4.2.5. Clock Multiplication and Division
An Intel® Arria® 10 PLL output frequency is related to its input reference clock source by a scale factor of M/(N × C) in integer mode. The input clock is divided by a pre-scale factor, N, and is then multiplied by the M feedback factor. The control loop drives the VCO to match fin × (M/N).
The Intel® Quartus® Prime software automatically chooses the appropriate scale factors according to the input frequency, multiplication, and division values entered into the Altera IOPLL IP core for I/O PLL and Arria 10 FPLL IP core for fPLL.
Pre-Scale Counter, N and Multiply Counter, M
Each PLL has one pre-scale counter, N, and one multiply counter, M. The M and N counters do not use duty-cycle control because the only purpose of these counters is to calculate frequency division.
Post-Scale Counter, C
Each output port has a unique post-scale counter, C. For multiple C counter outputs with different frequencies, the VCO is set to the least common multiple of the output frequencies that meets its frequency specifications. For example, if the output frequencies required from one I/O PLL are 55 MHz and 100 MHz, the Intel® Quartus® Prime software sets the VCO frequency to 1.1 GHz (the least common multiple of 55 MHz and 100 MHz within the VCO operating frequency range). Then the post-scale counters, C, scale down the VCO frequency for each output port.
Post-Scale Counter, L
The fPLL has an additional post-scale counter, L. The L counter synthesizes the frequency from its clock source using the M/(N × L) scale factor. The L counter generates a differential clock pair (0 degree and 180 degree) and drives the HSSI clock network.
The delta-sigma modulator (DSM) is used together with the M multiply counter to enable the fPLL to operate in fractional mode. The DSM dynamically changes the M counter factor on a cycle-to-cycle basis. The different M counter factors allow the "average" M counter factor to be a non-integer.
In fractional mode, the M counter value equals to the sum of the M feedback factor and the fractional value. The fractional value is equal to K/232 , where K is an integer between 0 and (232 – 1).
For a fPLL operating in integer mode, M is an integer value and DSM is disabled.
The I/O PLL can only operate in integer mode.