Use the pllena signal to enable or disable the PLL. When you deassert the pllena signal, the PLL does not drive any output clock signal and therefore it loses lock. All counters in the PLL, including the gated lock counter, return to the default state. When you assert the pllena signal, the PLL drives output clock signals and tries to gain lock. The single PLL enable port on each device is shared among all PLLs on the device. By default, the pllena signal is tied to VCC internally.
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