Unlike primary clocks, generated clocks must be defined in the transitive fanout
of their master clock, so that the timing engine can accurately compute their insertion
delay. Failure to follow this rule results in improper timing analysis and most likely
in invalid slack computation. For example, in the following figure gen_clk_reg/Q is being used as a clock for the next flop
(q_reg), and it is also in the fanout cone of the
primary clock c1. Hence gen_clk_reg/Q must have a create_generated_clock on it, rather than a create_clock.
Figure 1. Generated Clock in the Fanout Of Master Clock
create_generated_clock -name GC1 -source [get_pins gen_clk_reg/C] -divide_by 2
[get_pins gen_clk_reg/Q]