Manual Override - Altera Cyclone IV Device Handbook

Chapter 5: Clock Networks and PLLs in Cyclone IV Devices
Hardware Features
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. Choose the secondary clock frequency so the VCO operates in the
recommended frequency range. Also, set the M, N, and C counters accordingly to
keep the VCO operating frequency in the recommended range.
Figure 5–18
automatic loss of clock detection. Here, the inclk0 signal remains low. After the
inclk0 signal remains low for approximately two clock cycles, the clock-sense
circuitry drives the clkbad0 signal high. Also, because the reference clock signal is not
toggling, the switchover state machine controls the multiplexer through the clksw
signal to switch to inclk1.
Figure 5–18. Automatic Switchover Upon Clock Loss Detection
Note to
(1) Switchover is enabled on the falling edge of inclk0 or inclk1, depending on which clock is available. In this figure,
switchover is enabled on the falling edge of inclk1.

Manual Override

If you are using the automatic switchover, you must switch input clocks with the
manual override feature with the clkswitch input.
Figure 5–19
when controlled by clkswitch. In this case, both clock sources are functional and
inclk0 is selected as the reference clock. A low-to-high transition of the clkswitch
signal starts the switchover sequence. The clkswitch signal must be high for at least
three clock cycles (at least three of the longer clock period if inclk0 and inclk1 have
different frequencies). On the falling edge of inclk0, the reference clock of the counter,
muxout, is gated off to prevent any clock glitching. On the falling edge of inclk1, the
reference clock multiplexer switches from inclk0 to inclk1 as the PLL reference, and
the activeclock signal changes to indicate which clock is currently feeding the PLL.
October 2012 Altera Corporation
shows a waveform example of the switchover feature when using
inclk0
inclk1
muxout
clkbad0
clkbad1
activeclock
Figure 5–18:
shows an example of a waveform illustrating the switchover feature
(1)
(1)
Cyclone IV Device Handbook,
5–29
Volume 1