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Table of Contents

Definition

Static timing analysis (STA) is a method of validating the timing performance of a design by checking all possible paths for timing violations. STA breaks a design down into 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.

Another way to perform timing analysis is to use dynamic simulation, which determines the full behavior of the circuit for a given set of input stimulus vectors. Compared to dynamic simulation, static timing analysis is much faster because it is not necessary to simulate the logical operation of the circuit. STA is also more thorough because it checks all timing paths, not just the logical conditions that are sensitized by a set of test vectors. However, STA can only check the timing, not the functionality, of a circuit design.

How does STA work?

When performing timing analysis, STA first breaks down the design into timing paths. Each timing path consists of the following elements:

  • Startpoint. The start of a timing path where data is launched by a clock edge or where the data must be available at a specific time. Every startpoint must be either an input port or a register clock pin.
  • Combinational logic network. Elements that have no memory or internal state. Combinational logic can contain AND, OR, XOR, and inverter elements, but cannot contain flip-flops, latches, registers, or RAM.
  • Endpoint. The end of a timing path where data is captured by a clock edge or where the data must be available at a specific time. Every endpoint must be either a register data input pin or an output port.

The following figure shows the timing paths in a simple design example:

Timing Paths | Synopsys

A combinational logic cloud might contain multiple paths, as shown in the following figure. STA uses the longest path to calculate a maximum delay and the shortest path to calculate a minimum delay.

Multiple paths through combined logic | Synopsys

STA also considers the following types of paths for timing analysis:

  • 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.
Types of paths considered for timing analysis | Synopsys

After breaking down a design into a set of timing paths, an STA tool calculates the delay along each path. The total delay of a path is the sum of all cell and net delays in the path.

Cell delay is the amount of delay from input to output of a logic gate in a path. In the absence of back-annotated delay information from an SDF file, the tool calculates the cell delay from delay tables provided in the logic library for the cell.

Typically, a delay table lists the amount of delay as a function of one or more variables, such as input transition time and output load capacitance. From these table entries, the tool calculates each cell delay.

Net delay is the amount of delay from the output of a cell to the input of the next cell in a timing path. This delay is caused by the parasitic capacitance of the interconnection between the two cells, combined with net resistance and the limited drive strength of the cell driving the net.

STA then checks for violations of timing constraints, such as setup and hold constraints:

  • A setup constraint specifies how much time is necessary for data to be available at the input of a sequential device before the clock edge that captures the data in the device. This constraint enforces a maximum delay on the data path relative to the clock edge.
  • A hold constraint specifies how much time is necessary for data to be stable at the input of a sequential device after the clock edge that captures the data in the device. This constraint enforces a minimum delay on the data path relative to the clock edge.

The following example shows how STA checks setup and hold constraints for a flip-flop:

Setup and hold checks | Synopsys

For this example, assume that the flip-flops are defined in the logic library to have a minimum setup time of 1.0 time units and a minimum hold time of 0.0 time units. The clock period is defined in the tool to be 10 time units. The time unit size, such as ns or ps, is specified in the logic library.

By default, the tool assumes that signals are propagated through each data path in one clock cycle. Therefore, when the tool performs a setup check, it verifies that the data launched from FF1 reaches FF2 within one clock cycle, and arrives at least 1.0 time unit before the data gets captured by the next clock edge at FF2. If the data path delay is too long, it is reported as a timing violation. For this setup check, the tool considers the longest possible delay along the data path and the shortest possible delay along the clock path between FF1 and FF2.

When the tool performs a hold check, it verifies that the data launched from FF1 reaches FF2 no sooner than the capture clock edge for the previous clock cycle. This check ensures that the data already existing at the input of FF2 remains stable long enough after the clock edge that captures data for the previous cycle. For this hold check, the tool considers the shortest possible delay along the data path and the longest possible delay along the clock path between FF1 and FF2. A hold violation can occur if the clock path has a long delay.

If certain paths are not intended to operate according to the default setup and hold behavior assumed by the STA tool, you need to specify those paths as timing exceptions. Otherwise, the tool might incorrectly report those paths as having timing violations.

An STA tool may let you specify the following types of exceptions:

  • False path. A path that is never sensitized due to the logic configuration, expected data sequence, or operating mode.
  • Multicycle path. A path designed to take more than one clock cycle from launch to capture.
  • Minimum or maximum delay path. A path that must meet a delay constraint that you explicitly specify as a time value.

Does Synopsys offer a STA solution?

Synopsys' PrimeTime static timing analysis tool provides a single, golden, trusted signoff solution for timing, signal integrity, power and variation-aware analysis. The Synopsys PrimeTime suite includes: PrimeTime, PrimeTime SI, PrimeTime ADV, and PrimeTime PX. It delivers HSPICE® accurate signoff analysis which helps pinpoint problems prior to tapeout, thereby reducing schedule risk, ensuring design integrity, and lowering the cost of design.

This industry gold-standard solution improves your team’s productivity by delivering fast turnaround on development schedules for large and small designs while ensuring first-pass silicon success through greater predictability and the highest accuracy.

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