Get email delivery of the Cadence blog featured here
There are a couple new Differential Pair (Diff Pair) capabilities available with the SPB16.3 Allegro PCB Editor Constraint Manager - Differential Pair Renaming and Dynamic Phase Control for Differential Pairs.
Differential Pair Renaming
Prior to the SPB16.3 release, library and model-defined differential pairs are automatically named based upon the member nets of the differential pair. However, you might want to rename differential pairs based on specific naming conventions. Until now, you could rename only user-defined differential pairs in Constraint Manager. In the SPB16.3 release, Constraint Manager extends the support for renaming all types of differential pairs.Constraint Manager (CM) now supports renaming of all DiffPair types (user, library, and model) using commands. When renaming a library or model defined DiffPair, the rename dialog contains a new Use default button which allows you to revert to the auto-generated name. This button will not be visible for user-defined DiffPairs. In CM, model defined differential pairs are denoted as Type Dpr(M):
These differential pairs can now be renamed using Objects > Rename or RMB > Rename.
A dialog box is presented, allowing you to rename the differential pair.
The Use Default button resets the differential pair name back to its default value. Dynamic Phase Control for Differential Pairs
Differential Pair (Diff Pair) technology has evolved where more stringent checking is required in the area of phase control. This is evident on higher data rates associated with parallel buses such as QPI, SMI, PCI Gen 2, DDR, QDR and Infiniband. In the simplest of terms, Diff Pair technology is sending opposite and equal signals down a pair of traces. Keeping these opposite signals in phase is essential in assuring that they function as intended. As the current “Static Phase” is limited to a one time check across the entire Driver-Receiver path, a new “Dynamic Phase” check is introduced that performs phase checks at bend point intervals across the Diff Pair. The Dynamic Phase check is designed to meet the guidelines that suggest that the path lengths of the true and complement signals within the differential pair must differ by no more than “x mils” along the entire path of the net. If at any point on the net, the skew between true and complement exceeds “x mils”, this mismatch needs to be compensated within “y mils”. Representative values for x and y might be x = 20 and y = 600.
The constraints associated with Differential Pairs now support Static and Dynamic Phase. The margins of each constraint can be set independently using length or time. The Max Length (running skew) constraint for Dynamic Phase is limited to length only.
As an example, suppose your Dynamic Phase constraints are set as follows:
When the DRC is updated, it shows the following:
The beginning of the yellow pseudo line (closest to driver) is where the Diff Pair initially goes out of Phase (beyond the 20 mil tolerance). The DRC marker D-Y is placed at the initial ‘out of phase spec' location as measured from the Driver Pins. Notes:
Please share your experiences with this new SPB16.3 capability!
Jerry "GenPart" Grzenia