5.1.2.2 SDM Integer Boundaries and Instable Control Signal ffreq[17:0] Ranges

The range of the sigma delta modulator control word ffreq[17:0] is from 0 to 262143. The range from 16384 to 180224 must be used to allow stable operation of the SDM. Values outside this range are not used.

If certain ranges of ffreq[17:0] are used, the fractional-N PLL works close to frequencies at which integer boundary spurious arises that cannot be dithered by the sigma delta modulator. These ranges must, therefore, be avoided by choosing another XTAL frequency. In the configuration tool, a warning displays if these forbidden ranges are used.