EP2S90F1020C5 Datasheet, PDF (318/768 Pages) Altera Corporation – Stratix II Device Handbook, Volume 1

Spread-Spectrum Clocking
Figure 1–33. Stratix II and Stratix II GX Spread-Spectrum Circuit Block Diagram
refclk
÷n
PFD
Spread-
Spectrum
Counter
n count1
÷m
n count2
Up
Down
m count1
m count2
Figure 1–34 shows a VCO frequency waveform when toggling between
different counter values. Since the enhanced PLL switches between two
different m and n values, the result is a straight line between two
frequencies, which gives a linear modulation. The magnitude of
modulation is determined by the ratio of two m/n sets. The percent
spread is determined by:
percent spread = (f_VCOmax - f_VCOmin)/f_VCOmax = 1 [(m₂ × n₁)/(m₁ × n₂)].
The maximum and minimum VCO frequency is defined as:
f_VCOmax = (m₁/n₁) × f_REF
f_VCOmin = (m₂/n₂) × f_REF
Figure 1–34. VCO Frequency Modulation Waveform
count2 values
count1 values
VCO Frequency
1–54
Stratix II Device Handbook, Volume 2
Altera Corporation
July 2009