The Intel® Quartus® Prime software may report lower PLL VCO frequency than the PLL specifications for Arria® V, Cyclone® V, and Stratix® V devices in the following items.
- Altera PLL IP parameter editor > Advanced Parameters tab > PLL Output VCO Frequency
- Compilation Report > Fitter > Resource Section > PLL Usage Summary > PLL VCO Frequency
This is because a PLL in Arria V, Cyclone V, or Stratix V devices have a VCO post divider and the Intel Quartus Prime software reports VCO frequency after the VCO post divider. The raw VCO frequency before the VCO post divider meets the PLL specifications.
You can calculate the raw VCO frequency as follows:
<< Example 1 >>
Device : Cyclone V -C6, -C7, -I7 speed grades
PLL voltage-controlled oscillator (VCO) operating range : Min 600MHz, Max 1600MHz
Altera PLL IP parameters :
Reference Clock Frequency = 10.0 MHz
Output Clocks
outclk0
Desired Frequency = 300.0 MHz
Actual Frequency = 300.0 MHz
M, N, C, VCO post divider counter and PLL output VCO frequency are calculated as follows:
M counter = 30
N counter = 1
C counter = 1
VCO Post Divide Counter = 2
PLL Output VCO Frequency = Reference Clock Frequency * M / N / C = 10MHz * 30 / 1 / 1 = 300MHz
The raw VCO frequency before VCO post divider = PLL Output VCO Frequency * VCO Post Divide Counter = 300MHz * 2 = 600MHz
<< Example 2 >>
Device : Cyclone V -C6, -C7, -I7 speed grades
PLL voltage-controlled oscillator (VCO) operating range : Min 600MHz, Max 1600MHz
Altera PLL IP parameters :
Reference Clock Frequency = 10.0 MHz
Output Clocks
outclk0
Desired Frequency = 600.0 MHz
Actual Frequency = 600.0 MHz
M, N, C, VCO post divider counter and PLL output VCO frequency are calculated as follows:
M counter = 60
N counter = 1
C counter = 1
VCO Post Divide Counter = 1
PLL Output VCO Frequency = Reference Clock Frequency * M / N / C = 10MHz * 60 / 1 / 1 = 600MHz
The raw VCO frequency before VCO post divider = PLL Output VCO Frequency * VCO Post Divide Counter = 600MHz * 1 = 600MHz