a subdivision of a clock phase.

• Normally all pins of a clock phase are in group 0 and balanced together. If you label a set of pins as group 1, the skew phase splits into two skew groups - one with the user-specified pins and one with the normal pins - so groups can be balanced independently.

Skew anchor (sink point):

• A skew anchor is a clock endpoint pin that controls a downstream clock tree - for example, the clock input of a divide-by-2 generator, because the arrival time there affects all clocks in the generated domain starting at that register's Q pin.

Skew offset:

• A skew offset is a floating-point number describing phase relationships when clocks with different periods or different edges are placed into the same skew phase; it adjusts the arrival time of a specific clock phase so it can be compared to another phase in the same group.

KEY Clock phase, skew phase, skew group, skew anchor and skew offset give fine control over clock-tree balancing.

Skew Groups and Clock Phase

• A skew group is a set of clock pins declared as a group. By default all clock pins are in group 0, so each skew phase contains a single group.
• If the user creates a group of pins labelled, for example, group 1, the skew phase containing those pins splits into two skew groups: (i) the normal clock pins and (ii) the user-specified group.
• This is useful for separating clock pins with special requirements so they are not balanced with the default group.
• Skew optimization runs on a per-skew-group basis, after the basic clock has been inserted.

KEY A skew group subdivides a skew phase; default pins sit in group 0, and user groups are balanced independently.