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Figure 4-13:
clkena
Implementation with Clock Enable and Disable Circuit
This figure shows the implementation of the clock enable and disable circuit of the clock control block.
clkena
Clock Select
Multiplexer Output
GCLK/
RCLK/
FPLL_<#>_CLKOUT
D
D
Q
Q
R1
R2
The R1 and R2 bypass paths
are not available for the PLL
external clock outputs.
The select line is statically
controlled by a bit setting in
the .sof or .pof.
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.
Figure 4-14: Example of
clkena
Signals
This figure shows a waveform example for a clock output enable. The
clkena
signal is synchronous to the
falling edge of the clock output.
clkena
AND Gate Output
with R2 Bypassed
(ena Port Registered as
Falling Edge of Input Clock)
Clock Select
Multiplexer Output
AND Gate Output
with R2 Not Bypassed
(ena Port Registered as Double
Register with Input Clock)
Use the clkena signals to
enable or disable the GCLK
and RCLK networks or the
FPLL_<#>_CLKOUT pins.
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 Quartus II 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.
Altera Corporation
Clock Networks and PLLs in Cyclone V Devices
4-15
Clock Enable Signals
CV-52004
2014.01.10