Spread-Spectrum Clocking; Pll Specifications - Altera Cyclone IV Device Handbook

Chapter 5: Clock Networks and PLLs in Cyclone IV Devices

Spread-Spectrum Clocking

Figure 5–26
Figure 5–26. PLL Dynamic Phase Shift
PHASECOUNTERSELECT
The PHASESTEP signal is latched on the negative edge of SCANCLK (a,c) and must remain
asserted for at least two SCANCLK cycles. Deassert PHASESTEP after PHASEDONE goes low.
On the second SCANCLK rising edge (b,d) after PHASESTEP is latched, the values of
PHASEUPDOWN and PHASECOUNTERSELECT are latched and the PLL starts dynamic
phase-shifting for the specified counters, and in the indicated direction. PHASEDONE is
deasserted synchronous to SCANCLK at the second rising edge (b,d) and remains low
until the PLL finishes dynamic phase-shifting. Depending on the VCO and SCANCLK
frequencies, PHASEDONE low time may be greater than or less than one SCANCLK cycle.
You can perform another dynamic phase-shift after the PHASEDONE signal goes from
low to high. Each PHASESTEP pulse enables one phase shift. PHASESTEP pulses must be
at least one SCANCLK cycle apart.
f
For information about the ALTPLL_RECONFIG MegaWizard Plug-In Manager,
refer to the
Spread-Spectrum Clocking
Cyclone IV 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 PLL. PLLs of Cyclone IV devices can track a spread-spectrum input clock
as long as it is in the input jitter tolerance specifications and the modulation frequency
of the input clock is below the PLL bandwidth, that is specified in the fitter report.
Cyclone IV devices cannot generate spread-spectrum signals internally.

PLL Specifications

f
For information about PLL specifications, refer to the
chapter.
October 2012 Altera Corporation
shows the dynamic phase shifting waveform.
SCANCLK
PHASESTEP
PHASEUPDOWN
PHASEDONE
a
ALTPLL_RECONFIG Megafunction User
b
PHASEDONE goes low
synchronous with SCANCLK
Guide.
Cyclone IV Device Datasheet
5–41
c d
Cyclone IV Device Handbook,
Volume 1