DCO mode allows the user to dynamically change an output clock frequency in any existing device or OPN if required. The output frequency can be modifying by changing the N divider value which is assigned to the particular clock output. However manually writing to the N dividers is not a recommended way to use the DCO mode. There are certain factors which need to be considered.
The N divider which is associated with the DCO enabled output should be in fractional mode. If the N divider is integer in the original project file, then it needs to be changed to operate in the fractional mode. The register PIBYP[4:0] enables to change the N divider mode. Each bit in this register is assigned to one N divider. If N0 needs to be in fractional mode, write 0 to PIBYP. Note that a soft reset should be performed after changing the above mentioned register.
The next step involves calculating the frequency step word and then also the corresponding values for either the N divider numerator or the N divider denominator. All these calculations are explained in detail in the following application note:
In order to avoid all these complications, the best and easiest way to implement DCO mode is using Clock Builder Pro DCO mode tool. It will perform all the calculations and determine the required register values.