EP3SE50F780I3N Altera, EP3SE50F780I3N Datasheet - Page 196

• Manufacturer Part Number: EP3SE50F780I3N
• Description: Stratix III
• Manufacturer: Altera

PLLs in Stratix III Devices
Figure 6–32. Manual Clock Switchover Circuitry in Stratix III PLLs
6–46
Stratix III Device Handbook, Volume 1
clkswitch
inclk0
inclk1
f
Control Logic
Clock Switch
For more information on PLL software support in the Quartus II
software, refer to the
Guidelines
Use the following guidelines when implementing clock switchover in
Stratix III PLLs.
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Automatic clock switchover requires that the inclk0 and inclk1
frequencies be within 100% (2×) of each other. Failing to meet this
requirement causes the clkbad[0] and clkbad[1] signals to not
function properly.
When using manual clock switchover, the difference between
inclk0 and inclk1 can be more than 100% (2×). However,
differences in frequency and/or phase of the two clock sources will
likely cause the PLL to lose lock. Resetting the PLL ensures that the
correct phase relationships are maintained between input and
output clocks.
Applications that require a clock switchover feature and a small
frequency drift should use a low-bandwidth PLL. The low
bandwidth PLL reacts more slowly than a high-bandwidth PLL to
reference input clock changes. When the switchover happens, a low
bandwidth PLL propagates the stopping of the clock to the output
more slowly than a high-bandwidth PLL. However, be aware that
the low-bandwidth PLL also increases lock time.
After a switchover occurs, there may be a finite resynchronization
period for the PLL to lock onto a new clock. The exact amount of time
it takes for the PLL to re-lock depends on the PLL configuration.
The phase relationship between the input clock to the PLL and the
output clock from the PLL is important in your design. Assert
areset for at least 10 ns after performing a clock switchover. Wait
for the locked signal to go high and be stable before re-enabling the
output clocks from the PLL.
muxout
n Counter
altpll Megafunction User
refclk
Guide.
PFD
Altera Corporation
November 2007
fbclk