Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family
5–33
Spread-Spectrum Clocking
Figure 5–24 shows the dynamic phase shifting waveform.
Figure 5–24. Timing Diagram for Dynamic Phase Shift
SCANCLK
PHASESTEP
PHASEUPDOWN
PHASECOUNTERSELECT
PHASEDONE
a
b
c
d
PHASEDONE goes low
synchronous with SCANCLK
The PHASESTEPsignal is latched on the negative edge of SCANCLK(a,c) and must remain
asserted for at least two SCANCLKcycles. Deassert PHASESTEPafter PHASEDONEgoes low.
On the second SCANCLKrising edge (b,d) after PHASESTEPis latched, the values of
PHASEUPDOWNand PHASECOUNTERSELECTare latched and the PLL starts dynamic
phase-shifting for the specified counters, and in the indicated direction. PHASEDONEis
deasserted synchronous to SCANCLKat the second rising edge (b,d) and remains low
until the PLL finishes dynamic phase-shifting. Depending on the VCO and SCANCLK
frequencies, PHASEDONElow time may be greater than or less than one SCANCLKcycle.
You can perform another dynamic phase-shift after the PHASEDONEsignal goes from
low to high. Each PHASESTEPpulse enables one phase shift. PHASESTEPpulses must be
at least one SCANCLKcycle apart.
f
For information about the ALTPLL_RECONFIG MegaWizard Plug-In Manager, refer
to the Phase-Locked Loop Reconfiguration (ALTPLL_RECONFIG) Megafunction user
guide.
Spread-Spectrum Clocking
The Cyclone III device family 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. Cyclone III device family PLLs 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, which
is specified in the fitter report. The Cyclone III device family cannot generate
spread-spectrum signals internally.
PLL Specifications
f
For information about PLL specifications, refer to the Cyclone III Device Data Sheet and
Cyclone III LS Device Data Sheet chapters.
July 2012 Altera Corporation
Cyclone III Device Handbook
Volume 1
ALTERA [ ALTERA CORPORATION ]
