During synthesis with Design Compiler, set_drive and set_driving_cell model input port drive strength by computing transition time and additional port delay. The set_input_transition command lets you override these computed values with an explicit transition time, directly setting the port’s min_transition_fall, min_transition_rise, max_transition_fall, and max_transition_rise attributes. To clear a forced trnasition, use remove_attribute. This command is available across PrimeTime and IC Compiler with minor variations. Its syntax is:

set_input_transition transition port_list
    [-rise] [-fall] [-min] [-max]

Note that the port_list must immediately follow the transition value.

Assigning the Transition Value

Supply a non-negaitve transition whose unit matches the technology library’s time unit. Larger values indicate a weaker drive, while smaller values represent stronger drive.

Targeting Ports

Provide a list or collection of input or bidirectional ports. Wrap multiple objects in quotes or braces when using a list; collections do not require this.

Basic Usage

Consider a circuit where a clock is defined on port clk:

create_clock -period 10 [get_ports clk]

Input delays referenced to that clock are applied to ports d and c:

set_input_delay 0.5 -clock clk [get_ports d]
set_input_delay 0.5 -clock clk [get_ports c]

Assigning a transition to port d:

set_input_transition 0.5 [get_ports d]

Running report_timing with -transition_time on paths from d and c to the flip-flop reveals that d shows a transition of 0.5 (instead of 0), while neither port gains an extra input delay from the drive modeling.

Controlling Edge Sensitivity

Use -rise or -fall to apply the transition only to the corresponding edge. If both are omitted, the value applies to both edges.

set_input_transition -rise 0.6 [get_ports d]

This sets a 0.6 transition for rising edges on port d while the falling edge stays at 0.5. report_port -verbose displays all applied transition values.

Distinguishing Min and Max Analysis

-max associates the transition with maximum delay analysis (typically setup), and -min with minimum delay analysis (typically hold). Without either, the value applies to both.

set_input_transition -max 0.8 [get_ports c]
set_input_transition -min 0.6 [get_ports c]

Now report_timing for setup uses 0.8, while hold uses 0.6 on port c.

Differences from set_clock_transition

Before clock tree synthesis, set_clock_transition provides an estimate applied directly at flip-flop clock pins, ignoring clock-path delay. After clock tree synthesis and set_propagated_clock are in effect, apply set_input_transition on the clock input port so the tool propagates the transition through the real clock network.

Multicorner-Multimode Behavior

The command takes effect only within the active scenario, enabling per-scenario transition specifications.

## Deprecated Options

The `-clock` and `-clock_fall` options are obsolete and should be avoided in new designs.