Confirmation of stable 0 before the rising edge of CK is required before starting the noise window detection phase. Confirmation of the stable 0 tells that fine tap increment will start from left most edge of the noise.
For confirming the stable 0 few fine taps are incremented and if it still sees stable 0 for all the fine taps then it means it is in the stable 0 region before the noise of 0 to 1 transaction. If it does not find the stable 0 this way then one coarse tap is decremented, and stable 0 is confirmed again. If it does not find stable 0 even after decrementing coarse tap, it indicates error.