PLLs & Clock Networks
feedback for one of the dedicated external outputs, either one
single-ended or one differential pair. In this mode, one e counter
feeds back to the PLL FBINinput, becoming part of the feedback
loop.
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Normal mode: If an internal clock is used in this mode, it is
phase-aligned to the input clock pin. The external clock output pin
has a phase delay relative to the clock input pin if connected in this
mode. You define which internal clock output from the PLL should
be phase-aligned to the internal clock pin.
No compensation: In this mode, the PLL does not compensate for
any clock networks or external clock outputs.
Phase & Delay Shifting
Stratix GX device enhanced PLLs provide advanced programmable
phase and clock delay shifting. For phase shifting, you can specify a
phase shift (in degrees or time units) for each PLL clock output port or for
all outputs together in one shift. Phase-shifting values in time units are
allowed with a resolution range of 160 to 420 ps. This resolution is a
function of frequency input and the multiplication and division factors.
In other words, it is a function of the VCO period equal to one-eighth of
the VCO period. Each clock output counter can choose a different phase
of the VCO period from up to eight taps. You can use this clock output
counter along with an initial setting on the post-scale counter to achieve
a phase-shift range for the entire period of the output clock. The phase tap
feedback to the m counter can shift all outputs to a single phase or delay.
The Quartus II software automatically sets the phase taps and counter
settings according to the phase shift entered.
In addition to the phase-shift feature, the fine tune clock delay shift
feature provides advanced time delay shift control on each of the four
PLL outputs. Each PLL output shifts in 250-ps increments for a range of
–3.0 ns to +3.0 ns between any two outputs using discrete delay elements.
Total delay shift between any two PLL outputs must be less than 3 ns. For
example, shifts on outputs of –1 and +2 ns is allowed, but not –1 and
+2.5 ns. There is some delay variation due to process, voltage, and
temperature. Only the clock delay shift blocks can be controlled during
system operation for dynamic clock delay control.
Spread-Spectrum Clocking
The Stratix GX device’s enhanced PLLs use spread-spectrum technology
to reduce electromagnetic interference generation from a system by
distributing the energy over a broader frequency range. The enhanced
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Stratix GX Device Handbook, Volume 1
Altera Corporation
February 2005
ALTERA [ ALTERA CORPORATION ]
