net.group

What it is ¶

net.group(group, kind, net) yields one row per member net of each net group the design declares. group is the group’s name as the design tool wrote it. kind is diff_pair for a differential pair and match_group for a match group. net is the member net. A design that declares no groups yields no rows, and today only an Allegro board declares any.

For hardware engineers ¶

Say a layout engineer pairs USB1_DP with USB1_DM in the constraint manager, and puts a DDR byte lane’s eleven nets in one match group. The board records both groups. The router then couples the pair and tunes the lane to one length. This relation reads those groups back, so a review asks about the pairs the designer declared rather than the pairs a name happens to suggest.

A pair whose names follow no suffix convention still shows up here. The BeagleBone AI pairs DDR1_CLK0 with DDR1_CLK0N in a pair named DIFFPAIR2, and SSTX1+ with SSTX1-. A net can sit in a pair and a match group at once, as each DDR strobe pair sits in its byte lane.

For software engineers ¶

A projection of ir.Design.net_groups, one row per (group, net). It joins to everything keyed by net name. ?net is not unique, since a net in a pair and in a match group gives two rows. ?group is not unique either, since a group gives one row per member. A pair always has exactly two rows. The reader leaves out a pair table that reaches some other number of nets.

The relation reports what the design states, never a guess. diff-pair-naming reads the same declaration through Model.DeclaredPair before it falls back to the name vocabulary.

Go projector ¶

netGroupFacts in stdlib/relations/facts.go, over Model.NetGroups. Rows come in the reader’s order, which is sorted by group name and then by net. An empty result means the design declares no groups. It does not mean the design has no differential signals.

Datalog ¶

The declared pairs, one row per pair with both members:

net.group(?pair, "diff_pair", ?a), net.group(?pair, "diff_pair", ?b), ?a < ?b => ?pair, ?a, ?b

The match groups and how many nets each holds:

net.group(?g, "match_group", ?n) => ?g, count(?n)