For IDT's computing clocks, could you explain the VTTPWRGD# pin?

Vttpwrgd# is the system signal that either comes from Intel chipset or power sequence control circuit. The clock device relies on the Vttpwrgd# signal to latch CPU FSC_A signal which is a CPU frequency information signal. Before Vttpwrgd# trigger and FSC:A latch, the clock device will not enable clock outputs.