A driving-cell bound names a library cell that the tool treats as driving an input port. It bounds the transition the input can present.
The cell name refers to the teaching or project library in use. Stronger drive shortens the transition the tool assumes; weaker drive lengthens it.
A load bound states the capacitance the tool treats as hanging on an output port. It is an EXAMPLE parameter in femtofarads.
Larger loads slow the output transition the tool computes. Each output needs its own bound when the loads differ.
The listing below is illustrative tool-neutral SDC and is not run by any tool.
illustrative SDC sketch (not run by any tool):
set_driving_cell -lib_cell BUF [get_ports req]
set_load 50.0 [get_ports ack]Time is stated in nanoseconds and capacitance in femtofarads in this topic. Both are EXAMPLE units for hand arithmetic, not a claim about any tool or library.
The unit convention in force varies with the library and tool setup. State the units alongside every number so the constraint is read as intended.
Bound | What it states | EXAMPLE unit |
|---|---|---|
Driving cell | Which cell model drives the input | Cell name, such as BUF |
Load | Capacitance on the output | 50.0 fF |
Delays and periods | Time budgets from earlier lessons | ns |
It means 50.0 fF of capacitance on that output in this topic's EXAMPLE units. Larger values slow the computed transition.
The tool works without an edge bound, and its assumption varies. State the drive explicitly instead.