message: WARNING:Xst:xxxx – Found x-bit latch for the signal StN LATCH este asynchronous. XST does not recommends asynchronous circuits! If the designer wants to intentionally generate latches, the warning message can be ignored Otherwise, review the code

13 Slide 13 Note: Incomplete assignments in Sequential always statements Example: always @ (posedge CLK) if (Reset) Q <= 0; else if (CE) Q<=Q+1; What happens with Q if Reset = 0 and CE = 0? The counter will hold i.e. keeps its value No latch will be inferred, because anyway a register is inferred for the counter Incomplete assignments are allowed in sequential always statements Used to describe the role of the “Clock Enable” types of signals

14 Slide 14 1.Make INCOMPLETE ASSIGNMENTS 2.Exclude based on the SENSITIVITY LIST 3.Make COMBINATIONAL LOOPS Why these are not treaten as an error by the synthesizer? DO NOT: Verilog for Synthesis: Combinational always statements

15 Slide 15 DO NOT use the sensitivity list to exclude a signal! Example: We want to read the value of a signal ONLY WHEN another signal changes (B will be read only when A changes) always @ (A or C) //The always statement will not be executed //by the simulator when B changes Q<=(!C) | (A AND B); Behavioral Simulation Result: B=0 but still Q=1 CAN BE PRACTICALLY MADE SUCH A CIRCUIT, WITHOUT USING A MEMORY ELEMENT (i.e. REGISTER)? Excluding signals based on the SENSITIVITY LIST

16 Slide 16 DO NOT use the sensitivity list to exclude a signal! Example: We want to read the value of a signal ONLY WHEN another signal changes (B will be read only when A changes) always @ (A or C) //The always statement will not be executed //by the simulator when B changes Q<=(!C) | (A AND B); Synthesis result: Post-Translate simulation result: Q changes no matter whether B changes or not Excluding signals based on the SENSITIVITY LIST

17 Slide 17 DO NOT use the sensitivity list to exclude a signal! Example: We want to read the value of a signal ONLY WHEN another signal changes (B will be read only when A changes) always @ (A or C) //The always statement will not be executed //by the simulator when B changes Q<=(!C) | (A AND B); Synthesis result: During synthesis: WARNING:Xst:819 - c:/temp/test1/combinational.v line 21: The following signals are missing in the process sensitivity list: RECOMMENDATION: INSERT IN THE SENSITIVITY LIST ALL OF THE SIGNALS READ IN THE ALWAYS STATEMENT! This is a NECESARRY CONDITION for Behavioral and Post-Translate simulations to produce the same result! Simplified solution: always @ * Excluding signals based on the SENSITIVITY LIST

18 Slide 18 1.Make INCOMPLETE ASSIGNMENTS 2.Exclude based on the SENSITIVITY LIST 3.Make COMBINATIONAL LOOPS Why these are not treaten as an error by the synthesizer? DO NOT: Verilog for Synthesis: Combinational always statements

19 Slide 19 Example: module Test( input [3:0] A, output reg [3:0] Q); always @ (A or Q) Q<=Q + A; //Clearly a Combinational Loop From the simulator’s point of view: the always statement will run in an infinite loop (after finishes, Q changes, so the always statement is run again) If A is not 0, the Q immediately reaches its maximum value The Behavioral simulator will hang or issue an error But the circuit CAN BE SYNTHESIZED, EVEN WITHOUT A WARNING! The generated circuit is obviously useless (positive fedback – a latch?) The Post-Translate and Post-Route simulators will not be able to determine the value of Q, the output value will be always XXXX AVOID COMBINATIONAL LOOPS!!! Combinational Loops

20 Slide 20 Everybody can avoid a combinational loop such as below: Q<=Q + A; HOWEVER: What about in a longer code? always @ (A or B or C) begin … A<= B & … end …………. always @ (B or Q or C) begin … B<= Q | C |… end …………… always @ (Q or A or C) begin ….. if (A==…) Q<= end //A depends on B, B depends on Q and Q depends on A! It is a loop! What do you want to describe here? Try to clear up first the combinational logic You want to describe, then start writing the code! Combinational Loops

21 Slide 21 Note: Loops can be made in sequential always statements. In this case registers are inferred for keeping the value of Q: always @ (posedge CLK) begin Q<=Q + A; Synthesis result : Behavioral and post-translate simulation result: Counter with increment of A Combinational Loops

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