4.1.9. Clock Enable Signals
You cannot use the clock enable and disable circuit of the clock control block if the GCLK or RCLK output drives the input of a PLL.
The clkena signals are supported at the clock network level instead of at the PLL output counter level. This allows you to gate off the clock even when you are not using a PLL. You can also use the clkena signals to control the dedicated external clocks from the PLLs.
Cyclone® V devices have an additional metastability register that aids in asynchronous enable and disable of the GCLK and RCLK networks. You can optionally bypass this register in the Intel® Quartus® Prime software.
The PLL can remain locked, independent of the clkena signals, because the loop-related counters are not affected. This feature is useful for applications that require a low-power or sleep mode. The clkena signal can also disable clock outputs if the system is not tolerant of frequency overshoot during resynchronization.
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