Time Unit (tU) is adjustment phase granularity. In RoboClock family, the CY7B993V and CY7B994V have it ranging from 0.625ns to 1.25ns. The CY7B991 and CY7B992 have it ranging from 0.7ns to 1.5ns. The selectable skew is in discrete increments of time unit (tU) where more options are available in CY7B993V and CY7B994V devices. As mentioned in the datasheets, the value of a tU is determined by the FS setting and nominal output frequency (fNOM) with the equation to be used to determine the tU value given as tU = 1/(fNOM*N). Here, FNOM is the VCO operating range and N is the multiplication factor whose value is determined by the Frequency Select and N value table in the respective datasheets.