Kazi ECE 6811

ECE 681ECE 681VLSI Design VLSI Design AutomationAutomationKhurram Kazi*Khurram Kazi*

Lecture 10Lecture 10

Thanks to Automation press Thanks to Automation press THE button outcomes the THE button outcomes the Chip !!! Reality or MythChip !!! Reality or Myth

(*Mostly extracted from Synopsys PrimeTime manuals)(*Mostly extracted from Synopsys PrimeTime manuals)

Kazi ECE 681 2

Features of PrimetimeFeatures 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.

Kazi ECE 681 3

Physical Synthesis flow Physical Synthesis flow using PrimeTimeusing 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.

Kazi ECE 681 4

Typical Checks performed Typical Checks performed with PrimeTimewith 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

Kazi ECE 681 5

Some of the analysis featuresSome 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

Kazi ECE 681 6

Command InterfaceCommand Interface• PrimeTime offers two command

environments for timing analysis:

• pt_shell

• Graphical user interface (GUI).

Kazi ECE 681 7

pt_shell windowpt_shell window

Kazi ECE 681 8

pt_shell interfacept_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.

Kazi ECE 681 9

GUI windowGUI window

Kazi ECE 681 10

GUIGUI• 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.

Kazi ECE 681 11

Commonality between Primetime Commonality between Primetime and Design compilerand 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.

Kazi ECE 681 12

Why use PrimeTime on top Why use PrimeTime on top of DCof 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.

Kazi ECE 681 13

Overview of Static Timing Overview of Static Timing Analysis (STA)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.

Kazi ECE 681 14

Timing pathsTiming paths

Kazi ECE 681 15

Combinatorial logic may Combinatorial logic may have multiple pathshave multiple paths

•PrimeTime uses the longest path to calculate a maximum delay or the shortest path to calculate a minimum delay.

Kazi ECE 681 16

Other types of Timing pathsOther 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

Kazi ECE 681 17

Other types of paths Other types of paths depicteddepicted

Kazi ECE 681 18

Delay calculationsDelay 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.

Kazi ECE 681 19

Constraints CheckingConstraints 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.

Kazi ECE 681 20

Input and Output delaysInput and Output delays

Kazi ECE 681 21

Setup and Hold checksSetup and Hold checks

Kazi ECE 681 22

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

Kazi ECE 681 23

Hold time analysis when two Hold time analysis when two different clocksdifferent clocks

Kazi ECE 681 24

Single-Cycle Path analysisSingle-Cycle Path analysis

Kazi ECE 681 25

Setting the output loadSetting the output load

Kazi ECE 681 26

To run PTTo 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

Kazi ECE 681 27

A Sample PT scriptA Sample PT scriptset 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

Kazi ECE 681 28

Mini AssignmentMini 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).