Stratix Architecture
Figure 2–53. Clock Switchover Circuitry
CLK0_BAD
CLK1_BAD
Active Clock
SMCLKSW
Clock
Sense
Switch-Over
State Machine
CLKLOSS
CLKSWITCH
Δt
INCLK0
INCLK1
MUXOUT
PFD
n Counter
FBCLK
Enhanced PLL
There are two possible ways to use the clock switchover feature.
■
Use automatic switchover circuitry for switching between inputs of
the same frequency. For example, in applications that require a
redundant clock with the same frequency as the primary clock, the
switchover state machine generates a signal that controls the
multiplexer select input on the bottom of Figure 2–53. In this case, the
secondary clock becomes the reference clock for the PLL.
Use the clkswitchinput for user- or system-controlled switch
conditions. This is possible for same-frequency switchover or to
switch between inputs of different frequencies. For example, if
inclk0is 66 MHz and inclk1is 100 MHz, you must control the
switchover because the automatic clock-sense circuitry cannot
monitor primary and secondary clock frequencies with a frequency
difference of more than 20%. This feature is useful when clock
sources can originate from multiple cards on the backplane,
requiring a system-controlled switchover between frequencies of
operation. You can use clkswitchtogether with the lock signal to
trigger the switch from a clock that is running but becomes unstable
and cannot be locked onto.
■
Altera Corporation
July 2005
2–89
Stratix Device Handbook, Volume 1
ALTERA [ ALTERA CORPORATION ]
