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©2018 Integrated Device Technology, Inc.
August 30, 2018
VersaClock
®
6E Family Register Descriptions and Programming Guide
Figure 5. Spread Step and Period
As normally defined, ODx_period (dec) would be 1/ F SS, but the modulation period is defined instead as ½*1/ FSS for the most direct
calculation of ODx_step as will be seen below in the following Equations. An added benefit is that the up ramp and the down ramp are
guaranteed to be symmetric. Note that ODx_period does not have units of time; it is the dimensionless number of FOUT periods that fit in
a half period of FSS.
Calculate the step size.
Since the spread spectrum ramp as implemented only decreases the frequency of FOUT, then the actual offset for down spread is zero.
But if the spread is to be centered, an offset equal to half the peak modulation, SS%AMT *N, is to be subtracted from the value of
FRAC(N).
If FRAC(N) is a small positive value, it is possible that after the center spread offset is subtracted ODx_offset will be negative. In this
case, retain only the lower 30 bits of the 32 bit hex value and assign them to ODx_offfset[29:0].
In this manner it can be seen that ODx_offset is the value of FRAC(N), appropriately adjusted should center spread be enabled.
