EP2C20F256C8N Datasheet, PDF (202/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Hardware Features
areset
The PLL areset signal is the reset and resynchronization input for each
PLL. The areset signal should be asserted every time the PLL loses lock
to guarantee correct phase relationship between the PLL input and
output clocks. You should include the areset signal in designs if any of
the following conditions are true:
■ Manual clock switchover is enabled in the design
■ Phase relationships between input and output clocks need to be
maintained after a loss of lock condition
■ If the input clock to the PLL is not toggling or is unstable upon
powerup, assert the areset signal after the input clock is toggling,
staying within the input jitter specification
1
Altera recommends using the areset and locked signals in
your designs to control and observe the status of your PLL.
The areset signal is an active high signal and, when driven high, the
PLL counters reset, clearing the PLL output and causing the PLL to lose
lock. The VCO is also set back to its nominal frequency. The clock outputs
from the PLL are driven to ground as long as areset is active. When
areset transitions low, the PLL resynchronizes to its input clock as the
PLL relocks. If the target VCO frequency is below this nominal frequency,
then the PLL clock output frequency starts at a higher value than desired
during the lock process. In this case, Altera recommends monitoring the
gated locked signal to ensure the PLL is fully in lock before enabling the
clock outputs from the PLL. The Cyclone II device can drive this PLL
input signal from LEs or any general-purpose I/O pin. The areset
signal is optional. When it is not enabled in the Quartus II software, the
port is internally tied to GND.
pfdena
The pfdena signal is an active high signal that controls the PFD output
in the PLL with a programmable gate. If you disable the PFD by
transitioning pfdena low, the VCO operates at its last set control voltage
and frequency value with some long-term drift to a lower frequency. Even
though the PLL clock outputs continue to toggle regardless of the input
clock, the PLL could lose lock. The system continues running when the
PLL goes out of lock or if the input clock is disabled. By maintaining the
current frequency, the system has time to store its current settings before
shutting down. If the pfdena signal transitions high, the PLL relocks and
resynchronizes to the input clock. The pfdena input signal can be driven
by any general-purpose I/O pin or from LEs. This signal is optional.
When it is not enabled in the Quartus II software, the port is internally
tied to V_CC.
7–18
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2007