The 5T929 will lock to, and track, a valid CLKIN signal; LOCK will be low when this has occurred. If CLKIN fails, the 5T929 PLL will smoothly switch to lock to REFIN without generating any glitches on the output. The fact that the PLL is locked to REFIN rather than CLKIN is indicated by a high state on LOCK. When a valid input is then applied to CLKIN, the 5T929 will smoothly switch back to locking on CLKIN, and LOCK will go low.