PCB Design Blogs

Constraints are rules which ensure that the PCB design requirements are met both from a logical and a physical standpoint. Constraints can be defined on different objects – components, pins, or nets. In this post, we’ll focus on constraints defined on an electrical net. These constraints can be electrical, such as the length of a stub and the propagation delays; or, they can be physical, such as the minimum line width, static phase tolerance, and so on. All different constraints captured during the design phase help meet the electrical criteria of the final physical board. Capturing this data can be a daunting task but it doesn't have to be. As schematic designers, we need a quick and fast way to assign the initial constraints in the design before we send this data out to the board. We also need a way to check the validity of these constraints while reviewing the design.

Allegro^® System Capture provides an efficient way of managing these constraints. It provides a constraint editing window that can be docked right next to the canvas where the net connectivity is visible. This docked window showing constraints is commonly called Docked Constraint Manager and is specifically aimed at setting up the most commonly used constraint rules. You can always use the full spreadsheet Allegro^® Constraint Manager application from within Allegro System Capture to apply any type of constraint, just that the Docked Constraint Manager view provides a quick snapshot of the constraints on the selected nets. The following image shows a selected net and the constraints applied to it. As you can see, the in-context constraint view makes editing and reviewing easier. 

The user interface is similar to Allegro Constraint Manager, supports the most common rules, and is even more simplified. You can add constraints on one object at a time or add constraints for a group of objects simultaneously. Docked Constraint Manager also enables you to assign more advanced constraints such as net classes and match groups. As it is docked against the schematic canvas, the context in which constraints are being captured is never lost. It also enables easy access to constraints during design reviews. 

In the Docked Constraint Manager window, constraints are categorized into multiple tabs, which separate out the objects on which constraints are being captured. The main tab is for the net or XNet selected on the canvas. The rest of the tabs show details for the Net Class, Match Group, Differential Pair, and so on. Under each tab, the constraints are organized into different categories. 

You can quickly expand or collapse all these accordions depending on how much information you wish to see. For example, for every net, the docked window allows capturing of wiring constraints, vias, propagation delay, impedance, and relative propagation delay.

You can also launch Signal Explorer from Docked Constraint Manager. With a single click, you can work on the net topology and assign custom constraints and measurements. Any changes done to the constraints in Signal Explorer can then be updated back to the design and seen in docked constraint window.

Here is a video that shows how easy this is.

You can quickly pair pins on the given electrical net and add constraints on those pin pairs. Once these are added, the propagation delay, relative propagation delay, and impedance panels display these pin pairs along with the constraints captured on them. 

So, you see how convenient it is to use Docked Constraint Manager to define, view, and validate constraints at the time of design creation itself. It is a simple to use yet powerful widget available within Allegro System Capture and for everything else, there is the full-blown Allegro Constraint Manager, which is again accessible from Allegro System Capture.

That's all for now!

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