Introduction to Static Timing Analysis:

What is timing analysis? It checks the design for proper timing not correct functionality. It is a simulation method of computing expected timing of a digital circuit, without requiring the complete simulation of the circuit. Method of validating timing performance of a design by checking all possible paths for timing violation, under worst case conditions.

How STA is performed on the circuit? Design is broken down into sets of timing paths Calculate signal propogation delay along each path Checks for violations

Timing Path (Data Path)

Data Path (Timing)

Clock Path (Timing)

Asynchronous Path Functionality of the set/reset pin is independent of the clock edge. It’s level triggered and can start functioning at any point of time.

Basic Path Types

Setup and Hold Time Setup Time is the minimum time before the clock edge, when the input data should be stable. Hold Time is the minimum time after the clock edge, when the data should be held stable.

When will this circuit run from a timing perspective ?

Flop analysis with respect to Setup and Hold

What is Jitter? Short term variation of a signal w.r.t it’s ideal position in time. Jitter can lead to violations in timing margins, leads to compromise in reliability of digital systems. Digital Designer has to take jitter into account during STA.

Jitter uncertainty constraints:

Modified Equation after Jitter modeling:

Uncertainty adds on the timing constraints:

Delay of the combinational logic I:

Delay of the combinational logic II:

Logic I & II delay should be less than Setup timing Constraints

Setup Analysis with Multiple Clocks There will be two different clock sources for the two flops. The two clocks will be related to one other : a valid timing path between the two clocks. The two flops will be synchronous in nature but asynchronous to one another. Challenge is to design with uncommon rising edges.

Hold Time Analysis = Think reverse of Setup Hold Time Check is done on the same clock edge unlike Setup Time Delay of the Combinational logic should be greater than the Hold Time as the data needs to be held stable for a finite amount of time after clock edge, before new data replaces it. Hold Time analysis helps us retain functional credibility of the design and does not determine the frequency of the design.

KEY : Difference in Double launch edge

Launch and Capture Clock Edges