EP4CE40 datasheet(103/488 Pages) ALTERA

103 / 488 page

Chapter 5: Clock Networks and PLLs in Cyclone IV Devices

5–41

Spread-Spectrum Clocking

October 2012

Altera Corporation

Cyclone IV Device Handbook,

Volume 1

Figure 5–26 shows the dynamic phase shifting waveform.

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 ALTPLL_RECONFIG Megafunction User Guide.

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 Cyclone IV Device Datasheet

chapter.

Figure 5–26. PLL Dynamic Phase Shift

PHASEDONE goes low

synchronous with SCANCLK

SCANCLK

PHASESTEP

PHASEUPDOWN

PHASECOUNTERSELECT

PHASEDONE

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