ECE 681 VLSI Design Automation

1. ECE 681VLSI Design Automation Khurram Kazi* Lecture 10 Thanks to Automation press THE button outcomes the Chip !!! Reality or Myth (*Mostly extracted from Synopsys PrimeTime manuals)

2. Features of Primetime • PrimeTime is a full-chip, gate-level static timing analysis tool targeted for complex, multimillion-gate designs. • It is used for Static timing analysis for the pre and post layout gate level netlist. • During the pre-layout, it uses the wire load models specified in the library to estimate the net delays. • Any violations found during the pre-layout static timing analysis should be fixed (either resynthesizing or RTL recoding) prior to moving ahead for placement and routing. • Place and route produces a chip layout database from which accurate delay information or detailed parasitic information can be extracted. • This data, when back-annotated on the design in PrimeTime, results in a layout-accurate timing analysis. A successful validation of the circuit timing at this point leads to signoff of the completed design.

3. Physical Synthesis flow using PrimeTime The diagram only shows the steps related to timing analysis in the physical synthesis flow. It does not include unrelated steps such as formal verification, scan synthesis, and logic simulation.

4. Typical Checks performed with PrimeTime • Setup and hold • User-specified data-to-data timing constraints • Clock-gating setup and hold constraints • Minimum period and minimum pulse width for clocks • Design rules (minimum/maximum transition time, capacitance, and fanout) • Bus contention and floating net conditions

5. Some of the analysis features • Multiple clocks and clock frequencies • Multicycle path timing exceptions • False path timing exceptions and automatic false path detection • Simultaneous minimum/maximum delay analysis for setup and hold constraints • Analysis with on-chip variation of process, voltage, and temperature (PVT) conditions • Mode analysis (analysis with module-specific operating modes, • such as read mode or write mode for a RAM module) • Bottleneck analysis (reporting of cells that cause the most timing violations) • “What-if” analysis without modifying the original netlist, using inserted buffers, resized cells, and modified nets • Analysis of crosstalk effects between physically adjacent nets using the PrimeTime SI (signal integrity) option

6. Command Interface • PrimeTime offers two command environments for timing analysis: • pt_shell • Graphical user interface (GUI).

7. pt_shell window

8. pt_shell interface • The pt_shell interface is the command environment used most often by experienced PrimeTime users. • It is a text-only environment in which you enter commands at a prompt (pt_shell> ) and view responses and reports in text form. • The pt_shell interface is based on the Tcl scripting language, which means that you can use Tcl features such as procedures, lists, and array processing functions. • Routine tasks are typically done by executing scripts or procedures prepared beforehand.

9. GUI window

10. GUI • The GUI offers some visual analysis capabilities that are not available in pt_shell. • For example, you can view schematics of the design, display clock waveforms, and generate histograms of analysis results such as path slack, net capacitance, and bottleneck cost. • The console window within the top-level window lets you enter commands and view the text response, just like pt_shell.

11. Commonality between Primetime and Design compiler • They use the same technology libraries and read the same design data files in .db format. • They share many of the same commands, such as create_clock, set_input_delay, and report_timing. Shared commands are identical or very similar in operation. • They share the same delay calculation algorithms and generally produce identical delay results. • Timing reports generated by the two tools are very similar.

12. Why use PrimeTime on top of DC • Although Design Compiler has its own built-in static timing analysis capability, PrimeTime has better speed, capacity, and flexibility for static timing analysis, and offers many features not supported by Design Compiler such as timing models, mode analysis, and internal clocks.

13. Overview of Static Timing Analysis (STA) • PrimeTime breaks the design down into a set of timing paths, calculates the signal propagation delay along each path, and checks for violations of timing constraints inside the design and at the input/output interface. • Each path has a startpoint and an endpoint. • The startpoint is a place in the design where data is launched by a clock edge. • The data is propagated through combinational logic in the path and then captured at the endpoint by another clock edge.

14. Timing paths

15. Combinatorial logic may have multiple paths PrimeTime uses the longest path to calculate a maximum delay or the shortest path to calculate a minimum delay.

16. Other types of Timing paths • Clock path (a path from a clock input port or cell pin, through one or more buffers or inverters, to the clock pin of a sequential element) for data setup and hold checks • Clock-gating path (a path from an input port to a clock-gating element) for clock-gating setup and hold checks • Asynchronous path (a path from an input port to an asynchronous set or clear pin of a sequential element) for recovery and removal checks

17. Other types of paths depicted

18. Delay calculations • Once the paths are identifies, PT, calculates the delays. The total delay of a path is the sum of all cell and net delays in the path. • It estimates delays if the analysis is being performed prior to layout using wire load models • After layout, an external tool can accurately determine the delays and write them to a Standard Delay Format (SDF) file. PrimeTime can read the SDF file and back-annotate the design with the delay information for layout-accurate timing analysis. • PrimeTime can also accept a detailed description of parasitic capacitors and resistors in the interconnection network, and then accurately calculate net delays based on that information.

19. Constraints Checking • Once the path delays are calculated, PT, checks the delays against the constraints provided to it. For example it check the setup or hold timing violations. • The amount of time by which a violation is avoided is called the slack. • For example, for a setup constraint, if a signal must reach a cell input at no later than 8 ns and is determined to arrive at 5 ns, the slack is 3 ns. • A slack of 0 means that the constraint is just barely satisfied. • A negative slack indicates a timing violation.

20. Input and Output delays

21. Setup and Hold checks

22. Path Delay for Flip-Flops Using Different Clocks (setup analysis)

23. Hold time analysis when two different clocks

24. Single-Cycle Path analysis

25. Setting the output load

26. To run PT To run in GUI mode type: (you may want to set an alias for this locally) /opt/synopsys/pt_vT-2002.09/sparc64/syn/bin/primetime To run in shell mode type: /opt/synopsys/pt_vT-2002.09/sparc64/syn/bin/pt_shell

27. A Sample PT script set search_path {~kkazi/synopsys_examples/db} set link_path {*/opt/synopsys/syn/libraries/syn/lsi_10k.db} read_db ~kkazi/synopsys_examples/db/REG_FILE.db #link design check_timing set_operating_conditions WCCOM create_clock -name "Clk" -period 10 -waveform { "0" "5" }\ { "Clk" } # The clock latency value of 2.5 ns specifies that the clock delay # from the input port Clk to all the endpoints is fixed at 2.5 ns set_clock_latency 2.5 [get_clocks Clk] set_clock_transition 0.2 [get_clocks Clk] # Clock skew is approximated with 1.2 ns specified for set-up time set_clock_uncertainty 1.2 -setup [get_clocks Clk] # Clock skew is approximated with 0.5 ns specified for hold time set_clock_uncertainty 0.5 -hold [get_clocks Clk] #set_input_delay 2.0 -clock CLK [all_inputs] #set_output_delay 1.0 -clock CLK [all_outputs] set_load 50.0 [all_outputs] report_clock –skew report_port -input_delay report_port -output_delay check_timing set_wire_load_model -name 20x20 # Writing timing reports with different options to #different files report_timing -to [all_registers -data_pins] > pt1.rpt report_timing -to [all_outputs] > pt2.rpt #Writing the SDF file write_sdf REG_FILE.sdf

28. Mini Assignment • Choose the final synthesis results of the FIFO and perform PrimeTime analysis. Write a small paragraph about the similarities or differences in the two results (i.e. DC analysis and PrimeTime analysis).