Formal Verification of a Public Domain DDR2 Controller Design

1. “Formal verification of a public-domain DDR2 controller design”, VLSI Design 2008. Author:Abhishek Datta, Vigyan Singhal Speaker: Chia-Wei Chang

2. Introduction • Formal verification usage has been increasing in recent times. • However, formal verification can still not lay claim to mainstream acceptance as a necessary part of verification sign-off. • This can be attributed to a dearth of published material on three key aspects of effective formal verification • Structured verification planning. • re-usable verification IP. • Good verification strategy. Chia-Wei Chang

3. Introduction • Using the open-source OpenSPARC T1 DDR2 controller design as a context to demonstrate formal verification planning and implementation • Most memory controllers have common design strategies, even though they have been designed independently • By extension, the techniques used to tackle the complexity are applicable to the whole class of memory controller designs and can be leveraged for other design types Chia-Wei Chang

4. OpenSPARC-T1 • A highly integrated processor that implements the 64-bit SPARC V9 architecture • Multi-threading, multi-CPU • This paper focus at the DDR2 DRAM controllers • 144-bit interface per channel • 25 GB/sec peak total bandwidth • Open source. • Verilog RTL source • Documentation • Simulation-based regression environment Chia-Wei Chang

5. The DDR2 Controller Chia-Wei Chang

6. The DDR2 Controller • Existing high concurrency in the control path due to the multiple independent sources of command words to the DDR2 DIMMs • In order of priority, these sources are: • Periodic Refresh requests. • Pending CAS Requests. • Scrub Row-Address-Strobe (RAS) requests. • Incoming Read RAS requests. • Incoming Write RAS requests. • The data-path contains elements like ECC-based error correction which due to the arithmetic operations involving large data-paths is not ideal for formal analysis. Chia-Wei Chang

7. Formal Verification-planning • Identification • involves determination of design components that are in the formal verification. • Blocks that have concurrency with limited sequential depth are good candidates. • Target is DDR2 control path logic. • Description • involves a high-level functional specification of the target block. • Use JEDEC DDR2 protocol specification. Chia-Wei Chang

8. Formal Verification-planning • Interface description • Refers to the documentation of the inputs and outputs of the target block. • The DRAM section of the OpenSPARC micro-architecture document describes the interface in detail. • Requirements Checklist • refers to the creation of a set of English language properties derived from the high-level design description. • 1. Activate cannot be issued to a non-Idle bank. • 2. Read and write commands can only be issued to active banks. • 3. Check that the minimum Write to Read turnaround time is (CL-1) + (BL/2) + T_WTR … • formal properties • To translate the requirements to a formal description. • using a pre-verified set of DDR2 properties, packaged as verification IP, in this effort. Chia-Wei Chang

9. DDR2 Formal VIP • The re-use of proven, canned sets of properties is an important element of formal verification planning and re-use. • The DDR2 Formal VIP is articulated into module level and per-bank properties with minimum auxiliary code sharing between different properties. • Coding one or more monolithic FSMs and then writing properties that are derived from the states (or state transitions) in the large FSMs. • Minimize the shared logic between the different properties • The former approach is considered better for implementing constraints and the latter for assertions. Chia-Wei Chang

10. CSR Value Selection and Initialization • Initializing CSRs registers. • Module enumeration. Chia-Wei Chang

11. Identifying Formal Sweet-spots Chia-Wei Chang

12. Exploiting Design Symmetry • Design symmetry can be used to reduce analysis complexity. • The DDR2 controller is capable of accessing four DDR2 modules concurrently with a 4-bit chip-select. • The symmetry of the implementation means that if we verify the correctness for a single module while allowing the other modules to be accessed (but not monitored) then we have reasonable confidence in the completeness of the proof. Chia-Wei Chang

13. Abstracting Large Counters • Large counters can add substantial amount of sequential depth (diameter) to the formal analysis. As a result, the analysis might not converge. • Scrub Interval – The controller issues internal scrub commands at periodic intervals. The period is established by a 12-bit counter. • If we place a cut-point on “que_scrb_time” net (treat this net as if it were a primary input) This increases the likelihood of analysis convergence. Chia-Wei Chang

14. Handling Address Comparisons Chia-Wei Chang

15. Handling Address Comparisons Chia-Wei Chang

16. Applying Verification Patterns • Patterns can be defined as generalized solutions in a given engineering domain that find recurring application to the problems of that domain. • Pattern: Floating Pulse • Specify that a single bit value can be asserted for only one cycle in any cycle of an infinite sequence. • DDR2 property – • No more than 4 ACTIVATE commands may be issued to the DDR2 SDRAM within a window of T_FAW clock cycles. Chia-Wei Chang

17. Setup a pulse element. Chia-Wei Chang

18. Binding the pulse to (any and all) occurrence of an ACTIVATE command. • The pulse assertion marks the start of our sampling window. Chia-Wei Chang

19. The following assertion ties everything together. Chia-Wei Chang

20. Result Chia-Wei Chang

21. Conclusion • Presenting a way to apply the verification planning process and a set of abstraction techniques on a non-trivial open-source example • Showed how simulation-assisted initialization, exploiting design symmetry. • Using cut-points, design abstractions and verification patterns can be used to reduce complexity. • The techniques and abstractions presented have been found to be equally effective in the verification of other classes of designs. Chia-Wei Chang