Why doesn't the output data toggle when running a gate-level simulation (.sdf)?

In order to view output data that toggles at a high frequency in a third-party simulation tool, enable the transport switches in the simulation tool.

The following switches must be enabled:

+transport_int_delays

+transport_path_delays

+pulse_r/0

+pulse_e/0

For example, invoke VCS with the follow command:

vcs <my_test_bench.v> <my_design_file.v> +transport_int_delays +transport_path_delays +pulse_r/0 +pulse_e/0

Below is a description of each switch:

+transport_int_delays: Allows propagation of all pulses that are narrower than the SDF INTERCONNECT delay.

+transport_path_delays: Allows propagation of all pulses that are narrower that the inter-module path delay.

+pulse_e/<number>: Rejects (flags as error and drives X) any pulse with a width is less than or equal to the specified percentage of the module path delay . The <number> argument is a percentage in the range of 0 to 100.

+pulse_r/<number>: Rejects any pulse with a width less than the specified percentage of the inter-module path delay. The <number> argument is a percentage in the range of 0 to 100.

The above transport switches are not enabled by default. For further information on these switches, refer to your simulation tool's user manual.