Can I tie one of the phase-shifted clocks (CLK90, CLK180, or CLK270) to the feedback pin? What is the effect of doing this?
This is a legal connection (although it has not been tested); however, it is useless in practice.
If the DLL locks with one of the phase-shifted clocks that is driving the CLKFB pin, then that signal is no longer phase-shifted in relation to the CLKIN pin. Therefore, if you connect the CLK90 pin to CLKFB, the CLK90 pin will be exactly in phase with CLKIN; CLK180 will be 90 degrees out of phase, CLK270 will be 180 degrees out of phase, and CLK0 will be 270 degrees out of phase. The only real effect this action has is to invalidate the names of the CLK output signals.