Allegro Phy Constraint Set : MAX UNCOUPLED LENGTH does it refer to Lambda/4 ??

Hi grax,

Are you referring to the Electrical Constraint for Diff Pairs? This one is also called DIFFP_UNCOUPLED_LENGTH. If you hover the mouse on top of the column name, it'll tell you the name of the parameter you need to search. 

You can find information about it here: 

Title: Allegro X Platform Properties Reference -- Allegro X Platform Properties - DIFFP_UNCOUPLED_LENGTH
URL: https://support.cadence.com/apex/techpubDocViewerPage?xmlName=propref.xml&title=Allegro%20X%20Platform%20Properties%20Reference%20--%20Allegro%20X%20Platform%20Properties%20-%20DIFFP_UNCOUPLED_LENGTH&hash=pgfId-851311&c_version=23.1&path=propref/propref23.1/properties.html#pgfId-851311

DIFFP_UNCOUPLED_LENGTH

(formerly DIFFP_2ND_LENGTH)

The DIFFP_UNCOUPLED_LENGTH property, attached to a net, replaces the DIFFP_2ND_LENGTH property. Use this property to override the Max uncoupled length electrical constraint for differential pairs. This property sets the cumulative maximum distance that segments of one net in a differential pair can run inside or outside the coupling band. The value of this property is in design units. If you do not specify a value, Allegro X PCB Editor does not perform a coupling DRC check.

Routing a differential pair non-orthogonally causes a slight increment in edge-to-edge spacing and that increases the uncoupled length. You can use DIFFP_COUPLED_PLUS property to exclude such variations.

The pair is coupled when the lines are neither closer nor farther than two bands around the primary gap, defined by the DIFFP_COUPLED_PLUS and DIFFP_COUPLED_MINUS values. See Figure 1-1.

Each time a line becomes uncoupled (an uncoupling event), the uncoupled length is captured by Allegro X PCB Editor in this check. If you set the DIFFP_GATHER_CONTROL property to Include and the lines are uncoupled right from the driver pin, the beginning of each line is the first uncoupling event.

If you set the DIFFP_GATHER_CONTROL property to Ignore, the line segments that diverge as the pair of nets go from driver pins to the first coupling event and diverge again as they move from the last portion of coupling to receiver pins, are not included in the uncoupled length. In Figure 1-2, the dotted lines are not included in the uncoupled length check.

Note:

• If a differential pair has multiple driver and receiver pins (PINUSE is set to OUT [driver], IN [receiver], or BI [bidirectional]), each driver and receiver combination for each line is treated as an independent path for checking uncoupling. Consequently, a check is performed for each portion of etch between a driver and receiver and for the entire line.
• The Allegro X PCB Editor measures only nets and Xnets. It does not perform uncoupling checks on system extended nets (SXnets).

By default, the Max uncoupled length electrical constraint is empty. The constraint is visible and active in Allegro PCB Design XL, Allegro PCB Performance L, APD, and Allegro PCB SI.

Edits to the property at the net level bubble up to the differential pair group and the Xnet level. This property is used by automatic routing and DRC checking.