Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
5–17
Hardware Features
PLL Control Signals
You can use the following three signals to observe and control the PLL operation and
resynchronization.
pfdena
Use the pfdenasignal to maintain the last locked frequency so that your system has
time to store its current settings before shutting down. The pfdenasignal controls the
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.
areset
The aresetsignal is the reset or resynchronization input for each PLL. The device
input pins or internal logic can drive these input signals. When driven high, the PLL
counters reset, clearing the PLL output and placing the PLL out of lock. The VCO is
then set back to its nominal setting. When driven low again, the PLL resynchronizes
to its input as it re-locks.
You must include the aresetsignal in your designs if one of the following conditions
is true:
■
PLL reconfiguration or clock switchover enabled in your design
■
Phase relationships between the PLL input clock and output clocks must be
maintained after a loss-of-lock condition
1
1
If the input clock to the PLL is toggling or unstable upon power up, assert the areset
signal after the input clock is stable and within specifications.
locked
The lockedoutput indicates that the PLL has locked onto the reference clock and the
PLL clock outputs are operating at the desired phase and frequency set in the
Quartus II MegaWizard™ Plug-in Manager.
Altera recommends that you use the aresetand lockedsignals in your designs to
control and observe the status of your PLL.
This implementation is illustrated in Figure 5–13.
Figure 5–13. Locked Signal Implementation
locked
V
CC
OFF
D
Q
PLL
locked
areset
December 2011 Altera Corporation
Cyclone III Device Handbook
Volume 1
ALTERA [ ALTERA CORPORATION ]
