Many applications require output frequencies that are not integers and are represented using very specific decimal values. Clock generators and Jitter attenuators (Si534x/9x) devices are capable of generating all types of frequency outputs. When a required input-output configuration file is generated, Clock Builder Pro calculates the values of the M, N and P dividers according to the input-output frequencies.
The M and N dividers are in the form of a multiplication ratio. Calculating these values is very tricky when the output frequencies are not integers. In order to get the correct multiplication ratio, it is important to express the frequency values in an exact manner. This means that the decimal frequencies should not be rounded. They should be expressed in terms of fractions. Many times, the output frequency is in terms of repeating decimals. If these are rounded, then there is possibility of getting incorrect divider values.
For example, if a 33.3333… MHz output is required, it should be expressed as 100/3 MHz and not rounded to 33.333 MHz. The following table will show the difference in the M and N divider register values between 33.333 MHz and 100/3 MHz output frequency.
The left side shows register values when the output frequency is 33.333 MHz and the right side shows when it’s 100/3 MHz. As seen the M divider and N divider values are different and in order to get a perfect multiplication ratio, decimal frequencies should be expressed as fractions.