PLLs in Stratix II and Stratix II GX Devices
Programmable Duty Cycle
The programmable duty cycle allows enhanced and fast PLLs to generate
clock outputs with a variable duty cycle. This feature is supported on
each enhanced and fast PLL post-scale counter C[]. The duty cycle
setting is achieved by a low and high time count setting for the post-scale
counters. The Quartus II software uses the frequency input and the
required multiply or divide rate to determine the duty cycle choices. The
post-scale counter value determines the precision of the duty cycle. The
precision is defined by 50% divided by the post-scale counter value. The
closest value to 100% is not achievable for a given counter value. For
example, if the C0counter is ten, then steps of 5% are possible for duty
cycle choices between 5 to 90%.
If the device uses external feedback, you must set the duty cycle for the
counter driving the fbinpin to 50%. Combining the programmable duty
cycle with programmable phase shift allows the generation of precise
non-overlapping clocks.
Advanced Clear and Enable Control
There are several control signals for clearing and enabling PLLs and their
outputs. You can use these signals to control PLL resynchronization and
gate PLL output clocks for low-power applications.
Enhanced Lock Detect Circuit
The lock output indicates that the PLL has locked onto the reference clock.
Without any additional circuitry, the lock signal may toggle as the PLL
begins tracking the reference clock. You may need to gate the lock signal
for use as a system control. Either a gated lock signal or an ungated lock
signal from the locked port can drive the logic array or an output pin. The
Stratix II and Stratix II GX enhanced and fast PLLs include a
programmable counter that holds the lock signal low for a user-selected
number of input clock transitions. This allows the PLL to lock before
enabling the lock signal. You can use the Quartus II software to set the
20-bit counter value.
Altera Corporation
July 2009
1–29
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
