sta

Open-Silicon ConfidentialOpen-Silicon Confidential

Static Timing AnalysisJanuary 05

222Open-Silicon ConfidentialOpen-Silicon Confidential

Goals for today

►Introduction to STA and the basic idea behind the process.

►STA inputs and outputs.►Defining and applying constraints.►Analyze the results of STA.►Sample STA run and reports


Synchronous Design

► In Synchronous Design Clock Controls the Dataflow???

► On Every Clock Tick data moves from one State to Another

► Logic Delay between two state has to be Less then clock Period

► Almost all ASICs are Synchronous Designs


Synchronous Design: Example

► Out = 1’b1 if (A+B)>(C+D) else 1’b0

A

B

C

D

Clk

OutAdd

Add

Comp.


Timing Analysis

► Logic delay between any two Flops need to be smaller then Clock Period

► There are Two ways to verify ― Dynamic Timing analysis― Static Timing analysis


Dynamic Timing analysis

► Timing simulation with Series of Functional vectors

► Simulator will take in netlist, Gate delays

► Check if there is any Timing Violations


Dynamic Timing analysis

► Weakness of DTA― It is difficult to generate vectors which

cover all combinations of logic― Requires huge CPU and Memory ― Time consuming


Static Timing Analysis

► Design is broken down in sets of timing paths

► Delay of each path is calculated► Compare path delay to constraints

check if they have been met

► Why is it called STA?


Static timing analysis: Advantages

► Do not require any vectors► Runs very fast► More efficient in CPU and Memory

usage


What STA checks

► If design Can work at given Max Frequency► If Design can work in different operating

(PVT) conditions► If Design Satisfies Electrical constraints

(Trans, cap, fanout etc)► Signal Integrity of routing


Timing Paths

► Design is broken down in sets of signal paths

► Each path has startpoint and Endpoint ► Startpoint is Primary Input or Clock

pins of Register► Endpoint is Primary output or D pins of

registers► Register is a cell with setup/Hold time

requirement e.g Flop, Memory etc.


Timing path Types


Timing path Types cont…

► Primary input to register― Start From Input Pin, Ends at Register― Example In1 to FF1― Requires Setting input delay at input

pin relative to clock



► Register to register― Start from Clock pin of Register― Ends at D pin of Register― Example: Path between FF1 to FF2― Constrained by specification of the

clock



► Register to output― Start from Clock pin of Register― Ends at Output Pin― Example: Path between FF2 to Out1― Constrained by specification Output

delay on output port



► Input to output― Start from Input Pin― Ends at Output Pin― Example: Path between In2 to Out2― Constrained by specification Output

delay on output port and input delay on input port


Path Groups

► Paths are grouped according to clocks controlling their endpoints

► Set of paths associated with a clock is called as path group

► All paths with no particular clock goes to default path group


Timing Checks

► Typical Timing checks done are― Setup and Hold― Recovery and Removal― Minimum pulse width― Glitch detection (clock gating)


Setup and Hold

► Minimum time that a data input pin of a Sequential cell must be stable Before Clock Transition.

► Minimum time that a data input pin of a Sequential cell must be stable After Clock Transition.


Recovery and Removal

► Recovery: Minimum time that an asynchronous control input pin must be stable after deserted and before next clock edge

► Removal: Minimum time that an asynchronous input pin must be stable before being deserted and after the previous clock edge

Reset

Reset

Clock

Clock

Recovery

Removal


Minimum Clock Pulse Width

► Minimum Amount of time between two consecutive Rise and fall edge of the clock

Minimum High Pulse width

Minimum Low Pulse width


Glitch Detection (Clock getting check)

1 Flip Flop

CLK

En

Clk

Clk

En

FF.Clk


Constraints

► Constraints are Designer’s goals► Restriction put by technology library

also qualify as constraints► STA measures if these goals are met.► Typical constraints are clock, input/out

put delays, design specific timing requirements


Types Of Constraints


Defining clock

► Defines which are the clock ports and the clock attributes

► create_clock –period 10n –waveform {0 5} –name clk

Waveform starts at 0 with 50% duty cycle


Generated/Derived clock

► Internal clock is divide by 4

► create_generated_clock


Virtual clock

► This path is constrained by a clock (clk) outside of chip


Clock Distribution


Clock Parameters

► Clock skew► Clock uncertainty► Clock insertion delay► Clock jitter► Ideal and propagated clock

► How this parameters affects


Clock skew


Clock jitter


Clock latency/delay


Ideal and computed Clock

► Off chip clock effects are fixed throughouta. Source latencyb. Jitter

► On chip routing is estimated for ideal clocka. Budgeted network latencyb. Budgeted skew

► On chip routing is computed after CTSa. Actual insertion delayb. Actual skew


Setting Input Delay

► Input signal arrives at 2.5ns after rising edge of CK

► Set_input_delay


Setting Output Delay

► Output signal needs to arrive time at O after xy ns

► Set_output_delay


Input Drive strength

► Along with timing constraints one needs to specify design environment

► Set_driving_cell


Output load

► Setting output load

► Set_driving_cell


Check – Check- Check

► Check_timing will check if every thing is constrained


Timing Constraints Tutorial

► Derive top Level SDC from Given Data


False Path

A false path is a timing path that cannot propagate a signal.

► A path may exist in the circuit but never be used

► A functional path may exist but the timing is very slow or irrelevant

► A path may exist in the circuit but no combination of input vectors may ever exercise it


False path Example


False path Example:Paths Between Two clocks


MultiCycle Path

► A multicycle path is a timing path that is not expected to propagate a signal in one cycle.

► Normally, all paths are constrained for single-cycle timing.

► Multicycle paths are exceptions to the default single-cycle timing..

► Multicycle paths are defined by command set_multicycle_path.


MultiCycle Paths


Functional Modes

► Complex Designs can have multiple Time paths that are dependent on the enabled functional modes

► E.g Normal Functional mode and Debug Mode

► Case analysis provides a means of setting constant paths that you want the tool to ignore during timing analysis


Operating Conditions

► The operating conditions of a design usually include the process, voltage, and temperature ranges a design encounters.

► Operating conditions are defined in a technology library in an operating_conditions group.

► You can choose operation condition by set_operating_condition


Specify Design data and libraries

Specify Interconnect

Specify Clocks

Specify Input/Output

Delays

Specify Timing

Exceptions

ASIC Netlist

ASIC Libarary

SPEF Wire Load Models

Back-annotated

STA Methodology


STA Methodology

Check Timing

Update Timing

Generate Reports

Constraints Violators

Path Timing

Bottleneck

Reports


Delays

► Total Delay = Cell Delay + Interconnect Delay

► Cell Delay depend upon Input transition and output cap.

► Interconnect Delay depends upon capacitance and resistance

► User need to provide Interconnect R, C


Specify Interconnect

► Standard parasitic Exchange format (SPEF) file― Extracted R and C

► Standard Delay Format (SDF)― Accurate timing information from Delay

calculation tool► Wireload Models

― Pre-layout net delay estimates


Path Delay

► Actual path delay is sum of net and cell delays along timing the path


Net Delays

► Is the total time need to charge or discharge all the parasites of a Net

► Net parasites depends up on― Net Length― Net Routing― Net Fanout


On-Chip variation

► Intra die variation and intra wafer process variations.

► Temperature and voltage variation on the die.

► Deration of the appropriate path for the analysis so that additional margin is obtained.


SI effects

► Effect of signal propagation on one net, affecting another adjacent net.

► Causes delay push out and glitch in the victim net.

► PT-SI analysis


STA walk through


Goals for today

►Introduction to STA and the basic idea behind the process.

►STA inputs and outputs.►Defining and applying constraints.►Analyze the results of STA.►Sample STA run and reports

sta

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