4.2.5. Clock Multiplication and Division
Each Intel® Cyclone® 10 LP PLL provides clock synthesis for PLL output ports using M/(N × post-scale counter) scaling factors. 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). Each output port has a unique post-scale counter that divides down the high-frequency VCO. For multiple PLL outputs with different frequencies, the VCO value is the least common multiple of the output frequencies that meets its frequency specifications. For example, if output frequencies required from one PLL are 33 and 66 MHz, the Intel® Quartus® Prime software sets the VCO to 660 MHz (the least common multiple of 33 and 66 MHz in the VCO range). Then, the post-scale counters scale down the VCO frequency for each output port.
There is one pre-scale counter, N, and one multiply counter, M, per PLL, with a range of 1 to 512 for both M and N. The N counter does not use duty cycle control because the purpose of this counter is only to calculate frequency division. There are five generic post-scale counters per PLL that can feed GCLKs or external clock outputs. These post-scale counters range from 1 to 512 with a 50% duty cycle setting. The post-scale counters range from 1 to 256 with any non-50% duty cycle setting. The sum of the high/low count values chosen for a design selects the divide value for a given counter.
The Intel® Quartus® Prime software automatically chooses the appropriate scaling factors according to the input frequency, multiplication, and division values entered into the ALTPLL IP core.
Phase alignment between output counters is determined using the tPLL_PSERR specification.
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