STEP 1: Timing Path Fundamentals
This step explains the basic structure that Static Timing Analysis evaluates. Every timing check in STA is performed on a timing path. Understanding what a timing path is and how it is classified is essential before learning timing equations or tool usage.
What is a Timing Path
A timing path is a logical and electrical path through which data travels in a digital circuit under the control of a clock. In synchronous designs, data is launched by a source element, propagates through combinational logic and interconnect, and is finally captured by a destination element. Static Timing Analysis evaluates whether this data movement satisfies all timing requirements without simulating signal transitions.
A complete timing path consists of three main parts: a start point, a data propagation region, and an end point. The start point is where data is launched, the propagation region is where the data experiences delay, and the end point is where data is captured or observed.
Launch Flop to Capture Flop Structure
In a typical synchronous timing path, the start point is a launching flip-flop. On an active clock edge, the launching flip-flop releases data at its output after the clock-to-Q delay. This data then travels through a network of combinational logic gates and interconnects. Finally, the data reaches the input of a capturing flip-flop, which samples the data on its active clock edge.
This launch and capture mechanism forms the foundation of setup and hold timing checks. Static Timing Analysis ensures that the data launched by the source flip-flop arrives at the destination flip-flop within the allowed timing window defined by the clock and flip-flop requirements.
Register to Register Path
A register to register path is a timing path where data is launched from one flip-flop and captured by another flip-flop. This is the most common and most important timing path type in synchronous designs. Both the start point and the end point are clocked elements.
In this path, STA checks both setup and hold timing. The setup check ensures that the data arrives before the capture clock edge minus the setup time, while the hold check ensures that the data does not arrive too early after the launch clock edge.
Input to Register Path
An input to register path starts at a primary input of the design and ends at a flip-flop inside the design. The data is not launched by an internal flip-flop but by an external source, such as another chip or system.
In Static Timing Analysis, the launch time of the data is defined using input delay constraints relative to an external clock. The capturing element is an internal flip-flop. STA verifies that the incoming data meets the setup and hold requirements of the capturing flip-flop.
Register to Output Path
A register to output path starts at a flip-flop inside the design and ends at a primary output. In this case, the data is launched by an internal flip-flop and then propagates through combinational logic to the output port.
The capture of this data happens outside the design, so STA uses output delay constraints to represent the timing requirements of the external receiving device. The analysis ensures that data reaches the output within the required time window.
Input to Output Path
An input to output path starts at a primary input and ends at a primary output, passing only through combinational logic. There are no internal flip-flops in this path.
Such paths are typically analyzed for maximum and minimum delay constraints, or they may be marked as false paths if they are not functionally relevant. Static Timing Analysis evaluates these paths based on input and output delay constraints.
Data Path
The data path is the portion of the timing path through which data physically propagates. It includes the clock-to-Q delay of the launching flip-flop, all combinational logic delays, and all interconnect delays up to the data input of the capturing element.
The data path determines how long it takes for data to travel from the source to the destination. In STA, the data path delay is compared against timing requirements derived from the clock and flip-flop constraints.
Clock Path
The clock path is the portion of the circuit through which the clock signal travels to reach the launching and capturing elements. It includes the clock source, clock buffers, clock gating cells, and interconnect.
The clock path determines when data is launched and when it is captured. Differences in clock path delays between launch and capture elements result in clock skew, which directly affects setup and hold timing.
Maximum Path and Setup Timing
A maximum path is a timing path that is analyzed for setup timing. In this analysis, STA considers the worst-case, or maximum, delays of the data path. The goal is to ensure that even in the slowest conditions, data arrives early enough to meet the setup time requirement.
Setup timing analysis focuses on long paths with large combinational delay and is primarily concerned with meeting the clock period constraint.
Minimum Path and Hold Timing
A minimum path is a timing path that is analyzed for hold timing. In this analysis, STA considers the best-case, or minimum, delays of the data path. The goal is to ensure that data does not arrive too early at the capturing element and violate the hold time requirement.
Hold timing analysis focuses on short paths with very little combinational delay and is sensitive to fast operating conditions.
Relationship to Static Timing Analysis
Static Timing Analysis examines all timing paths in the design by classifying them into appropriate path types and analyzing them as maximum or minimum paths. By understanding timing path fundamentals, one can correctly interpret timing reports, identify critical paths, and understand the root cause of timing violations.
The objective of this step is to develop the ability to visualize a timing path from start point to end point and to clearly explain how data and clock signals interact along that path.