define_clock {Clock} -freq 100 -clockgroup default_clkgroup_0 -rise 0 -fall 5 -route 5
Which would be typical of the input constraint file for Synopsis (Scope source file)
or create_clock -period 10.000000 -waveform {0.000000 5.000000} Clock
which is a typical sdc file format - in both cases the frequency and duty cycle are declared in nS
Period speaks for istelf the two parameters in the waveform switch set the rising and falling edges, thereby setting the duty cycle and <clock> is substituted for your own signal name. SDC does not allow for any other parameters to be entered