How does a PLL behave when switching off the reference clock?

[LFrin]

I have observed a strange behavior of the FPGA PLL (Cyclone V GT), if you take away the reference clock, it still has an low output frequency.

[IDeyn · Accepted solution]

Hi LFrin!

In case of absence of the reference clock, PLL can output a low output frequency.

PLL, in fact, consists of VCO (voltage controlled oscillator) and PFD - phase and frequency detector, which compares two input clocks - reference and looped clock, typically divided version of VCO.

The output signal from PFD controls the VCO frequency.

So in case of taking away the reference clock, PFD should output constant error signal, which can force VCO frequency to its minimum frequency.

// You can see the schematic of PLL for example here - https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ug/altera_pll.pdf - Figure 1.

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Best regards,

Ivan