Signal Integrity Noise and Timing Budgets

Plan Your Budget

By Timothy Coyle

Noise and Timing Budgets Will Make You Rich

The whole point of running simulations and doing analysis on the resulting data is to make design decisions. However if you don’t know what your design goals are then your simulations will be worthless. You need to have a timing budget and a noise budget so you can make effective data driven decisions.

Noise Budget

The above image shows an example noise budget for an interface. It is important for the final budget to account for both simulated and non-simulated effects. This is just an example noise budget and the thresholds and noise components will depend on the technology being used and the types of simulations performed.

The goal of having a noise budget is when you have your simulated signal at the receiver input you can quantitatively decide if the interface will meet the input receiver electrical requirements.

The image above demonstrates how the noise margin is calculated at the input of a receiver. The signal needs to stay out of the receiver threshold region (between Vih and Vil) for the signal to be valid. To finish our example you need to take your simulated noise margin and look at your overall noise budget to make sure there is still margin for non-simulated effects.

Timing Budget

The other piece of the puzzle when planning your simulation budgets is timing. You need to know if the signal at the receiver input will meet the required setup and hold time.

In order to know if you meet an interface setup and hold time requirement timing equations are used that combine the simulated delay of the interface along with the datasheet delays of the driver and receiver. The results of the equations will show if you have positive margin for your setup and hold time requirements. The timing equations are different depending on the clock scheme (common clock, source synchronous, etc) and how the delay values are defined. Below is an example setup timing equation for a common clock interface.

Having a defined noise and timing budget before you start simulating will make the simulated results much easier to digest and more intuitive to understand when making design decisions.

References and Links

Here’s a link for a great book on defining and using noise and timing budgets for Signal Integrity analysis.

“Timing Analysis and Simulation for Signal Integrity Engineers”, Greg Edlund. ISBN # 978-0132365048.

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