Automatic Verification of Timing Constraints

1. Automatic Verification of Timing Constraints Asli Samir – JTag course 2006

2. Introduction • Today devices are challenging – million transistors & multiple gigahertz. • Unfortunately, the size and complexity of design constraints to build these designs are increasing exponentially. • One quarter of design projects undergo more than 10 iterations due to constraint issues . • Traditional design methods have largely ignored the design-constraint problem. Automatic Verification of Timing Constraints

3. Design Constraints Design constraints provide information on the design behavior and are usually generated by the designer. The timing constraints can be false-paths (FP) or multi-cycle paths (MCP), Automatic Verification of Timing Constraints

4. The Design and Verification Gap -- The number of transistors on a chip increases approximately 58% per year, according to Moore's Law. -- The design productivity, facilitated by EDA tool improvements, grows only 21% per year. Automatic Verification of Timing Constraints

5. A step before jtag, RTL level. GOAL Supplying an automatic way to verify the correctness of some timing constraints such as : Multicycle paths False paths Automatic Verification of Timing Constraints

6. MCP & FP • False paths (FP) and multi-cycle-paths (MCP) are timing exceptions that present a particularly difficult problem when trying to achieve timing closure in modern, high-performance designs. • Typically, these exceptions, as well as all timing constraints, are considered late in the design cycle and are specified in response to timing problems during verification. For optimum timing results, all timing exceptions must be guaranteed to be correct. We want to minimize the delay of the device Automatic Verification of Timing Constraints

7. What is Multi-Cycle Path • A multi-cycle path in a design is a register-to-register path through some combinational logic where if the source-register changes, the path will require N cycles (where N > 1) before the computation is propagated to the destination register. - A finite-state machine controls the outputs such that the addition is driven out in one clock cycle, while the multiplication is driven out a cycle later, (needs 2 clk cycles) Automatic Verification of Timing Constraints

8. Example (MCP) The circuit above takes the signals a_in and b_in and inputs them to both an adder and multiplier. A finite-state machine controls the outputs such that the addition is driven out in one clock cycle, while the multiplication is driven out a cycle later. In other words, it takes two cycles to produce the multiplication result. Automatic Verification of Timing Constraints

9. What is False Path - A false path is a path through a circuit that cannot be responsible for the circuit delay and no sequence of vectors result in the propagation of an event along the path. - There are two types of false paths: synchronous and asynchronous. Synchronous false paths are correct when the logic path cannot execute. Asynchronous false paths are correct if the source and the target flops of the paths are from asynchronous clock domains. Furthermore, a valid synchronization scheme is required. Automatic Verification of Timing Constraints

10. שינוי ב in1 לא מורגש ב out2 1 0 Example (Synchronous FP) Notice the register muxReg that controls the two muxes in the design is one-hot, and as result there is no path from the input in1 to the output out2. Therefore this path may be ignored when performing timing analysis. Automatic Verification of Timing Constraints

11. CLK A DA CLK B DB Example (Asynchronous FP) Clock Domain Crossing Paths (CDC) • Metastability D DA DB CLK A CLK B Automatic Verification of Timing Constraints

12. D CLK A CLK B DA Synchronizers D DB CLK A CLK B Circuits that conditions CDC signals in order to reduce the probability of metastability DA CLK B Automatic Verification of Timing Constraints

13. data_in sync data_out clk_a clk_b R1 R2 clk_b sync R1.q R2.q CDC Synchronization • Synchronizers • Flip-flops in sequence reduce probability of metastability • 2 D flip-flops is the most commonly used scheme • But leads to unpredictable delay in signal propagation דוגם אחרי 2 clk את הערך הנכון Automatic Verification of Timing Constraints

14. SDC – Synopsys Design Constraints • SDC file describes the “design intent” and surrounding constraints for synthesis, clocking, time, power, test and environmental and operating conditions. This file includes the design constraints Automatic Verification of Timing Constraints

15. Description of the problem • Textual mistakes . • Missing time-to-market requirement . • No method exists fo ensuring valid and consistent constraint formats in SDCs . • difficult verification . • Critical Impact . Automatic Verification of Timing Constraints

16. Description of the problem (cont) • Textual mistakes : Since the SDC files are entered in text format, it is possible for the designer to make mistakes that causes the constraints to be inconsistent with the design. For example, the SDC files might reference a signal name that is not actually in the design. • Missing time-to-market requirement : It is essential to eliminate fundamental problems as early in the design cycle as possible. If a designer applies flawed constraints to a design, he may be unable to mitigate problems that surface later without making sweeping changes to the design, which results in the project’s missing its time-to-market goals. Automatic Verification of Timing Constraints

17. Description of the problem (cont) • No method exists for ensuring valid and consistent constraint formats in SDCs. • difficult verification : The false-path and multi-cycle path exceptions that SDCs specify are difficult to verify in the design context. Automatic Verification of Timing Constraints

18. Description of the problem • Critical Impact : Incorrect timing constraints can leave chips with critical timing bugs that can cause recalls, re-spins, and redesigns, costing hundreds of thousands of dollars. Worse still, delays in getting to market and missed opportunities can be devastating. Automatic Verification of Timing Constraints

19. The automatic tool Solution Automatic Verification of Timing Constraints • Fortunately, Formal Verification can be used to analyze false path and multi-cycle path constraints and verify their correctness. • SolidTC, a timing constraint verifier from Averant, applies formal verification technology to the problem of verifying timing constraints in complex, multi-million gate designs. Automatic Verification of Timing Constraints

20. Flow Through The Tool It’s important to note this is a RTL too. When a path is reported as false, it is false under all delay assignments to gates and wires. Automatic Verification of Timing Constraints

21. Conclusion • Achieving timing closure is a critical factor in producing reliable, bug-free, high-performance designs. The key to this is thorough design verification, being sure not to inadvertently relax the design tests through the application of incorrect timing constraints. Automatic Verification of Timing Constraints

22. THANK YOU Automatic Verification of Timing Constraints