Figure 4-33: Automatic Clock Switchover Circuit Block Diagram
This figure shows a block diagram of the automatic switchover circuit built into the PLL.
Clock
Sense
Switchover
State Machine
Clock Switch
Control Logic
N Counter
inclk0
inclk1
Multiplexer
Out
clkbad[0]
clkbad[1]
activeclock
clkswitch
refclk
fbclk
clksw
PFD
When the current reference clock is not present, the clock sense block automatically switches to the backup
clock for PLL reference. You can select a clock source as the backup clock by connecting it to the
inclk1
port of the PLL in your design.
The clock switchover circuit sends out three status signals—
clkbad[0]
,
clkbad[1]
, and
activeclock
—from
the PLL to implement a custom switchover circuit in the logic array.
In automatic switchover mode, the
clkbad[0]
and
clkbad[1]
signals indicate the status of the two clock
inputs. When they are asserted, the clock sense block detects that the corresponding clock input has stopped
toggling. These two signals are not valid if the frequency difference between
inclk0
and
inclk1
is greater
than 20%.
The
activeclock
signal indicates which of the two clock inputs (
inclk0
or
inclk1
) is being selected as the
reference clock to the PLL. When the frequency difference between the two clock inputs is more than 20%,
the
activeclock
signal is the only valid status signal.
Glitches in the input clock may cause the frequency difference between the input clocks to be more
than 20%.
Note:
Use the switchover circuitry to automatically switch between
inclk0
and
inclk1
when the current reference
clock to the PLL stops toggling. You can switch back and forth between
inclk0
and
inclk1
any number of
times when one of the two clocks fails and the other clock is available.
For example, in applications that require a redundant clock with the same frequency as the reference clock,
the switchover state machine generates a signal (
clksw
) that controls the multiplexer select input. In this
case,
inclk1
becomes the reference clock for the PLL.
When using automatic clock switchover mode, the following requirements must be satisfied:
• Both clock inputs must be running when the FPGA is configured.
• The period of the two clock inputs can differ by no more than 20%.
If the current clock input stops toggling while the other clock is also not toggling, switchover is not initiated
and the
clkbad[0..1]
signals are not valid. If both clock inputs are not the same frequency, but their period
Altera Corporation
Clock Networks and PLLs in Cyclone V Devices
4-33
Automatic Switchover
CV-52004
2014.01.10