Altera Corporation
13–17
July 2005
Stratix GX Device Handbook, Volume 2
General-Purpose PLLs in Stratix & Stratix GX Devices
frequency, then the output frequency starts at a higher value than desired
as the PLL locks. If the system cannot tolerate this, the clkena signal can
disable the output clocks until the PLL locks.
The pfdena signals control the phase frequency detector (PFD) output
with a programmable gate. If you disable the PFD, the VCO operates at
its last set value of control voltage and frequency with some long-term
drift to a lower frequency. The system continues running when the PLL
goes out of lock or the input clock is disabled. By maintaining the last
locked frequency, the system has time to store its current settings before
shutting down. You can either use your own control signal or a clkloss
status signal to trigger pdfena.
The clkena signals control the enhanced PLL regional and global
outputs. Each regional and global output port has its own clkena signal.
The clkena signals synchronously disable or enable the clock at the PLL
output port by gating the outputs of the g and l counters. The clkena
signals are registered on the falling edge of the counter output clock to
enable or disable the clock without glitches.
Figure 13–7 shows the waveform example for a PLL clock port enable.
The PLL can remain locked independent of the clkena signals since the
loop-related counters are not affected. This feature is useful for
applications that require a low power or sleep mode. Upon re-enabling,
the PLL does not need a resynchronization or relock period. The clkena
signal can also disable clock outputs if the system is not tolerant to
frequency overshoot during resynchronization.
The extclkena signals work in the same way as the clkena signals, but
they control the external clock output counters (e0, e1, e2, and e3). Upon
re-enabling, the PLL does not need a resynchronization or relock period
unless the PLL is using external feedback mode. In order to lock in
external feedback mode, the external output must drive the board trace
back to the FBIN pin.