If you mean by the reference is the edge for which to measure the setup and hold time, then for negative edge triggered FFs, you should use the falling edge .
The moment we say it as a negative edge triggered FF, Negative or falling edge of clock become the reference point, we have to measure both setup and hold time only with reference to this falling edge of clock
Normally during synthesis, the tool will invert the clock and still use a posedge flop.
Not sure whether all tools will do this every time. But seen this with synopsys power compiler.
Hi Buddy,
Its simple..
For -ve edge triggered flipflops, timing parameters setup and hold time are calculated w.r.t -ve edge or the falling edge of the clock