PLLs in Stratix II and Stratix II GX Devices
Software Support
You can enter the desired down-spread percentage and modulation
frequency in the altpllmegafunction through the Quartus II software.
Alternatively, the downspreadparameter in the altpllmegafunction
can be set to the desired down-spread percentage. Timing analysis
ensures the design operates at the maximum spread frequency and meets
all timing requirements.
f
For more information about PLL software support in the Quartus II
software, see the altpll Megafunction User Guide.
Guidelines
If the design cascades PLLs, the source (upstream) PLL should have a
low-bandwidth setting, while the destination (downstream) PLL should
have a high-bandwidth setting. The upstream PLL must have a
low-bandwidth setting because a PLL does not generate jitter higher than
its bandwidth. The downstream PLL must have a high bandwidth setting
to track the jitter. The design must use the spread-spectrum feature in a
low-bandwidth PLL, and, therefore, the Quartus II software
automatically sets the spread-spectrum PLL bandwidth to low.
1
If the programmable or reconfigurable bandwidth features are
used, then you cannot use spread spectrum.
Stratix II and Stratix II GX devices can accept a spread-spectrum input
with typical modulation frequencies. However, the device cannot
automatically detect that the input is a spread-spectrum signal. Instead,
the input signal looks like deterministic jitter at the input of the
downstream PLL.
Spread spectrum can have a minor effect on the output clock by
increasing the period jitter. Period jitter is the deviation of a clock’s cycle
time from its previous cycle position. Period jitter measures the variation
of the clock output transition from its ideal position over consecutive
edges.
With down-spread modulation, the peak of the modulated waveform is
the actual target frequency. Therefore, the system never exceeds the
maximum clock speed. To maintain reliable communication, the entire
system and subsystem should use the Stratix II and Stratix II GX device
as the clock source. Communication could fail if the Stratix II or
Stratix II GX logic array is clocked by the spread-spectrum clock, but the
data it receives from another device is not clocked by the spread
spectrum.
Altera Corporation
July 2009
1–55
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
