Setting up auto phase alignment in differential pairs in CAD Cadence Allegro

Sometimes even your favorite programs can be tormenting - especially if you do not know how to properly configure this or that function. The Cadence Allegro PCB editor has such an awesomely useful option, High Speed, that really helps in tracing speed signals, DDR, and differential pairs. However ... not everything there turns out so smoothly and simply, if you do not know some of the nuances.

One of these functions is the ability to automatically align the static and dynamic phases in differential pairs, using the "humps", the so-called bumps. However, this tool will not work well without tuning, and get good results by simply running the tool with the Route - Auto-interactive phase tune command and clicking on a few diffs. couples will not work.

I bring to your attention my experience with this tool:

The first thing to do is set the PinUSE property on the component pins (pins) to IN at the input, OUT at the output. This is a prerequisite, since the phase equalization algorithm takes into account the direction of signal propagation. If both ends of the line can serve as a source and receiver of signals, then you need to independently choose what to consider as the source and what to be the receiver, and set the PinUse properties accordingly. In order to assign a property to a component pin, you must complete all the commands. While in General edit mode, in the Find tab, leave one checkmark opposite the Pins pins, and click LMB (with the left mouse button) on the pin. Then click RMB (right click) and select Property Edit from the drop-down menu. Then find the desired property in the property list: PINUSE. And finally, set its value accordingly to either IN,either in OUT.
How to get to the pin properties:




The sequence of actions for assigning additional properties to component pins.

Then you need to set the clearance for the gap between the differential conductors. pairs in Constraint Manager at least 0.01 mm or more. This eliminates the errors of defining a pair as Uncoupled (non-connected) when laying at 45 degrees and erroneously affixing markers of this Uncoupled state. In addition, the phase on such “Uncoupled” segments is not compensated. Those. a diffpara after a turn at 45 is considered Uncoupled if the gap is outside the tolerance, and naturally, the humps for phase equalization are not automatically set after each turn of the diff.pairs, as we would like.


Required differential tolerance settings in Constraint Manager

The addition of humps occurs on visible sections of the tracks. Those. if the trace is on the off layer, then the addition will not occur. And also the addition stops working when switching from layer to layer. Those. when activating the phase equalization tool, you need to look at the entire circuit from the source to the receiver, and after each vias (VIA), you must additionally click on the conductor located on another layer.

It is better to first deal with static phases, and only then with dynamic ones. Do not try to level everything all at once in 1 pass.

I recommend setting adequate boundaries for the size of the humps. It works well for me with the values ​​shown in the figure below:


Parameters of added compensation sections (humps)

After activating the tool itself, you need to pay attention to the Uncoupled bumps option. Without this option, the user will fail, because everything is added to the differential. a couple of objects will be considered uncoupled.


Do not forget to select this option to get the result.

As for the Pad Entry Shortening and Pad Entry Lengthening options, these are options that allow you to gain length or vice versa to remove excess by drawing additional segments near the contact pads. I do not recommend using them, since the engineer knows better how to approach the contact area, and fixing this place with a computer can be inefficient. It is better to draw the length of the humps on the trace segments, and do not touch the place of approach to the CP.

I hope that these tips will help users get the expected results when using this tool and not spoil their nerves by rummaging through the settings and trying to “get the program to work”.