asynchronous input to flip flop clock

Clocks will trickle through logic, but if you have a ripple clock, i.e. the output of a register drives the clock of another register, the clock won't go through that. A create_generated_clock needs to be applied to that register. Note that running Report Unconstrained Paths in the TimeQuest will identify this ripple clock as an unconstrained clock, so you don't have to know them all beforehand(considering it's someone else's design).