P240 datasheet(73/348 Pages) ALTERA

73 / 348 page

Chapter 5: Clock Networks and PLLs in the Cyclone III Device Family

5–13

Clock Feedback Modes

December 2011

Altera Corporation

Cyclone III Device Handbook

Volume 1

No Compensation Mode

In no compensation mode, the PLL does not compensate for any clock networks. This

provides better jitter performance because clock feedback into the PFD does not pass

through as much circuitry. Both the PLL internal and external clock outputs are

phase-shifted with respect to the PLL clock input.

Figure 5–9 shows a waveform example of the phase relationship of the PLL clock in

this mode.

Normal Mode

An internal clock in normal mode 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. The Quartus II software timing analyzer reports any phase difference between

the two. In normal mode, the PLL fully compensates the delay introduced by the

GCLK network.

Figure 5–9. Phase Relationship Between PLL Clocks in No Compensation Mode

Notes to Figure 5–9:

(1) Internal clocks fed by the PLL are phase-aligned to each other.

(2) The PLL clock outputs can lead or lag the PLL input clocks.

PLL Reference

Clock at the Input Pin

PLL Clock at the

(1), (2)

External PLL Clock

Outputs

(2)

Phase Aligned

P240 Datasheet(PDF) 73 Page - Altera Corporation